Wisconsin Department of Transportation Roundabout Guide

Transcription

Wisconsin Department of Transportation Roundabout Guide March 2013, update Facilities Development Manual
Chapter 11
Section 25
Design
Intersections at Grade
Wisconsin Department of Transportation
March 4, 2013
FDM 11‐25‐1 GENERAL ... ..............................................................................................................................................5
1.1 DESIGN CONSIDERATIONS .......................................................................................................................................5
1.1.1 RIGHT‐OF‐WAY CONSIDERATIONS .......................................................................................................................................6
1.2 URBAN INTERSECTIONS ...........................................................................................................................................6
1.3 RURAL INTERSECTIONS............................................................................................................................................7
1.3.1 INTERSECTIONS ON RURAL HIGH‐SPEED MULTILANE DIVIDED HIGHWAYS (“RURAL EXPRESSWAYS”)................................................7
1.3.1.1 Rural Expressway Intersection Safety Treatments .............................................................................................. 8
Table 1.1 Potential rural‐expressway intersection safety treatment .................................................................................. 9
Figure 1.1 Countermeasure matrices for TWSC expressway intersection.........................................................................10
Figure 1.2 Conflict Point Diagram for expressway 4‐legged intersection .......................................................................... 10
Figure 1.3 Conflict point Diagrams for expressway T‐intersections................................................................................... 11
1.3.1.2 Median width at unsignalized median openings on rural expressways............................................................
12
1.3.1.3 Median Signage and Delineation ...................................................................................................................... 13
1.3.2 J‐TURN INTERSECTION .................................................................................................................................................... 13
Figure 1.4 Conflict Point Diagram for J‐turn Intersection .................................................................................................. 14
1.4 TRUCK ROUTES AND ROUTES FOR OVERSIZED‐OVERWEIGHT (OSOW) VEHICLES.....................................................15
1.5 REFERENCES ............. ............................................................................................................................................16
LIST OF ATTACHMENTS .. ............................................................................................................................................17
Attachment 1.1 Selection Criteria for Rural High Speed Intersections (Posted Speed >= 50 mph)...............................17
FDM 11‐25‐2 DESIGN CRITERIA AND GUIDELINES.........................................................................................................18
2.1 DESIGN VEHICLES ..... ............................................................................................................................................18
2.1.1 OSOW VEHICLES .......................................................................................................................................................... 18
2.1.1.1 Single‐Trip Permit OSOW vehicles (OSOW‐ST) .................................................................................................. 19
20
2.1.1.2 Multiple‐Trip Permit OSOW vehicles (OSOW‐MT).............................................................................................
2.1.1.3 OSOW Vehicle Inventory Evaluation Overview ................................................................................................. 20
Figure 2.1 Checkbox to Control Rear Steering ................................................................................................................... 21
2.1.2 SELECTING VEHICLES FOR INTERSECTION DESIGN AND OSOW VEHICLE CHECKS ........................................................................ 21
Figure 2.2 Illustrative Turning Movements for Intersection Design and Check Vehicles..................................................22
Figure 2.3 Degrees of Encroachment ................................................................................................................................23
Figure 2.4 Effective Pavement width and effect on degree of encroachment ..................................................................23
Table 2.1 Default Intersection Design and Check Vehicles & Degree of Encroachment [DE] [A] ........................................24
Table 2.2 intersections Where Checking OSOW‐ST or OSOW‐MT Vehicles is Required [A] [B] [C] ........................................26
Figure 2.5 WisDOT’s Interim Policy on Checking Criteria for OSOW‐ST and OSOW‐MT Vehicles at intersections............27
2.2 PHYSICAL AND FUNCTIONAL AREAS OF AN INTERSECTION .....................................................................................27
Figure 2.6 Physical and Functional Areas of an Intersection ........................................................................................... 28
2.2.1 DOWNSTREAM FUNCTIONAL LENGTH OF INTERSECTION........................................................................................................ 28
Table 2.3 Downstream Functional Length of Intersection Minimum Requirements........................................................
28
29
Figure 2.7 Downstream Functional Lengths of Intersection.............................................................................................
2.2.2 UPSTREAM FUNCTIONAL LENGTH OF INTERSECTION ............................................................................................................. 29
Figure 2.8 Upstream Functional Length of Intersection Elements...................................................................................
30
Table 2.4 Upstream Functional Length of Intersection Elements d1, d2, and d3 [A]]] ...................................................... 31
Table 2.5 Queue Storage (d4) for STH Intersections [A] [B] [C] ........................................................................................... 32
2.3 TURN BAYS...............
............................................................................................................................................33
Figure 2.9 Turn Bay Elements and Correlation with Upstream Functional Length of Intersection .................................. 34
Table 2.6 Full‐Width Turn‐Lane Length for Urban Streets and Low Speed Rural [A] ........................................................ 35
2.3.1 LEFT TURN LANES ......................................................................................................................................................... 35
2.3.2 RIGHT TURN LANES ....................................................................................................................................................... 35
2.4 TAPER DESIGN..........
............................................................................................................................................35
2.4.1 LANE REDUCTION AT INTERSECTION .................................................................................................................................. 35
2.5 CORNER CLEARANCE TO DRIVEWAYS.....................................................................................................................36
Figure 2.10 Intersection Marginal Corner Clearances (See Table 2.7)...............................................................................
37
Figure 2.11 Intersection Median Corner Clearances ......................................................................................................... 37
Table 2.7 Marginal Corner Clearance Distances ............................................................................................................... 39
2.5.1 CORNER CLEARANCE FOR NON‐STH ROADS ....................................................................................................................... 39
Figure 2.12 Inadequate Corner Clearance on Sideroad ..................................................................................................... 40
2.6 INTERSECTION VERTICAL ALIGNMENT....................................................................................................................40
2.7 INTERSECTION SIGHT DISTANCE.............................................................................................................................40
2.8 ANGLE OF INTERSECTION.......................................................................................................................................40
2.8.1 ANGLE OF INTERSECTION FOR NEW INTERSECTIONS ............................................................................................................. 41
2.8.1.1 Intersection on tangent or on outside of curve: ................................................................................................ 41
2.8.1.2 Intersection on inside of curve .......................................................................................................................... 41
Table 2.8 Angle of Intersection for Intersection on Inside of Curve .................................................................................. 41
2.8.2 ANGLE OF INTERSECTION FOR EXISTING INTERSECTIONS ON NEW CONSTRUCTION AND RECONSTRUCTION PROJECTS ........................ 41
2.8.2.1 Intersection on tangent or on outside of curve ................................................................................................. 41
2.8.3 ANGLE OF INTERSECTION FOR EXISTING INTERSECTIONS ON 3R PROJECTS ................................................................................ 41
2.8.3.1 Intersection on tangent or on outside of curve ................................................................................................. 41
2.9 INTERSECTIONS ON CURVES ..................................................................................................................................41
2.10 REFERENCES ........... ............................................................................................................................................42
Attachment 2.1 .. WisDOT Vehicle Inventory of OSOW Vehicles ..................................................................................... 43
Attachment 2.2 Taper Length Criteria ........................................................................................................................... 43
FDM 11‐25‐3 INTERSECTION CONTROL EVALUATION ..................................................................................................44
FDM 11‐25‐5 LEFT‐TURN LANES ....................................................................................................................... 45
5.1 INTRODUCTION ........ ............................................................................................................................................45
5.2 WARRANTING CRITERIA ........................................................................................................................................45
Table 5.1 Operational Warrants for Left‐Turn Lanes at Intersections on Two‐Lane Highways ........................................46
5.3 DESIGN CRITERIA......
............................................................................................................................................46
5.3.1 WIDTHS ...................................................................................................................................................................... 46
Table 5.2 Median, Separator, and Turn Lane Widths for non‐slotted Left Turn Lanes on Low‐Speed Urban
Arterials (Minimum Widths)..............................................................................................................................................47
5.3.2 MEDIAN END TREATMENT .............................................................................................................................................. 47
5.3.3 LENGTH ....................................................................................................................................................................... 48
5.4 SPECIAL DESIGNS......
............................................................................................................................................48
5.4.1 SLOTTED LEFT‐TURN LANES ............................................................................................................................................ 48
Figure 5.1 Urban Slotted Left Turn Lane with Left‐Turn Island.........................................................................................
49
5.4.2 TWO‐WAY LEFT‐TURN LANE (TWLTL) ............................................................................................................................. 49
5.4.2.1 Conversion from 4‐Lane Undivided to 3‐lane TWLTL (“Road Diet”) ................................................................. 50
5.4.3 MULTIPLE LEFT TURN LANES ........................................................................................................................................... 51
5.4.3.1 Design Considerations for Multiple Left Turn lanes ......................................................................................... 51
5.4.3.1.1 Dual Left Turn Lanes.....................................................................................................................................................
52
Figure 5.2 Outside and Inside Lanes for Dual Left Turn Lanes..........................................................................................
52
Figure 5.3 Dual Left Turn Lane with Throat Widening on Departure Leg ‐ Design Vehicle & Single Unit Vehicle
Turning Together . ............................................................................................................................................................ 52
Figure 5.4 Dual Left Turn Lanes ........................................................................................................................................ 53
Table 5.3 Expanded Throat Width (W) Guidelines for Dual Left Turn Lanes .................................................................... 53
53
5.4.3.1.2 Triple Left Turn Lanes...................................................................................................................................................
5.4.4 SHARED LEFT‐TURN/THRU LANES AT SIGNALIZED INTERSECTIONS ............................................................................................ 54
5.5 TEE INTERSECTION BYPASS LANE ...........................................................................................................................54
5.6 REFERENCES ............. ............................................................................................................................................54
Attachment 5.1 Attachment 5.2 Attachment 5.3 Attachment 5.4 Urban Median Opening and Intersection Guidelines ......................................................................... 56
Median Openings and Left Turn Lanes in Urban Roadways ............................................................... 56
Details for Slotted Left Turn Lanes and Median Openings at Urban Intersections.............................
56
Median Opening With Left Turn Lane on Rural High‐Speed 4‐Lane Divided Highways ...................... 56
FDM 11‐25‐10 RIGHT‐TURN LANES .................................................................................................................... 57
10.1 INTRODUCTION ...... ............................................................................................................................................57
10.2 INTERSECTIONS IN RURAL AND DEVELOPING AREAS ............................................................................................57
10.2.1 STORAGE LENGTH ....................................................................................................................................................... 57
10.3 TWO‐WAY STOP‐CONTROLLED INTERSECTIONS ON URBAN LOW SPEED AND TRANSITIONAL ROADS ...................57
Figure 10.1 Guidelines for a Major‐road Right‐turn Bay at Urban Two‐way Stop‐controlled Intersections.....................
58
10.3.1 CORNER CURB RADIUS ................................................................................................................................................. 59
10.3.2 LANE WIDTH .............................................................................................................................................................. 59
10.3.3 LANE LENGTH ............................................................................................................................................................. 59
10.4 SIGNALIZED INTERSECTION CONSIDERATIONS .....................................................................................................59
10.4.1 DUAL RIGHT TURN LANES ............................................................................................................................................. 59
Figure 10.2 Dual Right‐Turn Lanes....................................................................................................................................
61
10.5 REFERENCES ........... ............................................................................................................................................62
FDM 11‐25‐15 TURNING ROADWAYS (CHANNELIZED RIGHT) ................................................................................ 64
15.1 CRITERIA ................ ............................................................................................................................................64
15.2 SPEED AND CURVATURE .....................................................................................................................................64
15.3 DESIGN GUIDES ...... ............................................................................................................................................64
Figure 15.1 Intersection Angle for Channelized Right Turn .............................................................................................. 64
15.4 REFERENCES ........... ............................................................................................................................................64
FDM 11‐25‐20 MEDIAN OPENINGS .................................................................................................................... 65
20.1 INTRODUCTION ...... ............................................................................................................................................65
Figure 20.1 Directional Median Opening..........................................................................................................................
65
Figure 20.2 Separator Overlap for Directional Median Opening ....................................................................................... 65
Figure 20.3 Examples of Directional Median Openings between Signalized Intersections ...............................................66
20.2 U‐TURNS ................ ............................................................................................................................................66
Figure 20.4 Directional Median Openings for U‐turns (a) Downstream from Signalized Intersection (b) Upstream
from Signalized Intersection..............................................................................................................................................66
20.3 LENGTH OF OPENING...........................................................................................................................................67
20.4 SPACING.................
............................................................................................................................................67
Table 20.1 Median openings – allowable locations (applicable to STH and connecting highways) ..................................68
20.5 REFERENCES ........... ............................................................................................................................................69
FDM 11‐25‐25 CHANNELIZATION ...................................................................................................................... 70
25.1 GENERAL ................ ............................................................................................................................................70
25.2 ISLANDS ................. ............................................................................................................................................70
25.2.1 OFFSETS .................................................................................................................................................................... 70
25.2.2 SIGNALIZED INTERSECTION CONSIDERATIONS .................................................................................................................... 70
25.3 PAVEMENT MARKINGS........................................................................................................................................71
25.4 REFERENCES ........... ............................................................................................................................................71
FDM 11‐25‐30 CURB RAMPS........................................................................................................................................72
FDM 11‐25‐35 AUXILIARY LANES ...................................................................................................................... 72
35.1 AUXILIARY LANES ... ............................................................................................................................................72
35.2 ACCELERATION LANES .........................................................................................................................................72
35.3 BUS STOPS ............. ............................................................................................................................................72
35.4 REFERENCES ........... ............................................................................................................................................72
FDM 11‐25‐40 RAILROAD CROSSINGS ................................................................................................................ 73
40.1 GENERAL ................ ............................................................................................................................................73
40.2 REFERENCES .......... ............................................................................................................................................73
FDM 11‐25‐45 FRONTAGE ROADS .................................................................................................................... 74
45.1 GENERAL ................ ............................................................................................................................................74
Figure 45.1 Frontage Road Offset Guidelines .................................................................................................................... 75
45.2 REFERENCES .......... ............................................................................................................................................75
FDM 11‐25 MASTER REFERENCE LIST ........................................................................................................................76
Facilities Development Manual
Chapter 11
Section 26
FDM 11‐26‐1 GENERAL Design
Roundabouts
March 4, 2013
1.1 GENERAL ................................................................................................................................................................ 6
1.2 MODERN ROUNDABOUT VS. OTHER CIRCULAR INTERSECTIONS .............................................................................. 7
Table 1.1 Distinguishing Characteristics of Modern Roundabouts......................................................................................
8
1.3 ADVANTAGES AND DISADVANTAGES......................................................................................................................
9
Table 1.2 Advantages and Disadvantages of Roundabouts vs. Other Alternatives. ............................................................9
1.4 DEFINING PHYSICAL FEATURES ............................................................................................................................. 10
Figure 1.1 Single‐lane Roundabout Features.....................................................................................................................
10
11
Figure 1.2 Multilane Roundabout Features.......................................................................................................................
Table 1.3 Roundabout Features ........................................................................................................................................12
1.5 ROUNDABOUT CATEGORIES ................................................................................................................................. 12
1.5.1 SINGLE‐LANE ROUNDABOUT ............................................................................................................................................ 12
1.5.1.1 Urban Single‐Lane Roundabouts ...................................................................................................................... 12
1.5.1.2 Rural Single‐Lane Roundabouts ....................................................................................................................... 12
1.5.2 MULTILANE ROUNDABOUTS ............................................................................................................................................ 13
1.5.2.1 Urban Multilane Roundabouts ......................................................................................................................... 13
1.5.2.2 Rural Multilane Roundabouts .......................................................................................................................... 13
1.5.3 COMBINATION ROUNDABOUTS ........................................................................................................................................ 13
1.6 THROUGH HIGHWAY DECLARATION (SS 340.01(67) & 349.07)...............................................................................
13
1.7 SPEED ZONE DECLARATIONS (SS 346.57 & 349.11) ................................................................................................ 13
1.8 REFERENCES ......................................................................................................................................................... 13
FDM 11‐26‐5 DESIGN PROCESS AND QUALIFICATIONS FEBRUARY X, 2013 ................................................................ 14
5.1 ROUNDABOUT DESIGN PROCESS AND QUALIFICATIONS........................................................................................ 14
5.2 ROUNDABOUT DESIGNER REQUIREMENTS............................................................................................................
14
5.3 INTERSECTION CONTROL EVALUATION, PROGRAM LEVEL SCOPING PHASE ........................................................... 15
5.4 THE 3‐STAGE ROUNDABOUT DESIGN PROCESS......................................................................................................
15
Figure 5.1 WisDOT 3‐Stage Design Process ....................................................................................................................... 16
5.4.1 STAGE 1, ROUNDABOUT DESIGN PROCESS ......................................................................................................................... 16
FDM 11‐26‐10 USER CONSIDERATIONS FEBRUARY X, 2013 ....................................................................................... 18
10.1 PEDESTRIAN AND BICYCLIST ACCOMMODATIONS ............................................................................................... 18
10.1.1 PEDESTRIANS .............................................................................................................................................................. 18
Table 10.1 Roundabout Advantages and Disadvantages for Pedestrians .........................................................................18
10.1.2 BICYCLISTS ................................................................................................................................................................. 19
10.2 TRANSIT, LARGE VEHICLE, OVERSIZE VEHICLES AND EMERGENCY VEHICLE CONSIDERATIONS .............................. 19
10.2.1 TRANSIT .................................................................................................................................................................... 19
10.2.2 LEGAL LARGE VEHICLES ................................................................................................................................................ 19
10.2.3 PERMITTED OVERSIZED OVERWEIGHT (OSOW) VEHICLES .................................................................................................. 20
10.2.4 EMERGENCY VEHICLES .................................................................................................................................................. 20
10.3 REFERENCES ....................................................................................................................................................... 20
FDM 11‐26‐15 AGENCY & PUBLIC COORDINATION FEBRUARY X, 2013 ...................................................................... 20
15.1 PUBLIC MEETINGS............................................................................................................................................... 20
15.2 PUBLIC OUTREACH RESOURCES & METHODS ...................................................................................................... 21
15.3 REFERENCES ....................................................................................................................................................... 22
FDM 11‐26‐17 SYSTEM CONSIDERATIONS FEBRUARY X, 2013 ................................................................................... 22
17.1 SYSTEM CONSIDERATIONS...................................................................................................................................
22
17.2 ADJACENT INTERSECTIONS AND HIGHWAY SEGMENTS AND COORDINATED SIGNAL SYSTEMS.............................
22
17.3 ROUNDABOUTS IN AN ARTERIAL NETWORK........................................................................................................
22
17.3.1 PLANNED NETWORK, ACCESS MANAGEMENT ................................................................................................................... 23
17.3.2 PLATOONED ARRIVALS ON APPROACHES .......................................................................................................................... 23
17.3.3 ROUNDABOUT DEPARTURE PATTERN .............................................................................................................................. 23
17.4 CLOSELY SPACED ROUNDABOUTS ....................................................................................................................... 23
17.5 ROUNDABOUT INTERCHANGE RAMP TERMINALS ............................................................................................... 24
17.6 TRAFFIC SIGNALS AT ROUNDABOUTS..................................................................................................................
24
17.7 AT‐GRADE RAIL CROSSINGS ................................................................................................................................ 24
17.8 REFERENCES ....................................................................................................................................................... 25
FDM 11‐26‐20 OPERATIONS FEBRUARY X, 2013....................................................
25
20.1 OPERATIONAL ANALYSIS REFERENCES AND METHODS ........................................................................................ 25
20.2 ROUNDABOUT OPERATION ................................................................................................................................ 25
20.2.1 PLANNING LEVEL ANALYSIS AND SPACE REQUIREMENTS ...................................................................................................... 25
Table 20.1 Typical Inscribed Circle Diameters and Estimated Daily Service Volumes .......................................................26
20.3.1 PLANNING ESTIMATES OF LANE REQUIREMENTS .............................................................................................................. 26
20.3.2 PEDESTRIAN EFFECTS ON ENTRY AND EXIT CAPACITY ......................................................................................................... 26
20.4 OPERATIONAL ANALYSIS METHODOLOGY...........................................................................................................
27
Figure 20.1 WisDOT Approved Method for Analyzing Roundabouts ................................................................................ 28
20.4.1 GATHER TRAFFIC VOLUMES, PEAK HOUR FACTORS, AND TRUCK PERCENTAGES ....................................................................... 28
20.4.2 ENTER FORECASTED TRAFFIC VOLUMES INTO TRAFFIC FLOW WORKSHEET .............................................................................. 28
20.4.3 DETERMINE NUMBER OF ENTRY LANES AND LANE CONFIGURATION, DRAW LANE CONFIGURATION SKETCH ................................. 28
Figure 20.2 Lane Configuration Options ............................................................................................................................ 29
29
Figure 20.3 Lane Configuration Sketch..............................................................................................................................
20.4.4 ANALYZE ROUNDABOUT LANE CONFIGURATION ................................................................................................................ 29
Table 20.2 Choosing Appropriate Analysis Tool ................................................................................................................ 30
20.4.5 HCS 2010 ANALYSIS ................................................................................................................................................... 30
Figure 20.4 Critical Headway .............................................................................................................................................30
Figure 20.5 Follow‐up Headway ........................................................................................................................................30
Table 20.3 Recommended Headway Values ..................................................................................................................... 31
Figure 20.6 Operational Analysis Process, Inputs, and Outputs ........................................................................................ 32
20.4.5.1 HCS 2010 Roundabout Analysis Module ........................................................................................................ 32
Table 20.4 Common Lane Configurations and their HCS 2010 Coding (Partial Listing) .....................................................32
20.4.6 SIDRA INTERSECTION ANALYSIS (US MODE)......................................................................................................................
33
20.5 SUPPLEMENTAL TOOLS FOR OPERATIONAL ANALYSIS & DESIGN ......................................................................... 33
20.5.1 SPECIAL CONSIDERATIONS ............................................................................................................................................. 33
Figure 20.7 Capacity Considerations of Flared Entries ...................................................................................................... 35
20.6 CAPACITY ANALYSIS OF AN EXISTING ROUNDABOUT .......................................................................................... 35
20.7 REFERENCES ....................................................................................................................................................... 35
LIST OF ATTACHMENTS ............................................................................................................................................... 35
Attachment 20.1 Traffic Flow Worksheet......................................................................................................................
35
FDM 11‐26‐25 ACCESS CONTROL FEBRUARY X, 2013.................................................................................................
35
25.1 ACCESS MANAGEMENT.......................................................................................................................................
35
25.2 FUNCTIONAL INTERSECTION AREA ...................................................................................................................... 36
25.3 CORNER CLEARANCE AND DRIVEWAY LOCATION CONSIDERATIONS....................................................................
36
25.4 PARKING NEAR ROUNDABOUTS..........................................................................................................................
37
25.6 REFERENCES ....................................................................................................................................................... 37
FDM 11‐26‐30 PRINCIPAL BASED DESIGN GUIDANCE FEBRUARY X, 2013...................................................................
37
30.1 INTRODUCTION .................................................................................................................................................. 37
30.2 DESIGN PRINCIPLES............................................................................................................................................. 37
30.2.1 DESIGNING WITH TRADE‐OFFS IN MIND ........................................................................................................................... 38
Table 30.1 Effects of Design Elements on Safety and Operations ..................................................................................... 38
30.2.2 STAGING AND EXPANDABILITY ........................................................................................................................................ 38
30.2.3 IMPACT OF COST REDUCTION ON ROUNDABOUTS .............................................................................................................. 39
30.3 ROUNDABOUT DESIGN PROCESS.........................................................................................................................
39
Figure 30.1 Roundabout Evaluation & Design Process......................................................................................................
40
30.4 GENERAL DESIGN STEPS & EXPLANATION ........................................................................................................... 40
30.5 DESIGN CONSIDERATIONS .................................................................................................................................. 43
30.5.1 ALIGNMENT OF APPROACHES AND ENTRIES ...................................................................................................................... 43
Figure 30.2 Entry Deflection ..............................................................................................................................................44
30.5.2 ASSESSING VEHICLE PATHS ........................................................................................................................................... 44
Figure 30.3 Determination of Entry Path Curvature..........................................................................................................
45
(See NCHRP 672 Exhibit 6‐49 for multilane entries and Exhibit 6‐50 for right turns)........................................................45
Table 30.2 Roundabout Radii ............................................................................................................................................45
30.5.3 SPEED CONSISTENCY .................................................................................................................................................... 45
30.5.4 DESIGN GUIDANCE FOR ALL TRUCKS ................................................................................................................................ 46
30.5.5 GEOMETRIC DESIGN GUIDANCE FOR LEGAL TRUCKS ........................................................................................................... 46
Table 30.3 Advantages and Disadvantages for Case 1 Roundabout Designs.....................................................................47
Table 30.6 Typical Design Parameters for Two‐Lane Roundabouts* ................................................................................ 49
30.5.5.1 Geometric Design Guidance for Case 1 Roundabouts .................................................................................... 49
Figure 30.4 Case 1 Roundabout Design (Single lane line dividing the entry lanes) ...........................................................50
30.5.5.2 Geometric Design Guidance for Case 2 Roundabouts.....................................................................................
50
Figure 30.5 Case 2 Roundabout Design (6‐ft gore pavement marking between lanes) ....................................................51
30.5.5.3 Geometric Design Guidance Common to Case 2 and Case 3 Roundabouts ................................................... 51
30.5.5.4 Additional Geometric Design Guidance for Case 3 Roundabouts .................................................................. 52
Figure 30.6 Case 3 Roundabout Design (6‐ft to 8‐ft gore pavement marking between lanes) .........................................53
30.5.6 VERTICAL CONSIDERATIONS FOR OSOW VEHICLES ............................................................................................................ 53
Figure 30.7 Typical Ground Clearance Problem Areas ...................................................................................................... 54
Figure 30.8 Cross‐Section Example....................................................................................................................................54
30.5.7 OVERTURNING CONSIDERATIONS FOR LARGE VEHICLES ....................................................................................................... 55
30.5.8 ROADWAY WIDTH ....................................................................................................................................................... 55
30.5.8.1 Entry Width ..................................................................................................................................................... 55
30.5.8.2 Entry Flare ....................................................................................................................................................... 55
30.5.9 EXIT TAPERS ................................................................................................................................................................ 55
Figure 30.9 Exit Lane Taper ...............................................................................................................................................56
30.5.10 CIRCULATORY ROADWAY WIDTH .................................................................................................................................. 56
30.5.11 CENTRAL ISLAND ....................................................................................................................................................... 56
30.5.12 ENTRY CURVES ......................................................................................................................................................... 56
Figure 30.10 Example of alternative pavement marking design for single entrance lane not in the NCHRP
Report 672.........................................................................................................................................................................57
30.5.13 NON‐MOTORIZED USERS ............................................................................................................................................ 57
58
30.5.13.1 Bike Ramp Entrance and Bike Ramp Exit Design Guidance..........................................................................
Figure 30.11 Bike Ramp Entrance & Exit ........................................................................................................................... 58
30.5.13.2 Pedestrian Facilities, Shared‐Use Paths, and Roundabout Sidepaths ........................................................... 58
30.5.13.3 Roadway Width, Clear Roadway Width of Bridges, and Underpasses between Closely Spaced
Roundabouts ................................................................................................................................................................. 59
Figure 30.12 Roundabout Sidepath ...................................................................................................................................61
30.5.14 SPLITTER ISLANDS ...................................................................................................................................................... 61
Figure 30.13 Typical Splitter Island....................................................................................................................................62
30.5.15 INTERSECTION SIGHT DISTANCE (ISD) AND LENGTH OF CONFLICTING LEG OF SIGHT TRIANGLE ................................................. 62
Table 30.7 Roundabout Intersection Sight Distance.........................................................................................................
63
Figure 30.14 Example of Roundabout ISD Clear Sight Window (Leg 2 ISD shown – other legs are similar) .....................64
30.5.16 ANGLES OF VISIBILITY ................................................................................................................................................. 64
30.5.17 RIGHT TURN LANES ................................................................................................................................................... 64
30.5.17.2 Partial Bypass Right Turn Lane (Figure 30.15b or c and NCHRP 672 Exhibit 6‐73) ....................................... 65
30.5.17.3 Exclusive Right Turn Lane (Figure 30.15a and NCHRP 672 Exhibit 6‐74) ...................................................... 65
Figure 30.15 Right turn bypass lanes.................................................................................................................................65
(See also NCHRP 672 Exhibit 6‐74) ....................................................................................................................................65
30.5.18 VEHICLE PATH OVERLAP AND METHODS TO AVOID PATH OVERLAP ..................................................................................... 65
Figure 30.16 Entry Path Overlap........................................................................................................................................66
30.5.18.1 Method for Checking Path Overlap .............................................................................................................. 66
Figure 30.17 Method for checking path overlap ............................................................................................................... 66
30.5.18.2 DESIGN METHOD TO AVOID PATH OVERLAP ................................................................................................................ 67
Figure 30.18 Multilane Entry Design .................................................................................................................................67
30.5.19 APPROACH DESIGN .................................................................................................................................................... 67
30.5.20 VERTICAL DESIGN ...................................................................................................................................................... 68
68
30.5.20.1 Approaches/Departures (Intersection Legs).................................................................................................
30.5.20.2 Circulatory Roadway .................................................................................................................................... 68
30.5.21 CURBING ................................................................................................................................................................. 68
30.5.21.1 Approach Curbs ............................................................................................................................................ 68
Figure 30.19 High‐Speed Roundabout Approach .............................................................................................................. 70
30.5.21.2 Curb and Gutter Separating the Circulatory Roadway from the Truck Apron..............................................
70
30.5.21.3 Curb at the Inside of the Truck Apron or Edge nearest the Central Island ................................................... 71
30.5.22 SPIRALS ................................................................................................................................................................... 71
Figure 30.20 Spiral.............................................................................................................................................................71
30.5.23 ENTRY ANGLE, PHI (Φ) ............................................................................................................................................... 71
Figure 30.21 Method 1 Phi Measurement.........................................................................................................................
72
73
Figure 30.22 Method 2 Phi Measurement.........................................................................................................................
30.5.24 CLEAR ZONE ............................................................................................................................................................. 73
30.5.25 COLORING AND STAMPING CONCRETE ........................................................................................................................... 74
30.6 PLAN PREPARATION ........................................................................................................................................... 74
30.6.2 ALIGNMENT PLANS ...................................................................................................................................................... 74
30.6.3 PROFILE INFORMATION ................................................................................................................................................. 74
30.6.4 TYPICAL SECTIONS ....................................................................................................................................................... 75
30.7 REFERENCES ....................................................................................................................................................... 75
FDM 11‐26‐35 SIGNING AND PAVEMENT MARKING FEBRUARY X, 2013 .................................................................... 75
35.1 SIGNING ............................................................................................................................................................. 75
35.1.1 REGULATORY SIGNS ..................................................................................................................................................... 75
Figure 35.1 Regulatory Signs .............................................................................................................................................76
35.1.2 WARNING SIGNS ........................................................................................................................................................ 76
Figure 35.2 Warning Signs .................................................................................................................................................77
35.1.3 GUIDE SIGNS .............................................................................................................................................................. 77
77
35.1.3.1 Intersection Destination/Direction Signs........................................................................................................
78
35.1.3.2 Overhead Lane Guide Signs............................................................................................................................
Figure 35.4 Overhead Lane Guide Signs ............................................................................................................................ 79
35.1.3.3 Exit Guide Signs – In Splitter Island ................................................................................................................ 79
Figure 35.5 Exit Signs.........................................................................................................................................................80
35.1.3.4 Junction Assemblies........................................................................................................................................
80
80
35.1.3.5 Route Confirmation Signs...............................................................................................................................
35.1.4 URBAN SIGNING CONSIDERATIONS ................................................................................................................................. 80
35.1.5 RURAL AND SUBURBAN SIGNING CONSIDERATIONS ............................................................................................................ 81
35.1.6 CLOSELY‐SPACED MULTIPLE ROUNDABOUTS ..................................................................................................................... 81
Table 35.1 Minimum Visibility Distance* .......................................................................................................................... 81
35.1.7 ROUNDABOUTS IN CLOSE PROXIMITY TO RAILROAD CROSSINGS............................................................................................ 81
35.1.8 WRONG WAY MOVEMENTS IN ROUNDABOUTS ................................................................................................................ 81
WIDE TURNING TRUCKS IN ROUNDABOUTS ............................................................................................................. 81
35.1.9 35.1.10 SHORT TERM AWARENESS TECHNIQUES......................................................................................................................... 81
35.1.11 MAINTENANCE OF SIGNS ............................................................................................................................................ 82
35.1.12 SIGNING INSTALLATION FOR OSOW VEHICLE ROUTES ...................................................................................................... 82
35.2 PAVEMENT MARKING.........................................................................................................................................
82
35.2.1 APPROACH MARKINGS ................................................................................................................................................. 83
35.2.2 CIRCULATORY ROADWAY MARKING ................................................................................................................................ 84
35.2.3 EXIT MARKING ............................................................................................................................................................ 84
35.2.4 BICYCLE MARKING ....................................................................................................................................................... 84
Figure 35.6 Bike Lane Roundabout Marking......................................................................................................................
85
35.2.5 MAINTENANCE OF PAVEMENT MARKING ......................................................................................................................... 85
Attachment 35.1 Attachment 35.2 Attachment 35.31 Attachment 35.32 Attachment 35.33 Example Pavement Markings for Typical Designs..............................................................................
85
Roundabout Pavement Marking Bid Item and Product Type ............................................................85
Sample Signing Layout for a Single‐lane Roundabout .....................................................................85
Sample Signing Layout for a Multilane Roundabout........................................................................ 85
Sample Signing Plan for Roundabout Ramp Terminals....................................................................85
FDM 11‐26‐40 LANDSCAPING AND MAINTENANCE FEBRUARY X, 2013 ..................................................................... 85
40.1 LANDSCAPING .................................................................................................................................................... 85
40.2.1 LANDSCAPE DESIGN ..................................................................................................................................................... 87
40.2.1.1 Owned, Operated, and Maintained by WisDOT ............................................................................................. 87
Figure 40.1 Low‐Maintenance Central Island Landscaping ............................................................................................... 88
40.2.1.2 Owned by WisDOT but Maintained by Others ............................................................................................... 88
40.2.1.3 Local Roads and Connecting Streets...............................................................................................................
88
40.3.2 LANDSCAPE MAINTENANCE ........................................................................................................................................... 88
40.3.2.1 Owned, Operated, and Maintained by WisDOT ............................................................................................. 88
40.3.2.2 Owned by WisDOT but Maintained by Others ............................................................................................... 89
89
40.3.2.3 Local Roads and Connecting Streets...............................................................................................................
40.4 SHARED‐USE PATH MAINTENANCE ..................................................................................................................... 89
FDM 11‐26‐45 WORK ZONE TRAFFIC CONTROL FEBRUARY X, 2013 ........................................................................... 89
45.1 WORK ZONE TRAFFIC CONTROL .......................................................................................................................... 89
45.1.1 PAVEMENT MARKINGS ................................................................................................................................................. 89
45.1.2 SIGNING .................................................................................................................................................................... 89
45.1.3 LIGHTING ................................................................................................................................................................... 89
45.1.4 CONSTRUCTION STAGING .............................................................................................................................................. 90
45.1.5 PUBLIC EDUCATION ..................................................................................................................................................... 90
FDM 11‐26‐50 DESIGN AIDES FEBRUARY X,2013 ....................................................................................................... 90
50.1 EXAMPLE PLAN SHEETS ................................................................................................................................................... 90
50.2 CREATING ROUNDABOUT FASTEST PATHS (B‐SPLINE CURVES) AND USING AUTOTURN SOFTWARE ..................... 91
50.3 OSOW VEHICLE INVENTORY EVALUATION OVERVIEW..........................................................................................
91
Figure 50.1 Check Box for Override Angle ......................................................................................................................... 91
Attachment 50.1 Creating Roundabout Fastest Paths (Spline Curves) in AutoCAD Civil 3D..........................................92
Attachment 50.2 Creating Roundabout Fastest Paths (Spline Curves) in MicroStation Version 8i ...............................92
Attachment 50.3 Guide for Using AutoTURN in AutoCAD Civil 3D & MicroStation Version 8i......................................92
Chapter 11
Section 25
Design
Intersections at Grade FDM 11-25-1 General
March 4, 2013
1.1 Design Consideration
Design an intersection to either rural or urban design criteria depending on its location and the type of existing or
planned development in the area. Design intersections located to serve a present or future residential or
commercial area to urban standards with specific consideration of the current or eventual need for traffic signals,
roundabouts, cross walks, pedestrian signals, expected traffic volumes and size of vehicles expected. Consult
with the region planning staff to determine the type of development planned in the area of the intersection.
It is very important to include Traffic Operations personnel early in the scoping of a project. Volumes, storage,
geometric, and R/W needs should be addressed. It can then be determined if further involvement of Traffic
Operations is needed.
Try to keep the size of intersections to a minimum. Designing intersections for large trucks requires large corner
radii, which substantially increases the size of the intersection. Larger intersections generally have greater crash
potential, are more difficult to delineate, may be more confusing for drivers and other users, require more rightof-way, and significantly increase pedestrian and bicycle crossing times and distances.
References for this chapter include Chapter 9 of the AASHTO GDHS1 and other sources as noted.
Specific factors and features to consider are:
- Safety - some factors that affect intersection safety include:
- Number of approaches
- Number of potential conflict points
- Type of traffic control and advance signing (see FDM 11-25-3; also, see the TGM and the
TSDM and consult with Traffic Operations)
- Approach sight distance, i.e., the visibility of the intersection to an approaching driver (see FDM
11-10-5)
- Intersection Sight distance (see FDM 11-10-5)
- Intersection skew angle (see FDM 11-25-2.8)
- Whether the intersection is located on a curve (see FDM 11-25-2.9)
- Street lighting
- Turn Lanes (see FDM 11-25-2.2 and 2.3; also see FDM 11-25-5 and FDM 11-25-10)
- Auxiliary lanes (see FDM 11-25-35)
- Access management (see FDM 11-25-2.5. FDM 11-25-20, FDM 11-5-5, FDM chapter 7 and
HMM Chapter 91)
- Intersection radii and channelization (see FDM 11-25-10 and FDM 11-25-25; also see SDD
9A1)
- Functional classes of the intersecting roadways (see FDM 11-25-2, Table 2.1; also see FDM 11-15-1
Attachment);
- Designated Long Truck Routes, 75' Restricted Truck Routes, 65' Restricted Truck routes and
statewide Oversized-overweight (OSOW) Freight Network (FN) (see FDM 11-25-1.4).
- Topography and surrounding land uses - examples:
- The length of the crossroad available for traffic generating development including potential
extensions
- In urban and suburban or transitional areas, there is the potential for development to occur
along the highway or adjacent frontage roads. Traffic from this development will feed into the
crossroad.
- Commercial or industrial zoned areas may attract truck terminals or other truck generators.
- Schools, parks, residential developments are examples of destinations that should anticipate
1
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004.
Page 1
FDM 11-25 Intersections at Grade
bicycle, pedestrian and transit increases as well as motor vehicles..
- Corridor Considerations
- The design of an individual intersection will not only need to provide a safe environment with
adequate capacity, but will also need to reflect the needs of adjacent intersections and the
corridor as a whole. As such, isolated intersection designs may need to include features not
dictated by capacity alone. These features should be coherent with the overall facility, examples
of which may include: turn lanes, separation of turn lanes from adjacent through lanes, raised
medians, islands, and separated bicycle facilities. Right-of-way may also need to be preserved
for future corridor-based improvements.
- Traffic characteristics:
- Current and expected daily traffic volumes and turning movements (see FDM 3-10-10)
- Current and expected Design hour volumes and turning movements (see FDM 3-10-10)
- Composition of traffic - including trucks and buses (and bicycles) (see FDM 3-10-10)
- OSOW vehicles - including on roads that are not currently on the OSOW Freight Network, but
which contain an OSOW origination point, or a recurring OSOW destination (e.g., a
manufacturing plant or a gravel pit) (see FDM 11-25-1.4 and FDM 11-25-2.1.1).
- Design vehicle (see FDM 11-25-2.1)
- Vehicle speeds
- Level of Service (see FDM 11-25-3 and FDM 11-5-3)
- Traffic Control Warrants and Design:
- See FDM 11-25-3 for guidance on determining the appropriate type of intersection control
- In general, terms, any intersection, urban or rural, that meets the criteria for a four-way stop
condition or a traffic signal, also qualifies for evaluation as a modern roundabout. For more
information on roundabouts, see FDM 11-26.
- Consult with the region traffic section on the design and location of traffic signals. Applicable
references include:
- FHWA Manual of Uniform Traffic Control Devices (MUTCD) at http://mutcd.fhwa.dot.gov/,
- Wisconsin Supplement to the MUTCD (WMUTCD) on DOTNET
http://www.dot.wisconsin.gov/business/engrserv/wmutcd.htm and consultant extranet
https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/index.shtm,
- WisDOT’s Traffic Signal Design Manual (TSDM) on DOTNET at
http://dotnet/dtid_bho/extranet/manuals/tsdm/index.shtm and consultant extranet
https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/index.shtm),
- WisDOT’s Traffic Guidelines Manual (TGM) (on DOTNET at
http://dotnet/dtid_bho/extranet/manuals/tgm/index.shtm and consultant extranet
https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/index.shtm)
- Traffic signals or a roundabout may not be immediately warranted on a project but may be
warranted within the project‘s design life. See FDM 11-50-50.2 for guidance. .
- Crash experience – including numbers, rates, locations, types, and severity;
- Road user types - motorists, transit, bicyclists, and pedestrians
- Sidewalk approaches and crosswalks (see FDM 11-46-5 and FDM 11-46-10)
- Pedestrian crossing distance and Pedestrian Clearance Time
- Geometry and cross-sections of the approach roadways and the intersection;
- Drainage requirements (see FDM chapter 13)
- Proximity and traffic volumes of driveways and other roads (see FDM 11-25-2.5; also see FDM 11-5-5
and FDM 11-25-20; Refer to FDM 11-30-1 regarding ramp terminal spacing)
- Right-of-way requirements (see FDM 11-25-1.1.1)
- Cost and Potential impacts
1.1.1 Right-of-Way Considerations
Public right-of-way at STH intersections needs to accommodate design geometrics (for existing & future
conditions), operations-related infrastructure, and adequate sight distance. All WisDOT maintained signal &
electrical equipment must either be located within the public right-of-way or within a permanent limited easement
(PLE). Such signal equipment typically includes cabinet bases, signal/lighting bases, vehicle detection,
Page 2
associated conductor runs, and possibly temporary signal support guy-lines. Place this equipment in locations
where it is less likely to be struck by an errant vehicle - because this can reduce crash frequency and severity,
as well as maintenance costs. Also, consider the placement of this equipment in relation to existing or future
sidewalks or shared-use paths.
Also, consider future capacity expansion. Examples include right- & left-turn lanes, widened medians, sidewalk,
bike lanes, roundabouts or interchanges. Because of these issues, involve Regional Traffic Engineering and
Planning (e.g. bike/pedestrian coordinator, access management coordinator) staff in identifying required right-of
way at signalized intersections early in the design process.
1.2 Urban Intersections
At-grade urban intersections consist of a variety of types that cannot be grouped by a class of highway. Factors
that influence intersection design are peak-hour traffic volumes, type and size of turning vehicles, traffic control,
turning roadways, auxiliary lanes, number of lanes, divided or undivided cross section, pedestrian traffic, and
right of way limitations. The proximity of commercial and industrial sites may require special designs.
Intersection geometry and operations need to accommodate all roadway users - including pedestrians and
bicyclists - and provide safe travel and crossing (see FDM 11-46 for guidance on bicycle and pedestrian
accommodations). Minimize the size of the intersection and the pedestrian crossing distance by designing
intersection radii as small as possible. If the design vehicle is larger than a Single Unit (SU truck or a bus),
consider using a two-or three centered curve. Use templates or automated programs to determine the vehicle
path and then develop a two-or three-centered curve that closely emulates this path. Look at a range of vehicle
turning radii and select the best fit for the design vehicle while minimizing the size of the intersection. 2
A legal crosswalk exists at intersections, including “Tee” intersections, where the side road has sidewalks on
one or both sides of the street and the through street has sidewalk on the opposite side of the street from the
side road, whether the crosswalk is pavement marked or not3. FDM 11-46-10 further describes curb ramp
installation requirements and other conditions when curb ramp installation may be desirable.
When possible, prohibit parking near the intersection on routes identified on the Long Truck Operators Map and
the OSOW Freight Network to avoid conflicts with turning traffic. Large vehicles require greater turning radii and
wider sweeping paths to negotiate corners. Review whether parking, roadside utilities, or street furniture will
impede long truck and OSOW movements. This is of particular concern at the intersection of multiple state trunk
highways in established urban environments. Certain OSOW loads (such as a bridge girder) will encroach
beyond the face of curb even when the transport axles stay within the street. Refer to FDM 11-20-1 for
additional Parking Lane and Border guidance.
1.3 Rural Intersections
SDD 9A1 a & b illustrate six types of rural at-grade intersection: A1, A2, B1, B2, C and D. This SDD applies to
two-lane undivided and multilane divided high speed rural highways. The intersection type will indicate the
length of a turn lane and shall apply to both the left turning and the right turning traffic entering the same side
road leg. The lengths of the turn lanes are for deceleration only. If additional storage is needed to accommodate
queuing Design Hour Traffic, or there is a high volume of truck turning movements, then provide a longer turn
lane based on needed storage. Attachment 1.1 lists the criteria for using each type of intersection. FDM 11-25
Attachment 5.4 shows the median opening and non-slotted turn lanes on rural expressways.
Consider other roadways users such as pedestrian, bicyclists and transit users based on existing and future
land uses. Even though these users are not typically as prevalent in rural and high speed settings as they are in
urban settings, this may change with changing land uses. See FDM 11-46, “Complete Streets”, for guidance on
pedestrian and bicycle accommodations. See FDM 11-25-35.3 for guidance on bus stops at intersections.
1.3.1 Intersections on Rural High-Speed Multilane Divided Highways (“Rural Expressways”)4
A rural high-speed (≥50 mph), multilane, divided highway with partial access control is typically referred to as a
2
ORDOT Highway Design Manual (2) ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon Department of Transportation, 2008. ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf., Ch. 9, pp.14-15, “Intersection and Interchange Design”
3
Per s.340.01 (10) (b), Wis. Stats. 4
From Maze et al in NCHRP Report 650 (3) NCHRP Report 650: Median Intersection Design for Rural High-
Speed Divided Highways. Transportation Research Board of the National Academies, 2010. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., p.4, Background; p.147, Conclusions; pp. 1-3, Summary Page 3
“rural expressway”5. Rural expressways are generally a hybrid design between a freeway and a conventional
two-lane rural arterial roadway. Like freeways, rural expressways are typically four-lane divided facilities (i.e.,
two lanes in each direction separated by a wide, depressed, turf median), which may have grade separations
and interchanges. Like conventional two-lane undivided rural arterials, expressways have partial access control
allowing at-grade intersections and limited driveway access with the potential for signalization. Expressways
provide many of the mobility, travel efficiency, economic and safety benefits of freeways at a far lower cost.
However, increased at-grade intersection crashes and increased intersection crash severity diminish the
expected safety benefits of expressways.
The typical rural expressway intersection is an at-grade two-way stop controlled (TWSC) with the stop control on
the minor (usually two-lane) roadway. Expressway interchanges are generally limited to locations that meet
traffic volume warrants and/or that have a disproportionate rate of serious crashes, and where the additional
expenditure can be justified.
TWSC rural expressway intersections often experience safety problems long before the design life of the facility
and even before meeting traffic signal volume warrants. The percentage of total expressway crashes which
occur at TWSC intersections increases as the mainline traffic volumes increase and that all intersection crashes
increase and become more severe as minor roadway volumes increase. Right-angle collisions are the
predominant crash type at conventional TWSC rural expressway intersections. The most problematic of these
(with respect to severity) tend to be those occurring in the far-side intersection (i.e., after the minor road driver
has traveled through the median). The underlying cause of these collisions in most cases is not failure to yield,
but the inability of the driver stopped on the minor road approach to judge the arrival time of approaching
expressway traffic (i.e., gap selection).
The traditional approach to addressing safety problems at expressway intersections - after addressing potential
design issues such as insufficient sight distance - is to improve the traffic-control devices, implement traffic
signal control (if warranted), - and eventually construct an overpass or interchange. Traffic signals in rural areas
are discouraged for several reasons including violation of driver expectations and difficulty in servicing and
maintaining signals in remote locations. Signals also hamper the intended mobility of expressways. In addition,
traffic signals do not always improve safety - they may only change the crash type distribution. The construction
of an interchange reduces the cost advantage of building an expressway as compared with building a freeway,
and the mix of at-grade intersections and interchanges tends to violate driver expectations.
6
1.3.1.1 Rural Expressway Intersection Safety Treatments
Safety treatments for rural expressway intersections fall into three broad categories:
1. Conflict-point management strategies,
2. Gap selection aids, and
3. Intersection recognition devices.
Table 1.1 provides a listing of safety treatments by category. In general, select the most appropriate safety
countermeasure based on the crash types occurring at each location. The conflict-point management strategies
and the gap selection aids seem to have the most potential to improve safety at rural expressway intersections
because they address the apparent underlying cause of many crashes at TWSC rural expressway intersections
(i.e., far-side gap selection by crossing and left-turning minor road drivers).
5
Some roadways in Wisconsin are “designated expressways” per Wis Stat 84.295. The term “rural expressway”
is used herein to describe a rural high-speed (≥50 mph), multilane, divided highway with partial access control, regardless of whether the roadway is “designated expressway”.
6
Speed Divided Highways. Transportation Research Board of the National Academies, 2010. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., pp. 44-63) Page 4
Table 1.1 Potential rural-expressway intersection safety treatment7
Category
Subcategory
No. Safety Treatment
1. Conversion of entire expressway corridor to freeway
2. Isolated conversion to grade separation or interchange
Removal/ Reduction
Through Access
Control
3. Close low-volume minor road intersections and use frontage roads [See FDM
11-25-45]
4. Close median crossovers (right-in, right-out access only)
5. Convert four-legged intersection into T-intersection or initially construct Tintersections instead of four-legged intersections (Use a one-quadrant
interchange [A] if necessary)
Conflict Point
Management
Strategies
Replacement of High
Risk Conflict-points
1. J-turn intersections (indirect minor road crossing and left-turns) [A][See below]
Relocation or Control
2. Provide free right-turn ramps for exiting expressway traffic
2. Offset T-intersections (indirect minor road crossing)
1. Provide left/right-turn lanes or increase their length
3. Minimize median opening length
1. Provide clear sight triangles [See FDM 11-10-5]
Vehicle Detection
(Intersection Sight
Distance
Enhancements)
2. Modify horizontal/vertical alignments on intersection approaches
3. Realign skewed intersections to reduce or eliminate skew [See above]
4. Move minor road stop bar as close to expressway as possible
5. Provide offset right-turn lanes
6. Provide offset left-turn lanes [See FDM 11-10-5 and FDM 11-25-5]
Gap Selection
Aids
Judging Arrival Time
1. Intersection decision support system (IDS) or other dynamic device [A]
2. Roadside markers/poles (static markers at a fixed distance) [A]
1. Provide right-turn acceleration lanes for merging traffic
2. Expressway speed enforcement near intersections
Merging/Crossing Aids
(Promoting Two-Stage
Gap Selection)
3. Widen median to provide for adequate vehicle storage [See below]
4. Add centerline, yield/stop bars, and other signage in the median [See below]
5. Extend left edge lines of expressway across median opening [A]
6. Public education campaign teaching two-stage gap selection
Intersection Treatments
All Approaches
1. Provide overhead control beacon reinforcing two-way stop control
2. Provide intersection lighting
1. Enhanced (overhead/larger/flashing) intersection approach signage
1. Provide diagrammatic freeway-style intersection guide signs
Intersection
Recognition
Devices
Expressway
Approaches
2. Use of a variable median width (wider in intersection vicinity) [See below]
3. Change median type in vicinity of intersection
1. Use STOP-AHEAD pavement marking and in-lane rumble strips
Minor Road
Approaches
2. Provide a stop bar (or a wider one)
3. Provide divisional/splitter island at mouth of intersection
4. Provide signage/marking for prevention of wrong-way entry
[A] SEEG and SWB approval is required. Coordinate with SWB on design and evaluation.
7
From Maze et al in NCHRP Report 650 (3) NCHRP Report 650: Median Intersection Design for Rural HighSpeed Divided Highways. Transportation Research Board of the National Academies, 2010.
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., Table 19 on p. 47). (NCHRP references from
“NCHRP Report 650” are reproduced with permission of the TRB through the National Academy of Sciences
(NAS))
Page 5
Figure 1.1 Countermeasure matrices for TWSC expressway intersection8
Conflict-point management strategies are those treatments that remove, reduce, relocate, or control the conflictpoints that occur at a traditional TWSC rural expressway intersection. Conflict-points represent the locations
where vehicle paths cross, merge, or diverge as they move from one intersection leg to another. A typical fourlegged TWSC rural expressway intersection has 42 conflict-points, as shown in Figure 1.2 - assuming opposing
left-turn paths do not overlap. Conflict-point management strategies can be expensive - and controversial
because of movement restrictions and re-direction.
8
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., Figure 117, p. 148) (NCHRP references from
(NAS))
Page 6
Figure 1.2 Conflict Point Diagram for expressway 4-legged intersection9
Intersection conflict-point analysis is a well understood means of comparing the expected safety of alternative
intersection designs, which suggests that the more conflict-points an intersection design has, the more
dangerous it will be. This approach is useful but limited because it assumes the crash risk is equal at each
conflict point when, in fact, the crash risk associated with each conflict point varies depending on the complexity
and volumes of the movements involved. The conflict-points with the greatest crash risk (i.e., those accounting
for the largest proportion of crashes) at TWSC rural expressway intersections tend to be the far-side conflictpoints involving minor road left-turns and crossing maneuvers (i.e., Conflict-points 15, 16, 19, 21, 22, and 25 in
Figure 1.2).
The key to the effectiveness of conflict-point treatments is in eliminating the high-risk conflict-points. The
conflict-point management treatments with the most potential to improve rural expressway intersection safety
are those that eliminate the far-side conflict-points associated with minor road left-turns and crossing maneuvers
or replace them with conflict-points of lower risk and/or severity.
9
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., Figure 2 on p.5) (NCHRP references from
(NAS))
Page 7
Figure 1.3 Conflict point Diagrams for expressway T-intersections10
Gap selection aids are those countermeasures intended to aid a driver in selecting a safe gap into or through
the expressway traffic stream. Gap selection is a complex process. The driver must detect an oncoming vehicle,
assess the size of the gap (i.e., time-to-arrival of the approaching vehicle) and determine whether there is
enough time/space to complete their desired maneuver. The driver must then proceed and physically enter or
cross through the expressway traffic stream.
Right-angle collisions are the primary safety issue at TWSC rural expressway intersections. The predominant
cause of these crashes seems to be the failure of minor road drivers to detect approaching expressway traffic or
their inability to adequately judge the speed and distance (i.e., arrival time) of oncoming expressway vehicles.
These gap selection issues may be exacerbated by the presence of certain intersection geometric features
(e.g., horizontal/vertical curvature on the mainline, intersection skew, median width, etc.); driver age, driver
behavior (e.g., one-stage gap selection); and increasing traffic volumes on both of the intersecting roadways.
Intersection recognition devices are treatments that improve intersection conspicuity for drivers on either the
minor road or expressway. Many TWSC rural expressway intersections are not readily visible to approaching
drivers, particularly from the uncontrolled expressway approaches. As a result, crashes occur because
approaching expressway drivers are unaware of the intersection and are not prepared to react to potential
conflicts. Crashes also occur because drivers approaching on a sideroad do not stop at a stop sign because
10
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., Figure 31, p.49 and Figure 65, p.86) (NCHRP
references from “NCHRP Report 650” are reproduced with permission of the TRB through the National
Academy of Sciences (NAS))
Page 8
they do not recognize that they are approaching a stop-controlled intersection. Providing greater intersection
recognition reduces the likelihood of stop sign running, and alerts the expressway driver to proceed through the
intersection with caution.
Traditionally, these treatments are the first countermeasures used when right-angle crashes begin to occur at
TWSC rural expressway intersections because they are relatively low-cost and easy to deploy. However, lack of
intersection recognition (i.e., STOP sign violation) is not the major contributing factor in the majority of right
angle crashes occurring at TWSC rural intersections. Therefore, these treatments do not address the
predominant cause of right-angle crashes, which seems to be gap selection.
1.3.1.2 Median Width at Unsignalized Median Openings on Rural Expressways
The median width at a rural expressway intersection is usually the median width for the entire expressway
corridor. However, the major function of a median differs between intersections versus at intersections. The
major function of the median between intersections is to separate opposing expressway traffic; the major
function of the median at intersections is to provide a refuge area for left-turning and U-turning expressway
traffic as well as for left-turning and crossing traffic from the minor road. A median width of 40-feet or wider is
adequate for expressway drivers to experience a sense of separation from opposing traffic. However, research
has shown that wider medians are safer at unsignalized TWSC rural expressway intersections, most likely
because wider medians allow for two-stage gap selection (i.e., a minor road driver can safely stop in the median
area to evaluate the adequacy of the gap in expressway traffic coming from the right before completing a
11
crossing or left-turn maneuver). A wider median at an intersection also serves as an intersection recognition
device for expressway traffic by emphasizing the presence of the upcoming intersection.
The minimum median width at an intersection needed for two-stage gap selection is the length of the design
vehicle plus 3-feet of clearance to the expressway thru-lanes from both the front and the rear of the vehicle.
However, some drivers may perceive this as being too narrow because it places them across the expressway
left-turn lane(s). These drivers may feel that they have no option but to complete the crossing or left-turning
maneuver in one stage. Therefore, it is desirable to provide additional median width so that vehicles stored in
the median do not block the expressway left-turn lane approaching from their right but still have a minimum 3
foot clearance from the expressway thru-lanes. Additional median width may also be desirable to allow more of
the deceleration to take place within the median.
The standard median width of 50 or 60-feet will provide storage for cars or small trucks, but is not adequate for
storing long trucks or combinations of connected farm equipment. Provide a wide median where possible if the
divided highway intersects a side road on a curve or at any location to accommodate long trucks or
combinations of farm machinery. The median should be at least 100 feet wide, up to approximately 150 feet
wide to accommodate long trucks like the WB-65 or combinations of farm machinery that produce a long train of
connected equipment.
Median roadways wider/longer than 150 feet can cause problems as well. Consider appropriate signing to
prevent Wrong Way entry onto the expressway facility.
There are fewer operational problems at rural unsignalized intersections as the median width increases, but the
rate of undesirable maneuvers increases as the median opening length increases.12 In other words, the
geometrics of a wide median in combination with a smaller median opening help create the impression that
there is not much choice in traversing the median except to follow the path the designer intended. Median
delineation is another way to emphasize this desired path.
1.3.1.3 Median Signage and Delineation13
Median signage and delineation have four major objectives:
1. Inform minor road drivers that they have reached a divided highway intersection;
2. Establish the right-of-way between median and far-side expressway traffic;
3. Communicate the appropriate gap selection process (i.e., one or two-stage); and
4. Define the proper travel paths through the median roadway.
11
See Harwood et al in NCHRP Report 375 (4) NCHRP Report 375: Median Intersection Design. TRB, National Research Council, 1995.
12
See Harwood et al in NCHRP Report 375 (4) NCHRP Report 375: Median Intersection Design. TRB, National Research Council, 1995.
13
Speed Divided Highways. Transportation Research Board of the National Academies, 2010. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., pp. 58-59) Page 9
If a median is wide enough to store a passenger car, then stop or yield bars in conjunction with STOP or YIELD
signs may be used to establish right-of-way and to communicate the appropriate two-stage gap selection
behavior to the minor road driver. Generally, median yield control is encouraged unless the selected design
vehicle can be completely stored within the median area. Do not use this marking and signing if the median is
not wide enough to store a passenger car, i.e., if all vehicles require one-stage gap selection.
On median roadways wider than 120 feet, provide double yellow pavement marking to separate the opposing
traffic and provide stop bars and STOP signs at each end of the median roadway. These signs/markings
effectively provide a measure of depth perception to communicate to the minor road driver that the median is
wide enough for vehicle storage, thereby promoting two-stage gap selection behavior. Often, rural expressway
intersections with wide medians have large expanses of pavement that can make it difficult for drivers to decide
what path to follow and to anticipate the paths other drivers will take. The double yellow median centerline
should help to provide visual continuity with the centerline of the minor road approaches and to define the
desired vehicle paths through the median roadway. Slotted left turn lanes are generally not desirable for this
configuration.
1.3.2 J-Turn Intersection
The J-turn is an example of a reduced-conflict intersection that WisDOT has used on expressways. Justify
selection of a J-turn or other reduced-conflict intersections (or interchanges) using the Intersection Control
Evaluation (ICE) process described in FDM 11-25-3. J-turn implementation on WisDOT projects will be on a
pilot basis for the time being. Regions must coordinate with BPD and BTO in the evaluation and design.
The J-turn intersection combines a directional median (which allows direct left-turn exits from the expressway,
but prohibits sideroad traffic from entering the median) with downstream median U-turns. Left turning and
crossing traffic from the sideroad makes these maneuvers indirectly by turning right, weaving to the left, making
a downstream U-turn, and then returning to the intersection to complete their desired maneuver.
14
Since there is no indication that U-turns at unsignalized median openings constitute a safety concern , the Jturn intersection design effectively replaces the high risk, far-side conflict-points associated with direct minor
road left-turns and crossing maneuvers (i.e., Conflict-points 15, 16, 19, 21, 22, and 25 in Figure 1.2) with less
risky conflict-points associated with right-turns, U-turns, and weaving maneuvers. The J-turn intersection
reduces the total number of intersection conflict-points at a typical TWSC rural expressway intersection from 42
to 24 (as shown in Figures 1.2 and Figure 1.4, respectively).
Figure 1.4 Conflict Point Diagram for J-turn Intersection15
TWSC rural expressway intersections most likely to benefit from J-turn intersection conversion include:
- Intersections with a history of far-side right-angle collisions, collisions within the median, and/or “left
turn leaving” collisions;
14
See Potts et al in NCHRP Report 524 (5) NCHRP Report 524: Safety of U-Turns at Unsignalized Median Openings. Transportation Research Board of the National Academies, 2004. http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rpt_524.pdf. 15
Speed Divided Highways. Transportation Research Board of the National Academies, 2010. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf., Figure 48, p. 65) (NCHRP references from “NCHRP Report 650” are reproduced with permission of the TRB through the National Academy of Sciences (NAS)) Page 10
- Intersections with high volumes of traffic on the mainline creating infrequent safe gaps for direct
crossing or left-turn maneuvers, while still having frequent enough gaps for safe right-turn entry
- Intersections with relatively low volumes of traffic crossing or turning left from the minor roads; and
- Intersections with poor horizontal and/or vertical alignment
Limited experience with the J-turn intersection design on rural expressways in Maryland and North Carolina
have shown that the design can offer superior safety performance as compared with a typical TWSC rural
expressway intersection. The implementation at the four sites examined completely eliminated far-side right
angle collisions and improved overall safety.
There are some potential issues in using J-turns at high-speed rural expressway intersections because J-turns
are a relatively recent application:
- Design guidance and standards are still evolving. - There are no traffic volume or level-of-service
warrants.
- Signing and marking – a J-turn essentially creates three (3) separate intersections and drivers need
clear and timely direction in order to make the correct decision.
- Public acceptance
J-turn design considerations include:
- Operational and safety comparison with other intersection alternatives using the ICE process
described in FDM 11-25-3
A J-turn is essentially three separate intersections. Each of these intersections are evaluated
separately but compared collectively to other intersection alternatives
- Intersection Sight Distance (ISD)
The ISD for the mainline left turn into the side road is based on Case F; the ISD for the u-turn locations
is based on Case B1; the ISD for the sideroad right turns is based on Case B2 (see FDM 11-10-5.1.4)
- Separation between the sideroad intersection and the u-turn locations - this distance represents a
trade-off between providing sufficient space for safe/functional weaving, U-turn storage, and approach
signing, while minimizing the travel distance/time of the indirect left-turn and crossing maneuvers. Use
the following guidelines:
- As a rule of thumb, provide 7-10 seconds per lane16 to the begin taper for the U-turn lane - and
check the adequacy during design (e.g., a vehicle crossing 2-lanes at 70 mph requires 1450
feet using 7-sec per lane; and 2060-feet using 10-sec per lane);
- Do not place median openings within the functional length of intersection of any of the three
intersections comprising the j-turn;
- Provide adequate distance for advance signing
- Do not locate u-turns opposite driveways or streets
- Check weaving
- Geometry
- Provide positive offsets for opposing left turn lanes
- Accommodate u-turning vehicles. Possible treatments include increased median width, loons,
and jughandles;
- Consider positive offsets for right turn lanes
- Side road islands and directional median islands need to reinforce left-out and thru movement
restrictions
- Checking and accommodating OSOW vehicles if required (see Table 2.2 and Figure 2.5;
coordinate with the region freight operations unit)
- Accommodate bicyclists and pedestrians if appropriate
1.4 Truck Routes and Routes for Oversized-Overweight (OSOW) Vehicles
There are three (3) categories of truck routes on the STH:
1. “Designated Long Truck Routes” (no overall length limitation; MAX 53' trailer w/ 43' king pin to rear
axle; MAX 28’-6” trailers on double bottoms).
2. “75' Restricted Truck Routes” (75-ft overall length limitation; MAX 53' trailer, 43' king pin to rear axle;
16
(6) Innovative Intersection Designs. (DRAFT PowerPoint presentation for 2009 ACEC/WisDOT Transportation
Improvement Conference). SRF Consulting Group, Inc., 2009.
Page 11
no double bottoms).
3. “65' Restricted Truck Routes” (65-ft overall length limitation; MAX 48' trailer, no double bottoms).
See SS 348 and Administrative Code Trans 276 for requirements and definitions for these routes. Trans 276
has a listing of “Designated Long Truck Routes” and for a listing of 65’ Restricted Truck Route (Note: there are
non-STH routes on this list as well). If a STH is not listed as either a “Designated Long Truck Route” or a “65'
Restricted Truck Route” then it is a “75' Restricted Truck Route”. The “Wisconsin truck operator map” includes
these identified routes and is available at http://www.dot.wisconsin.gov/travel/maps/truck-routes.htm.
All Federally Designated Truck Routes in Wisconsin are Wisconsin “Designated Long Truck Routes” as listed in
23 CFR 658, Appendix A, but not all Wisconsin “Designated Long Truck Routes” are Federally Designated
Truck Routes. The design requirements for Federally Designated Truck Routes differ somewhat from other
Wisconsin “Designated Long Truck Routes” (see FDM 11-15-1, FDM 11-20-1).
Oversized-overweight (OSOW) vehicles are those vehicles that exceed the maximum requirements for a route.
These vehicles require a permit17.The required permits fall into two general categories: (1) single-trip; and (2)
multiple-trip. See FDM 11-25-2.1.1 for more information on OSOW vehicles.
WisDOT has established a statewide OSOW Freight Network (FN) for the use of permit OSOW vehicles, based
on routes that these vehicles have used in the past, and on projected requirements. The statewide OSOW FN
is a subset of “Designated Long Truck Routes”, i.e., all roads on the FN are on “Designated Long Truck
Routes”, with the exception of a few roadways that may be an origination point or a recurring destination point,
such as a manufacturing plant or a gravel pit.
The OSOW Freight Network map is available at the following link,
http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf. See FDM 11-25-2.1.1 for a discussion and
description of OSOW vehicles.
See the following sections of FDM 11-25 for additional design guidance for intersections on the OSOW FN:
FDM 11-25-1 General
1.1 Design Considerations
1.2 Urban Intersections
1.3 Rural Intersections
1.3.2 Jturn intersection
1.4 Truck Routes and Routes for OversizedOverweight (OSOW) Vehicles FDM 11-25-2 Design Criteria and Guidelines 2.1 Design Vehicles
2.1.1 OSOW vehicles
2.1.1.1 SingleTrip Permit OSOW vehicles (OSOWST)
2.1.1.2 MultipleTrip Permit OSOW vehicles (OSOWMT)
2.1.1.3 OSOW Vehicle Inventory Evaluation Overview
2.1.2 Selecting Vehicles for Intersection Design
Table 2.2 intersections Where Checking OSOWST or OSOWMT Vehicles is Required
Figure 2.5 WisDOT’s Interim Policy on Checking Criteria for OSOWST and OSOWMT
Vehicles at intersections
2.2 Physical and Functional Areas of an Intersection
2.2.2 Upstream Functional Length of Intersection
Table 2.5 Queue Storage (d4) for STH Intersections [A] [B] [C]
2.3 Turn Bays
Table 2.6 FullWidth TurnLane Length for Urban Streets and Low Speed Rural [A]
2.5 Intersection Vertical Alignment
2.7 Angle of Intersection FDM 11-25 Attachment 2.1 WisDOT Vehicle Inventory of OSOW Vehicles 17
SS 348.25(1) states “No person shall operate a vehicle on or transport an article over a highway without first
obtaining a permit therefore as provided in s. 348.26 or 348.27 if such vehicle or article exceeds the maximum
limitations on size, weight or projection of load imposed by this chapter.”
Page 12
FDM 11-25-3 Intersection Control Evaluation
3.2.2 Alternative Selection
Table 3.1 Intersection Control Evaluation Alternative Selection
3.2.3 Appendices to Attach FDM 11-25-5 LeftTurn Lanes 5.2 Warranting Criteria
5.3 Design Criteria
5.3.2 Median End Treatment
5.4 Special Designs
5.4.1 Slotted LeftTurn Lanes FDM 11-25-10 RightTurn Lanes 10.2 Intersections in Rural and Developing Areas
10.2.1 Storage Length
FDM 11-25-15 Turning Roadways (Channelized Right) 15.1 Criteria
FDM 11-25-25 Channelization 25.2 Islands FDM 11-25-40 Railroad Crossings 40.1 General
1.5 References
1. A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, Washington, DC,
2004.
2. ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon Department of
Transportation, 2008. ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf.
Accessed 8-6-2010.
3. Maze, T. H., J. L. Hochstein, R. R. Souleyrette, CTRE - Iowa State University, H. Preston, and R. Storm.
NCHRP Report 650: Median Intersection Design for Rural High-Speed Divided Highways. Transportation
Research Board of the National Academies, Washington, DC, 2010.
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf. Accessed 5-18-2010. 18
4. Harwood, D. W., M. T. Pietrucha, M. D. Wooldridge, R. E. Brydia, and K. Fitzpatrick. NCHRP Report 375:
Median Intersection Design. TRB, National Research Council, Washington, DC, 1995.
5. Potts, I. B., D. W. Harwood, D. J. Torbic, K. R. Richard, J. S. Gluck, H. S. Levinson, P. M. Garvey, and R.
S. Ghebrial. NCHRP Report 524: Safety of U-Turns at Unsignalized Median Openings. Transportation Research
Board of the National Academies, Washington, DC, 2004.
http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rpt_524.pdf.
18
[Dec 3, 2012 email from Ellen Chafee, Editor, CRP-TRB] The TRB through the National Academy of Sciences (NAS)
grants permission to use the material listed below from Maze et al. (2010) NCHRP Report 650:Median Intersection Design
for Rural High-Speed Divided Highways and J. A. Bonneson and M. D. Fontaine (2001) NCHRP Report 457: Engineering
Study Guide for Evaluating Intersection Improvements in a proposed revision to Chapter 11, Section 25 of Wisconsin DOT’s
Facilities Development Manual (FDM 11-25).
NCHRP Report 650 Table 19 p. 47
NCHRP Report 650 Figure 117p. 148
NCHRP Report 650 Figure 31 p. 49
NCHRP Report 457 Figure 2.6 p. 23
NCHRP Report 457 Figure 2-6.xls Interactive spreadsheet in online version
Permission is also granted for any subsequent versions of the Work, including versions made for use with blind or physically
handicapped persons, and all foreign-language translations of the Work prepared for distribution throughout the world.
Permission is given with the understanding that inclusion of the material will not be used to imply Transportation Research
Board, AASHTO, Federal Highway Administration, Transit Development Corporation, Federal Transit Administration, Federal
Aviation Administration, or Federal Motor Carriers Safety Administration endorsement of a particular product, method, or
practice.
Permission is also provided on condition that appropriate acknowledgment will be given as to the source material.
Page 13
6. Eyler, D. Innovative Intersection Designs. (DRAFT PowerPoint presentation for 2009 ACEC/WisDOT
Transportation Improvement Conference). SRF Consulting Group, Inc., Plymouth, MN, Feb. 25, 2009.
LIST OF ATTACHMENTS
Attachment 1.1
Selection Criteria for Rural High Speed Intersections
FDM 11-25-2 Design Criteria and Guidelines March 4, 2013
2.1 Design Vehicles
AASHTO19 has established four (4) general classes of standard vehicles:
1. Passenger
cars - includes passenger cars of all sizes, sport/utility vehicles, minivans, vans, and pick
up trucks.
2. Buses - include inter-city (motor coaches), city transit, school, and articulated buses
3. Trucks
- includes single-unit trucks, truck tractor-semitrailer combinations, and truck tractors with
semitrailers in combination with full trailers
4. Recreational
vehicles - includes motor homes, cars with camper trailers, cars with boat trailers, motor
homes with boat trailers, and motor homes pulling cars.
For purposes of geometric design, each standard vehicle has larger physical dimensions and a larger minimum
turning radius than those of almost all vehicles in its class. 20
For intersection geometric design, the most important attribute of a vehicle is its turning radius, which affects the
pavement corner radius, left-turn radii, lane widths, median openings, turning roadways, and ultimately, the size
of the intersection. A vehicle may also affect the choice of traffic control device and the need for auxiliary
lanes.21
The turning radius of a vehicle determines the ease and comfort of making the turning maneuver. The smaller
the turning radius, the larger the off-tracking of the vehicle and the slower the speed. Forcing large vehicles to
use very small turning radii forces the driver to perform a very slow maneuver. Tighter radii are typically chosen
for low speed and/or urban intersections, while larger radii are selected for higher speeds and rural
intersections. 22,23
See the following sections in chapter 9 of the 2004 AASHTO GDHS24 for guidance on turning paths, clearances,
encroachments and assumed speed of turning vehicles at intersections:
Right-turning vehicles:
- Types of Turning Roadways; pp.583-621
- Turning Roadways with Corner Islands; pp.634-639
- Free-Flow Turning Roadways at Intersections; pp.639-639 Left-turning vehicles:
19
AASHTO GDHS 2004 (1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO,
2004., Ch. 2, p.15, “Design Vehicles / General Characteristics” 20
Florida Intersection Design Guide 2007 (7) Florida Intersection Design Guide. Florida DOT, 2007. http://www.dot.state.fl.us/rddesign/FIDG-Manual/FIDG2007.pdf. sect. 3.4, “Design Vehicles”
21
MADOT Highway Department Project Development & Design Guide (8) MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design. Massachusetts Department of Transportation - Highway Division, 2006. http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf., Sect. 6.3.3, “Motor Vehicles
22
ORDOT Highway Design Manual (2) ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon Department of Transportation, 2008. ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf., Ch. 9, pp.14-15, “Intersection and Interchange Design” 23
ILDOT Bureau of Design and Environment Manual 2002 (9) ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002. http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. sect. 36-1.08(a), “Design Vehicles Types” 24
Page 14
- Median Openings; pp.689-704
- Auxiliary Lanes; pp.713-723
2.1.1 OSOW Vehicles
See FDM 11-25-1.4 for a discussion of the OSOW Freight Network (FN). OSOW vehicles are non-standard
vehicles that exceed the maximum requirements for a route and that require a permit25. OSOW vehicles fall into
two general categories:
1 Single-trip permit OSOW vehicle (OSOW-ST) (see FDM 11-25-2.1.1.1)
2 Multiple-trip permit OSOW vehicle (OSOW-MT) (see FDM 11-25-2.1.1.2) (Note: Although a combine is
listed as an OSOW-MT, it is an implement of husbandry and does not require a permit.)
The OSOW vehicle inventory on Attachment 2.1 shows vehicles of various configurations for which templates
are available for use with truck turning software to check if the OSOW vehicles will be able to negotiate an
intersection.
Figure 2.5 shows WisDOT’s interim policy for checking OSOW-ST and OSOW-MT vehicles at intersections.
Table 2.2 shows intersections where checking OSOW-ST and OSOW-MT vehicles is required. See FDM 11-25
2.1.1.1 and FDM 11-25-2.1.1.2 for guidance on accommodating OSOW vehicles.
2.1.1.1 Single-Trip Permit OSOW Vehicles (OSOW-ST)
Single-trip permit OSOW vehicles (OSOW-ST) are very large loads that exceed legal length, height, weight
and/or width. The permits are on a load specific and route-specific basis. These vehicles generally have an
overall length greater than 110 feet, and typically are required to incorporate rear steering maneuverability.
Escorts are typically required.
There are five (5) representative Single-trip permit OSOW vehicles (OSOW-ST) shown on the WisDOT vehicle
inventory (see Attachment 2.1, pages 1-2):
1. 5-axle expandable-deck lowboy (DST Lowboy)
2. Wind Tower Upper-Mid Section, 79.5’ L x 11.5’ W
3. Wind Tower Section, 78’ L x 14.7’ W
4. 55 Meter Wind Blade
5. 165’ Beam
It is estimated that if these five (5) vehicles are accommodated by the intersection then other OSOW-ST
vehicles will be accommodated as well.
On new construction, reconstruction and pavement replacement projects, identify and check the specific through
and turning movements of OSOW-ST vehicles at each intersection on the OSOW Freight Network(FN) (or on
non-FN routes where OSOW-ST vehicles are known to travel), including intersections with other FN routes (see
Table 2.2). Examples include:
- Turning movements onto county or local roads to the OSOW-ST origin such as a manufacturing plant
or gravel pit
- Freeway interchange off-on ramp terminals at the crossroad for a through movement,
- A turning movement where it is known that the OSOW-ST loads will turn.
- Through or turning movements at a roundabout (see FDM 11-26)
- Through movement from a stop-controlled side road across a non-stop controlled mainline
There may be special design considerations to accommodate OSOW-ST vehicles. The frequency of these
OSOW-ST loads is critical when considering the type of special design that may be used. Some examples of
special designs to accommodate OSOW-ST vehicles include:
- Curbs that are traversable (e.g., sloping face curbs that are 4-inches or lower) by OSOW-ST vehicles,
or
- Allow counter directional travel on a right-turn bypass lane, or
- Provide a gated bypass lane just for the OSOW-ST vehicles to use.
25
SS 348.25(1) states “No person shall operate a vehicle on or transport an article over a highway without first
obtaining a permit therefore as provided in s. 348.26 or 348.27 if such vehicle or article exceeds the maximum
limitations on size, weight or projection of load imposed by this chapter.”
Page 15
-
-
-
-
-
Full depth shoulders
Wide shoulders
Stabilized/paved areas behind curbing
Relocation of signals, poles, signs, street appurtenances, etc
Removable signs and street appurtenances26
On new construction, reconstruction and pavement replacement projects being designed with Civil 3D
software and using a 3D model, design pavement grades and cross slopes to ensure sufficient vehicle
body clearance so that vehicles can make the required movements without “hanging up”. This is
particularly important for the 5-axle expandable-deck lowboy (DST Lowboy).
Coordinate the OSOW Freight Network intersection maneuverability check with the Regional freight operations
unit.
2.1.1.2 Multiple-Trip Permit OSOW Vehicles (OSOW-MT)
Multiple-trip permit OSOW vehicles (OSOW-MT) exceed the legal semi truck criteria to use the highway system.
The permits are not load specific or route specific. Multiple Trip permits authorized by 348.27(2) and (7) may
travel on any road or over any bridge (including culverts), unless the roadway or structure has been restricted in
a manner consistent with various laws authorizing local or State personnel to restrict, e.g., weight posting. The
envelope for these multiple trip permits are: 16’ high; 15’ wide; 100’ long and 170k gvw27. Since OSOW-MT
vehicles have an overall length of less than 110 feet they are not required to incorporate rear steering
maneuverability. Escorts are typically not required.
There are about 15,000 to 17,000 multi-trip permits issued on an annual basis, which account for 300,000 to
400,000 loads per year. There are three (3) representative OSOW-MT vehicles shown on the WisDOT OSOW
vehicle inventory (see Attachment 2.1, page 1)
1. 80’ Mobile Home
2. WisDOT WB-92 (formerly WisDOT WB-67-Long)
3. Combine*
* A combine is a representative vehicle for implements-of-husbandry (IOH). Although shown as a
Multiple-trip permit OSOW vehicle (OSOW-MT), implements-of-husbandry do not require permits. The
primary reason the combine is in the OSOW inventory is because it has a 20’ width and can be
problematic at single lane roundabouts (or any narrow roadway) for signs, power poles, light poles to
make sure they are far enough from the roadway.
On new construction, reconstruction and pavement replacement projects identify and check the specific through
and turning movements of OSOW-MT vehicles at STH intersections with an STH or a truck route or at truck
route intersections with an STH or a truck route (unless restricted as noted above). Also, check OSOW-MT
movements at the same intersections as OSOW-ST movements (see FDM 11-25-2.1.1.1 and Table 2.2).
The WB-92 (formerly WB-67-Long) is a very challenging vehicle to accommodate at an intersection because of
its length and its lack of rear steering. Check if each required check movement of the WB-92 vehicle will fit
through an intersection or make turns at an intersection without tires going over the curbs or having to remove
signals, light poles or signposts. Lane encroachments and full use of roundabout truck aprons are acceptable.
Describe and document in the DSR the required WB-92 check movements that cannot be accommodated at an
intersection without excessive impacts. Also, discuss possible alternative routes for those movements
Coordinate the intersection maneuverability check with the Regional freight operations unit.
2.1.1.3 OSOW Vehicle Inventory Evaluation Overview
Use AutoTURN or AutoTrack software for OSOW horizontal evaluation (see FDM 11-26-50, Attachment 50.3)
and AutoTrack Pro for low clearance evaluation (DST lowboy). Refer to these links for videos and assistance in
using these tools.
This is the link to the AutoTURN Pro tutorial videos:
ftp://ftp.dot.wi.gov/dtsd/bpd/methods/ground-clearance-training
26
NYSDOT Highway Design Manual (10) NYSDOT Highway Design Manual ch. 5: Basic Design. New York
State DOT, 2006. https://www.nysdot.gov/portal/page/portal/divisions/engineering/design/dqab/hdm/hdm
repository/chapt_05.pdf., Sect. 5.7.1.3, “Oversized Vehicles”
27
gvw = gross vehicle weight
Page 16
The following OSOW-ST vehicles in the OSOW library have rear steering capabilities:
- 55 Meter Wind Blade
- 165' Beam
- Wind Tower Section, 78’L x 14.7’W
The easiest of these three is the Wind Tower Section 78’L x 14.7’W because the rear steering is linked to the
front. Just drive the vehicle and the rear steers itself.
The 55 Meter Wind blade and the 165' Beam are a little more complicated because they have rear steering that
is completely independent of what the front axle is doing. For those, when you initiate a swept path command,
you will see a check box called "Override Angle". You need to check that box to control the steering of the rear
axles (see Figure 2.1). In AutoCAD Civil 3D, the rear steering is then controlled by holding the Ctrl key and
using the wheel on your mouse as you move through the swept path.
Figure 2.1 Checkbox to Control Rear Steering
2.1.2 Selecting Vehicles for Intersection Design28 and OSOW Vehicle Checks
Turning movements control the operations, safety, and efficiency of an intersection. If intersection geometry
restricts vehicles from properly completing turning maneuvers then capacity is reduced, crash potential
increases and the intersection will potentially break down. Each leg of an intersection handles the turning
movements of various vehicle types with varying degrees of encroachment.
Intersection Design Vehicle (IDV). An Intersection Design Vehicle for an intersection turning movement is the
largest standard vehicle that frequently makes that turning movement. An Intersection Design Vehicle makes
the turning movement without encroaching onto other lanes (including a contiguous bike lane between a right
turn lane and a travel lane - as illustrated in Figure 2.2 on the EB approach leg) and without encroaching onto
the shoulder or gutter. Such designs help reduce collisions and operational delays from lane encroachments.
(Note: A right-turning Intersection Design Vehicle may encroach onto a bike lane that is contiguous to the gutter,
i.e., to the right of a right-turning vehicle- as illustrated in Figure 2.2 on the EB departure leg).
Intersection Check Vehicle (ICV). An Intersection Check Vehicle for an intersection turning movement is larger
than the Design Vehicle and makes the turn less frequently than the Design Vehicle. An Intersection Check
28
ORDOT Highway Design Manual (2) ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon Department of Transportation, 2008. ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf., Ch. 9, pp.14-15, “Intersection and Interchange Design” NYSDOT Highway Design Manual (10) NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, 2006. https://www.nysdot.gov/portal/page/portal/divisions/engineering/design/dqab/hdm/hdm
repository/chapt_05.pdf., Sect. 5.7.1, “Design Vehicle” I
ILDOT Bureau of Design and Environment Manual 2002 (9) ILDOT Bureau of Design and Environment Manual
ch. 36: Intersections. Illinois DOT, 2002. http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. sect. 36-2.01(c), “Encroachment” MADOT Highway Department Project Development & Design Guide (8) MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design. Massachusetts Department of Transportation - Highway Division, 2006. http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf., Sect. 6.7.2, “Pavement Corner Radius”; Sect. 6.7.2.1, “Simple Curb Radius” Page 17
Vehicle makes the turning movement by swinging wide and encroaching onto other traffic lanes (including bike
lanes) without disrupting traffic significantly. Desirably an Intersection Check Vehicle will not encroach into
opposing travel lanes or leave the roadway (i.e., drive up on the curb or encroach beyond the shoulder), but this
is not always practical or cost effective - particularly for OSOW vehicles or for turns made from/to low-speed,
low-volume local streets in urban areas.
For design purposes, assume that parking stalls are occupied and therefore unavailable for the movements of
Intersection Design Vehicles and Intersection Check Vehicles.
Figure 2.2 illustrates the concept of Intersection Design Vehicle vs. Intersection Check Vehicle.
Figure 2.2 Illustrative Turning Movements for Intersection Design and Check Vehicles
Figure 2.3 illustrates and defines the possible degrees of encroachment for intersection turning movements. The
acceptable degree of encroachment for a particular vehicle type varies significantly depending on roadway type
and balances the operational impacts to turning vehicles with the safety of all other users of the street.
Figure 2.4 illustrates “effective” pavement width on approach and departure legs. The “effective” pavement width
is the pavement width usable under the permitted degree of encroachment. At a minimum, effective pavement
width is always the right-hand lane and therefore usually at least 11-12 feet, on both the approach and
departure legs. Typically, legs with on-street parking have an effective pavement width that ranges from about
20-feet, if there is no bike accommodation, to about 25-feet if there is a bike accommodation. The effective width
may include encroachment into adjacent or opposite lanes of traffic, where allowed.
Page 18
Table 2.1 shows the default Design Vehicle for intersection turning movements, based on the functional
classifications of the intersecting highways. Potentially, each turning movement at an intersection could have a
different Design Vehicle.
Table 2.1 also shows Check Vehicles and their acceptable degrees of encroachment (see Figure 2.3), based on
the functional classifications of the intersecting highways.
Use Table 2.1 in conjunction with Figure 2.3 and 2.4 as a starting point for planning and design. Verify the
acceptable degree of encroachment during the project development process. Considerations include traffic
volumes, one-way or two-way operations, urban/rural location, construction impacts, right-of-way impacts and
the type of traffic control.
Figure 2.5 shows WisDOT’s interim policy on checking criteria for OSOW-ST and OSOW-MT vehicles at
intersections. Table 2.2 shows intersections where checking OSOW-ST and OSOW-MT vehicles is required.
See FDM 11-25-2.1.1.1 and 2.1.1.2 for guidance on accommodating OSOW vehicles.
Page 19
Figure 2.3 Degrees of Encroachment29
Figure 2.4 Effective Pavement width and effect on degree of encroachment30
29
Adapted from MADOT Highway Department Project Development & Design Guide (8) MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design. Massachusetts Department of Transportation - Highway Division, 2006. http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf., Sect. 6.7.2, “Pavement Corner Radius”, Exh. 6-15, “Typical Encroachment by Design Vehicle”
30
MADOT Highway Department Project Development & Design Guide (8) MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design. Massachusetts Department of Transportation - Highway Division, 2006. http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf., Sect. 6.7.2, “Pavement Corner Radius”, Exh. 6-17, “Effective Pavement Widths” Page 20
Table 2.1 Default Intersection Design and Check Vehicles & Degree of Encroachment [DE] [A]
For Turn Made
Intersection Design Vehicle
[DE=A1] [C], [D],
Intersection Check
Vehicle(s) [DE] ] [C] [D]
From (Approach) [B]
Onto (Departure) [B]
Ramp
Major Arterial or
Minor Arterial or
Collector or
Local
WB-65 [E] [F]
Major Arterial or
Minor Arterial or
Collector or
Local
Ramp
WB-65 [E] [F]
Major Arterial or STH or
Truck Route
Truck Route
WB-65 [E] [F]
Truck Route
Minor Arterial
WB-40 SU-40 [F]
Truck Route
Collector
WB-40 [F]
Truck Route
Local
SU-30 [F]
Minor Arterial
Truck Route
WB-40 SU-40 [F]
Minor Arterial
Minor Arterial
WB-40 SU-40 [F]
WB-65 [A2] [H]
Minor Arterial
Collector
WB-40 [F]
WB-65 [A2] [H]
Minor Arterial
Local
SU-30 [F]
Collector
Truck Route
WB-40 [F]
WB-65 [B2] [G], [I]
Collector
Minor Arterial
WB-40 [F]
WB-65 [B2] [G], [I]
Collector
Collector
SU-30 [F]
Collector
Local
SU-30 [F]
Local
Truck Route
SU-30 [F]
WB-65 [A2] [J]
WB-65 [A2] [J]
WB-65 [A2] [J]
WB-65 [A2] [I]
WB-40 [A2] [H]
WB-65 [B2] [G], [I]
WB-40 [A2] [H]
WB-65 [B2] [G], [I]
WB-40 [A2]
[H]
WB-65 [B3] [G]
Wb-40 [B2]
WB-65 [C2] [G]
Notes for Table 2.1:
A. Intersection geometrics shall be designed using turning templates or software such as AutoTURN or
Auto Track. Submit the intersection plan with turning template overlay to the Regional Traffic Unit for
review.
Coordinate with the Regional freight operations unit if there will be OSOW vehicles using an intersection. See Table 2.2 for intersections where checking OSOW-ST and OSOW-MT vehicles is required. See Figure 2.5 for WisDOT’s interim policy on checking criteria for OSOW-ST and OSOW-MT vehicles
at intersections.
See FDM 11-25-2.1.1.1 and FDM 11-25-2.1.1.2 for guidance on accommodating OSOW vehicles.
The OSOW Freight Network map is available at the following link, http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf. See also FDM 11-25-1.4. Page 21
B. Functional Classification Systems Maps can be found on the dotnet at http://dotnet/dtim
bop/function/functionalmaps-rural.htm and at http://dotnet/dtim-bop/function/functionalmaps
urban.htm.
They can also be found at http://www.dot.wisconsin.gov/projects/planresources/functional.htm
Truck routes are shown on the “Wisconsin truck operators map”, which is available at http://www.dot.wisconsin.gov/travel/maps/truck-routes.htm. See also, FDM 11-25-1.4. C. See Figure 2.2, 2.3 and 2.4 for definitions and illustrations of Degree of Encroachment (DE)
D. A smaller Intersection Design Vehicle than shown in Table 2.1 may be appropriate at some locations
but must be justified in the DSR. Conditions that might justify consideration of a smaller Intersection
Design Vehicle include:
- Right-of-way is limited
- Trucks are prohibited on cross streets
- Current and projected Traffic counts show a small number of both the default Intersection
Design Vehicle and vehicles that are larger than the default Intersection Design Vehicle (<1/day
total) making the turn(s)
- Cross street volume is minimal (< 400 AADT) and the route is unlikely to be used as a detour
route for a nearby higher volume roadway.
For 3R projects, the Intersection Design Vehicle may be site specific, if necessary, and may have a
less restrictive turning radius than those for new construction and reconstruction projects. 31
A larger Intersection Design Vehicle than shown in Table 2.1 may be appropriate at some locations
but must be justified in the DSR. Conditions that might justify consideration of a larger Intersection
Design Vehicle include:
- Current and projected Traffic counts show a significant number of vehicles that are larger than
the default Intersection Design Vehicle making the turn(s)
- The encroachment of even a few large vehicles will cause significant traffic disruption
The following conditions apply if an Intersection Design Vehicle other than shown in Table 2.1 is used:
- Use the default Intersection Design Vehicle from Table 2.1 as an Intersection Check Vehicle,
and verify that it can make the turn(s) - by encroaching onto other traffic lanes if necessary without significantly disrupting traffic. For signalized intersections, if the default Intersection
Design Vehicle is a WB-65, verify that the WB-65 can make the turn(s) with a DE=A2.
- The SU or school bus design vehicles are the smallest Intersection Design Vehicles used in the
design of intersections on the STH. This design reflects that, even in residential areas, garbage
trucks, delivery trucks, and school buses will be negotiating turns with some frequency.
- Verify that WB-65 trucks can physically make the turns at an intersection of two truck routes
without backing up and without impacting curbs, parked cars, utility poles, mailboxes, traffic
control devices, or any other obstructions, regardless of the selected Intersection Design
Vehicle or allowable encroachment.”
E. Check right turns with a WB-67 vehicle using DE=A1 - except encroaches onto curb flag
F. At signalized intersections, DE=A2 is acceptable for left turns from a single left turn lane if:
- Left turns are only allowed during protected phase, or
- There are no opposing vehicles (e.g., on the non-crossing leg of a T-intersection)
G. At signalized intersections, for the WB-65 Intersection Check Vehicle, use a preferred degree of
encroachment (DE) = A2, with a minimum DE as shown.
H. A Degree of Encroachment (DE) = A3 may be acceptable for right turns by an Intersection Check
Vehicle if there is a right-turn lane on the approach. This allows the vehicle to wait outside of the
approach travel lane until traffic clears from the opposing lane on the departure leg. Use only if this is
an infrequent occurrence and does not cause backups or other traffic disruptions.
I. At right-turn lanes with a contiguous bike lane between the turn lane and the travel lane, check the swept path of the WB-65 Intersection Check Vehicle to see if it is possible to avoid encroaching into the bike lane without significantly disrupting traffic or going outside of the roadway. Otherwise, 31
(9) ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002.
http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. sect. 36-1.08(b), “Selection”
Page 22
consider:
- accepting infrequent bike lane encroachments but consider a warning sign that right turning
large trucks pull left before turning.
- If bike lane encroachment is frequent enough to be potentially dangerous, consider:
- parking restrictions and/or a larger curb radius
- Mark as a shared bike/right-turn lane instead of a separate bike lane and right-turn lane
- Re-design to reduce or eliminate the conflict
Table 2.2 Intersections Where Checking OSOW-ST or OSOW-MT Vehicles is Required [A] [B] [C]
For Movement Made
OSOW Vehicles to Check
From (Approach Leg)
To (Departure Leg)
FN - or
FN - or
OSOW-ST & OSOW-MT
non-FN with KNOWN USE by
OSOW-ST vehicles - or
non-FN - with KNOWN USE by
OSOW-ST vehicles - or
check all applicable turning movements and
thru movements
Ramp
Ramp
non-FN STH - or
non-FN STH - or
OSOW-MT
non-FN truck route
non-FN truck route
check all applicable turning movements and
thru movements
Notes for Table 2.2:
A. See Figure 2.5 for WisDOT’s interim policy on checking criteria for OSOW-ST and OSOW-MT vehicles
at intersections.
B. Coordinate with the Regional freight operations unit if there will be OSOW vehicles using an
intersection. See FDM 11-25-2.1.1, 2.1.1.1 and 2.1.1.2 for additional guidance and requirements on
checking OSOW-ST and OSOW-MT vehicles. Also,
C. The OSOW Freight Network map is available at the following link,
http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf. See also, FDM 11-25-1.4.
Truck routes are shown on the “Wisconsin truck operators map”, which is available at http://www.dot.wisconsin.gov/travel/maps/truck-routes.htm. See also, FDM 11-25-1.4. Page 23
Figure 2.5 WisDOT’s Interim Policy on Checking Criteria for OSOW-ST and OSOW-MT Vehicles at Intersections (included as Exhibit 2.1) 2.2 Physical and Functional Areas of an Intersection
Figure 2.6 shows the Physical and Functional Areas of an intersection. The Physical Area of an Intersection is the pavement area where the intersecting roads coincide. The points of Page 24
curvature of the intersection radii define the outer boundaries of the area32.
The Functional Area of an Intersection includes the physical area, but also extends upstream and downstream
from the physical area for a distance equal to the functional length of intersection, along all of the intersection
roadways. It includes any auxiliary lanes and their associated channelization.
Figure 2.6 Physical and Functional Areas of an Intersection 33
2.2.1 Downstream Functional Length of Intersection
The downstream functional length of intersection is the length of road downstream from an intersection - as
measured from the sideroad edge of pavement on the downstream side of the intersection - needed to reduce
conflicts between through traffic and vehicles entering and exiting the roadway. See Table 2.3 for the minimum
requirements. See Figure 2.7 for illustrations of downstream functional length of intersection.
Table 2.3 Downstream Functional Length of Intersection Minimum Requirements
Traffic Control on Approaches
(Upstream Thru Road Leg / Upstream Intersection Leg)
No control / No control or Stop Sign
Signalized / Signalized
Roundabout / Roundabout
Downstream Functional Length
Stopping Sight Distance (SSD) based on thru road design
speed
Stopping Sight Distance (SSD) based on 25 mph
Stop Sign / Stop Sign
Stop Sign / No control and unchannelized turn
No upstream leg (e.g., non-crossing leg of T-intersection) /
Stop Sign
No control and unchannelized turn
Stop Sign / No control and channelized turn
No control and channelized turn
Stopping Sight Distance (SSD) based on the greater of 25 mph
or the speed of the channelized turn
32
AASHTO GDHS 2004 (1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., pp. 556-557) 33
See TRB Access Management Manual (11)Access Management Manual. Transportation Research Board, 2003., Figure 8-12, p 132 (TRB references from the “Access Management Manual” are reproduced with permission of the Transportation Research Board) Page 25
Figure 2.7 Downstream Functional Lengths of Intersection
The downstream functional length is also a parameter for access control in determining acceptable locations for
median openings and minimum separation between private accesses and public road intersections (i.e., corner
clearance - see FDM 11-25-2.5). Drivers making a turn at an intersection need adequate space to complete the
maneuver before encountering vehicles turning into a downstream driveway. The left turn is the more complex
maneuver because the driver is making it without positive guidance and must adjust speed, path, and direction.
2.2.2 Upstream Functional Length of Intersection
The upstream functional length of intersection is composed of four (4) elements as shown in Figure 2.8:
d1 = distance traveled at operating speed during the driver’s perception–reaction time (PRT). See Table
2.4.
d2 = distance traveled as a vehicle clears a thru-lane and enters a turn lane by moving laterally 9-feet while
braking. This is a more complex and demanding driving task than changing lanes only or braking only.
See Table 2.4.
This element does not apply (i.e., d2=0-feet) to vehicles continuing in a thru-only lane or a shared turnlane/thru-lane because there is no lateral movement).
d3 = distance traveled by vehicles in a turn lane while braking to a stop after a lateral shift from thru lane.
For vehicles in a shared turn-lane/thru-lane or vehicles in a stopped/signalized thru-only lane, it is the
distance traveled while braking to a stop after PRT. See Table 2.4.
This element does not apply (i.e., d3=0-feet) to vehicles continuing in an unstopped/unsignalized thruonly lane because there is no deceleration).
d4 = queue storage length. Typically, use Highway Capacity Manual (HCM) or other modeling software to
compute the queue storage requirement, but other methods are available. Confer with the Region
traffic engineer on the appropriate software and/or method. Note that the decelerating vehicle is the
last vehicle in the queue. See Table 2.5 and Table 2.6 for queue storage requirements.
This element does not apply (i.e., d4=0-feet) to vehicles continuing in an unstopped/unsignalized thruonly lane because vehicles do not stop.
On the OSOW Freight Network (FN), the storage distance (d4) may need to be adjusted to
accommodate one OSOW vehicle, depending on load frequency. Increased storage distance would
Page 26
not be required at intersections with non-FN routes.
Figure 2.8 Upstream Functional Length of Intersection Elements34
34
Adapted from (12) Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006., p.5-43, Figure 5-20
Page 27
Table 2.4 Upstream Functional Length of Intersection Elements d1, d2, and d3 [A]]]
Perception-Reaction Distance
Maneuver Distance
d1
(feet)
d2
(feet)
d3
(feet)
des (min)
des (min)
des (min)
Speed
mph
Rural
Urban /
Suburban
Turn lane
Thru lane
[B]
[C] [E]
[C] [F]
[C] [G]
[D] [H]
[C] [I]
25
90 (55)
55 (35)
75 (75)
25 (25)
100 (75)
30
110 (65)
65 (45)
95 (95)
75 (50)
145 (105)
35
130 (75)
75 (50)
110 (110)
100 (75)
195 (145)
40
145 (90)
90 (60)
130 (130)
150 (100)
255 (185)
45
165 (100)
100 (65)
150 (150)
200 (150)
325 (235)
50
185 (110)
110 (75)
165 (165)
250 (175)
400 (290)
55
200 (120)
120 (80)
185 (185)
325 (225)
485 (355)
60
220 (130)
130 (90)
205 (205)
400 (300)
580 (420)
65
240 (145)
145 (95)
225 (225)
475 (350)
680 (495)
70
255 (155)
155 (105)
240 (240)
575 (425)
785 (575)
Page 28
Notes for Table 2.4
A
See Table 2.5 for guidance on Upstream Functional Length of Intersection element d4 (Queue storage length)
B
Use operating speed of travel lanes (except, not < 25 mph and not > design speed) - either as observed or as
calculated using HCM or other appropriate method - Confer with the Region traffic engineer. Assume that free flow
speed does not exceed Design Speed.
C
All dimensions rounded to nearest 5-feet
D
All dimensions rounded to nearest 25-feet
E
Desirable distance based on a perception-reaction-time (PRT) of 2.5s.
Minimum distance based on a perception-reaction-time (PRT) of 1.5s.
F
Desirable distance based on a perception-reaction-time (PRT) of 1.5s.
Minimum distance based on a perception-reaction-time (PRT) of 1.0s.
G
Applies only to turn-lanes
2
The d2 distance is based on an assumed deceleration rate of 5.8 fps based , which is based on a vehicle moving
laterally 9-feet at an assumed lateral shift rate of 3 to 4 fps, while reducing its speed by 10 mph.
A vehicle is assumed to have cleared the thru traffic lane when it has moved laterally 9-feet .The speed differential
between the turning vehicle and following thru vehicles is 10 mph when the turning vehicle clears the thru traffic lane. .35
H
Applies only to turn-lanes
Distance to decelerate from [Speedminus10 mph] to [stop]
Desirable d3 distance based on a deceleration rate of 6.7 fps2, which is the observed 85th-percentile rate.
Minimum d3 distance based on a deceleration rate of 9.2 fps2, which is the observed 50th-percentile rate.
I
Applies only to shared turn-lane/thru-lanes or stopped/signalized thru-only lanes
Distance to decelerate from [Speed] to [stop]
Desirable d3 distance based on a deceleration rate of 6.7 fps2, which is the observed 85th-percentile rate.
Minimum d3 distance based on a deceleration rate of 9.2 fps2, which is the observed 50th-percentile rate.
35
Research shows that the crash rate is 3.3 times higher for a 20 mph speed differential than for a 10 mph
speed differential; 23 times higher for a 30 mph speed differential; and 90 times higher for a 35 mph speed differential, as documented by Stover & Koepke (12) Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006., p.5-37).
Crashes resulting from excessive speed differential can occur up to several hundred feet from the intersection
as well as at the intersection itself. Page 29
Table 2.5 Queue Storage (d4) for STH Intersections 36 [A] [B] [C]
Design Class
Rural
A2, A3
Rural
other
Urban transitional/highspeed
UA2, UA3
Urban transitional/highspeed
other
Urban low-speed
3, 4, 5
Urban low-speed
other
Thru-only Lanes
Left Turn
Right Turn
Approach
Control
des (min)
des (min)
des (min)
No control
no storage required
greater of 90th pctl or 4-veh
(greater of 90th pctl or 2-veh
90th pctl
Stop Sign
Signalized
50th pctl
(Check 95th pctl for backup into
adjacent intersection, etc.)
No control
no storage required
Stop Sign
50th pctl
No control
no storage required
Stop Sign
50th pctl
No control
no storage required
50th pctl
No control
no storage required
50th pctl
No control
no storage required
Roundabout
greater of 95th pctl or 2-vehicles
(greater of 90th pctl or 2-vehicles)
greater of 90th pctl or 4- veh
(greater of 90th pctl or 2- veh
90th pctl
(greater of 95th pctl or 2-vehicles
greater of 90th pctl or
2-vehicles
90th pctl
greater of 90ctl or 4- veh
(greater of 90th pctl or 2- veh
90th pctl
greater of 90ctl or 4 vehicles [D]
(greater of 90th pctl or 2-vehicles) [D]
Signalized
Stop Sign
90th pctl
Signalized
Stop Sign
2-vehicles
Signalized
Stop Sign
Signalized
Signalized
all
greater of 95ctl or *4 vehicles [D]
(greater of 90th pctl or *2-vehicles) [D]
2 vehicles [D]
90th pctl
greater of 90th pctl or 2-vehicles [D]
(greater of 85th pctl or 2-vehicles) [D]
50th pctl
greater of 95th pctl or 2-vehicles [D]
(greater of 90th pctl or 2 vehicles) [D]
see FDM 11-26
Notes for Table 2.5
A
pctl = percentile
36
Adapted from Bonneson & Fontaine in NCHRP Report 457 (13) NCHRP Report 457: Engineering Study
Guide for Evaluating Intersection Improvements. TRB, National Research Council, 2001.
http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf., pp.23-25); see also Stover & Koepke (12)
Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006., pp. 5-50 to 5
53)
Page 30
B
Assume vehicle length = 25-feet
C
On the OSOW Freight Network (FN), storage distance (d4) may need to be adjusted to accommodate one OSOW
vehicle, depending on load frequency. Increased storage distance is not be required at intersections with non- FN
routes.
D
one (1) vehicle if peak turning volume < 20 vph
An intersection approach may have a different upstream functional length for the thru lane(s), left-turn bay, and
right-turn bay because of different queue storage requirements for those lanes. Each lane of a multi-lane
approach can have a different upstream functional length. The upstream functional length for a thru lane is the
longer of the functional length calculated for the thru lane and the functional length(s) calculated for the turn
bay(s) adjacent to that thru lane.
The upstream functional length of intersection is not a static dimension, particularly on urban roads. It can vary
because operating speeds and queue storage requirements vary during the course of a day. For example,
during peak conditions, the queue storage requirement (d4) might be longer because there are more turning
vehicles; but the PRT (d1) and maneuver distances (d2 & d3) might be shorter because operating speeds are
lower. The opposite might be true during non-peak conditions.
Use the upstream functional length of intersection to design and evaluate turn bay lengths (see Table 2.5, “Turn
Bays” for additional guidance).
In addition, upstream functional length of intersection is a parameter for access control when determining
acceptable locations for median openings and minimum separation between private accesses and public road
intersections (i.e., corner clearance). See the sections below on “Median Opening Locations” and “Driveways
and Corner Clearance”.
2.3 Turn Bays
Turn bay length includes both the approach taper and the full width turn lane (see Figure 2.9). Providing
adequate turn bay length is important because it minimizes deceleration in the thru travel lanes by turning
vehicles 37, and it reduces the probability of “spillback” into the travel lane by queued turning vehicles.
Use the following guidance for determining turn bay length:
- Use the upstream functional length to design and evaluate turn bay lengths (see Figure 2.9 for the
correlation of Upstream Functional Length of Intersection and Turn Bay elements).
- Calculate for both the peak and non-peak conditions and use the longer of the two to determine the
length of turn bay.
- See Table 2.4 for functional length elements d1, d2, and d3 (i.e., PRT and deceleration)
- See Table 2.5 for functional length element d4 (i.e., queue storage);
- See Table 2.6 for full-width turn lane lengths
- See Attachment 2.2 for turn bay taper lengths.
- If possible, provide a turn bay length that meets desirable criteria. A design based on desirable criteria
will maximize the safety, operational efficiency and capacity of an intersection approach – and provide
a margin of error when conditions exceed design assumptions.
- If it is not possible to meet desirable criteria because of physical constraints or existing development
then, if possible, provide a turn bay length that exceeds minimum criteria.
- If it is not possible because of physical constraints or existing development to exceed minimum criteria
then provide a turn bay length that meets minimum criteria.
- If it is not possible to meet minimum criteria, look at removing or relocating the physical constraint. If
that is not possible, it may be necessary to close the median opening or restrict movements if it is not
possible to provide a proper left turn lane. As a last resort, with the approval of the Regions access
coordinator and traffic engineer, provide a shorter turn bay rather than no turn bay at all. Try to provide
enough queue storage to minimize spillback into the thru lanes.
37
As documented by Stover & Koepke (12) Transportation and Land Development, 2nd edition. Institute of
Transportation Engineers, 2006., p.5-37, Table 5-12): The crash rate is 3.3 times higher for a 20 mph speed
differential vs. a 10 mph speed differential; 23 times higher for a 30 mph speed differential vs. a 10 mph speed
differential; and 90 times higher for a 35 mph speed differential vs. a 10 mph speed differential
Page 31
Figure 2.9 Turn Bay Elements and Correlation with Upstream Functional Length of Intersection
Page 32
Table 2.6 Full-Width Turn-Lane Length for Urban Streets and Low Speed Rural [A] 38
Left Turn Lane
Approach Control
Rural
Right Turn Lane
Urban
Rural
Posted speed<=30 mph
d3+d4 (d4)
No control (i.e., unstopped)
d3+d4
[B] [C] [D]
Urban
d3+d4 (d4)
d3+d4
Posted Speed>30 mph
d3+d4
[B] [C] [D]
[B] [C] [D]
Posted Speed>30 mph
d3+d4
[B] [C] [D]
d4
Stop Sign
[B] [C] [D] [E] [F]
d3+d4 (d4)
d3+d4
Signalized
[B] [C] [D] [F]
d3+d4 (d4)
d3+d4
Posted Speed>30 mph
d3+d4
[B] [C] [D] [F]
[B] [C] [D] [F]
Posted Speed>30 mph
d3+d4
[B] [C] [D] [F]
Notes
A
See FDM 11-25 Attachment 1.1 for guidance on high-speed rural turn lanes
B On the OSOW priority network, full-width turn lane length may need to be adjusted to accommodate one OSOW
vehicle, depending on load frequency. Increased length would not be required at intersections on OSOW secondary routes. C See FDM 11-25-2.2.2, “Upstream Functional Length of Intersection” for definitions of dimensions d3 and d4.
See Table 2.4 for d3 dimension see Table 2.5, for d4 dimension.
D Vertical Alignment: A crest vertical curve can hide the beginning of a turn bay. Avoid this by extending the full width
turn-lane so that the turn lane is perceptible from the PRT distance. Do this by lengthening the full width turn-lane
rather than lengthening the taper.
E Both thru and turning vehicles decelerate on the approach to a stop sign, which minimizes the potential speed differential.
F Length of queue in the adjacent thru lane: The thru lane queue can sometimes block entry into a turn bay. This can
have a negative effect on the operation and capacity of the intersection if it occurs on a regular basis. Avoid this by
extending the length of full-width turn-lane so that it is at least as long as the longest expected queue in the adjacent
thru lane. (This is normally more critical for a left turn bay than a right turn bay).
2.3.1 Left Turn Lanes
See FDM 11-25-5 for additional guidance on left-turn lanes.
2.3.2 Right Turn Lanes
See FDM 11-25-10 for additional guidance on right-turn lanes.
2.4 Taper Design
Tapers commonly used around at-grade intersections can be classified as follows.
- Shifting taper
- Merge taper
- Add lane taper
- Turn bay taper (see FDM 11-25-2.3)
38
Adapted from (13) NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements.
TRB, National Research Council, 2001. http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf.
Page 33
- Shoulder taper
See Attachment 2.2 for descriptions of these features as well as guidance for designing them. Much of the
guidance in Attachment 2.2 comes from the FHWA MUTCD39 and the AASHTO GDHS 200440.
2.4.1 Lane Reduction at Intersection
It is more desirable to continue a full-width thru lane beyond an intersection and then terminate the lane with a
lane-drop taper (i.e., merging taper) than to terminate the lane at the intersection as a turn-only lane (i.e., “trap”
lane).
The table in Attachment 2.2 shows both the desirable and minimum length of tangent section that is to precede
a merging taper on the downstream side of an intersection. The desirable distance provides enough room for
placing two signs (W9-1R and W4-2R) upstream from the merge point. The minimum distance provides enough
room for placing only one sign (W4-2R).
The minimum tangent length comes from the Condition ‘A’ column of Table 2C-4 of the Wisconsin Supplement
to the MUTCD (available on both the dotnet and the extranet at
http://www.dot.wisconsin.gov/business/engrserv/wmutcd.htm) and represents the distance between the W4-2R
sign and the start of the merge taper. This distance varies according to the posted speed of the road. The
desirable tangent length equals the minimum tangent length plus 200-feet.
WisDOT’s standard practice is to provide for two signs in advance of a merging taper. The first sign is the W9
1R and is located at the desirable distance upstream from the start of the merging taper - either on the signal
pole on the downstream side of the intersection or on a separate post just beyond a non-signalized intersection.
The second sign (W4-2R) is located at the minimum distance upstream from the start of the merging taper and
200 ft downstream from the first sign. For example, at a posted speed of 55 mph, a W9-1R sign is located 950
feet ahead of the start of the merging taper; and a W4-2R sign is located 750-feet ahead of the start of the
merging taper.
Consider a longer tangent distance if the approach roadway has less than the minimum Stopping Sight Distance
(SSD) required by FDM 11-10-5.
2.5 Corner Clearance to Driveways
Traffic conflicts occur when the paths of vehicles intersect and may involve merging, diverging, stopping,
weaving, or crossing movements. Each conflict point is a potential collision. Each new access point introduces
conflicts and friction into the traffic stream. As conflicts increase, driving conditions become more complex,
drivers are more likely to make mistakes, crash potential increases, and the resulting friction translates into
longer travel times and greater delay. Conversely, simplifying the driving task contributes to improved traffic
operations and reduces collisions. Separating conflict areas helps to simplify the driving task and contributes to
improved traffic operations and safety. 41
The functional area of an intersection is the critical area where motorists are responding to the intersection,
decelerating, and maneuvering into the appropriate lane to stop or complete a turn. Access connections too
close to intersections can cause serious traffic conflicts that impair the function of the affected facilities. Drivers
need sufficient time to address one potential set of conflicts before facing another.
Driveways are, in effect, intersections. Their design and location merit special consideration because crashes
are disproportionately higher at driveways. Ideally, driveways are not located within the functional area of an
intersection or in the influence area of an adjacent driveway”42
“Corner clearance represents the distance that is provided between an intersection and the nearest driveway.”43
Marginal corner clearance (Figure 2.10) is the distance between an intersection and the nearest driveway along
the same side of the highway. Median corner clearance (Figure 2.11) is the distance between an intersection
and the nearest median opening for a driveway. See FDM 11-25-20.4 for median opening location criteria and
requirements.
39
(14) Manual on Uniform Traffic Control Devices. Federal Highway Administration, 2009., chapters 3 & 6
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., pp.715-716) 41
(11)Access Management Manual. Transportation Research Board, 2003., pp.8, 143
42
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., p.729) 43
(11)Access Management Manual. Transportation Research Board, 2003., p.155) 40
Page 34
Figure 2.10 Intersection Marginal Corner Clearances44 (See Table 2.7)
Figure 2.11 Intersection Median Corner Clearances45
Inadequate corner clearances can result in traffic operation, safety, and capacity problems. These problems can
be caused by blocked driveway ingress and egress, conflicting and confusing turns at intersections, insufficient
weaving distances, and backups from a downstream driveway into an intersection.
Use the following guidance for corner clearance on STH’s:
1. If possible, provide a driveway on corner parcels from the side road instead of from the STH. This
requires safe and convenient alternative access and reasonable internal site circulation
2. If it is necessary to provide a driveway from the STH, then limit a corner parcel to one (1) driveway on
the STH. If possible, locate this driveway at or beyond the corner clearance requirement shown in
Table 2.7. If the corner parcel has insufficient frontage, then it may be possible to accomplish this by
consolidating driveways with an adjacent property. Follow the guidance in FDM 11-20 Attachment 10.1
for driveway placement near a property line.
3. If it is necessary to provide driveway access from the STH and it is not possible to construct the
driveway at or beyond the corner clearance requirement shown in Table 2.7, then limit a corner parcel
to one (1) driveway on the STH that meets all of the following conditions:
- Locate the driveway as far from the intersection as possible. Follow the guidance in FDM 11-20,
44
(11)Access Management Manual. Transportation Research Board, 2003., Figure 9-10, p 157 (TRB references
from the “Access Management Manual” are reproduced with permission of the Transportation Research Board)
45
Adapted from Stover & Koepke (12) Transportation and Land Development, 2nd edition. Institute of
Transportation Engineers, 2006., p.6-24 to 6-35 and Figure 6-19). © 2012 Institute of Transportation
Engineers, 1627 Eye Street, NW, Suite 600, Washington, DC 20006 USA, www.ite.org. Used by
permission.
Page 35
-
-
-
-
-
-
-
Attachment 10.1 for driveway placement near a property line. Always consider consolidating
driveways to increase the corner clearance distance.
Do not allow left-turn ingress and egress at driveways within the functional area of intersection
on the STH, except as provided in Table 20.1. Provide a physical (nontraversable) median on
the STH to preclude left turns into or out of driveways. For divided highways, this means not
allowing a median opening for a driveway within the functional area of intersection, except as
provided in Table 20.1. For undivided highways, this means providing short sections of a
median divider and/or adopting a driveway design that discourages or prevents left turn
maneuvers.
Do not locate a driveway inside a right-turn bay unless all of the following apply:
- Alternative access is not possible,
- The driveway is low-volume (<15 vpd),
- A non-traversable median prevents left turns into or out of the driveway,
- Vehicles cannot maneuver into the left-turn lane from the driveway, and
- The successive separate right-turn bays would either be undesirably short and/or too
close together.
If possible, restrict a nearside driveway to right in if it is within the queue storage limits of the
downstream intersection.
If possible, restrict a far-side driveway to right out if it is closer than stopping sight distance from
the upstream intersection.
Do not locate a driveway within the physical area of the intersection (see Figure 2.6). Provide at
least 25-feet between the PC of the intersection curb radius and the PC of the driveway curb
radius.
Do not locate a nearside driveway at or downstream from the stop bar for the downstream
intersection. Provide at least 25-feet between the stop bar and the PC of the driveway curb
radius.
Do not locate a driveway within the limits of a legal crosswalk, or within the limits of a curb ramp
for a crosswalk.
4. If possible, relocate the driveway if joint or alternate access becomes available that meets or exceeds
corner clearance requirements.
Page 36
Table 2.7 Marginal Corner Clearance Distances
Corner
Clearance
Description
AApproach
(nearside) on
the STH
Urban
The upstream functional length for the STH)(see
FDM 11-25-2.2.2)
Rural
Desirable
The greater of the upstream functional length of
intersection for the STH (see FDM 11-25-2.2.2) or
the distance for private intersections from FDM
11-5 Attachment 5.1
Minimum
The distance for private intersections from FDM
11-5 Attachment 5.1
B-
Desirable
Desirable
Departure
(farside) on
STH
The greater of the downstream functional length
of intersection for the STH (see FDM 11-25-2.2.1)
or the upstream functional length for the proposed
driveway Minimum
The greater of the downstream functional length
of intersection for the STH, or the distance for
private intersections from FDM 11-5 Attachment
5.1
The downstream functional length of intersection
for the STH (see FDM 11-25-2.2.1)
Minimum
The distance for private intersections from FDM
11-5 Attachment 5.1
C-
STH side road
Approach
(nearside) on
the side road
The corner clearance requirement is equal to that of corner clearance “A.”
Non-STH side road
Desirable
the upstream functional length of intersection for the side road
Minimum
If the observed or estimated queue of vehicles on a crossroad approach will frequently block a
driveway entrance (as shown in Figure 2.12), then provide a corner clearance that is greater than the
longest expected queue. This will reduce the probability of a backup into the intersection by vehicles
making a left turn into the driveway. For additional guidance, see the TRB Access Management
46
47
Manual starting on p. 155 and also ITE’s Transportation and Land Development starting on p. 6-28 .
if the above corner clearances aren’t possible, apply the conditions of requirement 3 from the section,
“Corner clearances on STHs”
D-
STH side road
Departure
(farside) on the
side road
The corner clearance requirement is equal to that of corner clearance “B.”
Non-STH side road
Drivers making a turn onto a sideroad from a STH need adequate space to complete the maneuver
before encountering vehicles turning into a downstream driveway on the side road. The left turn from
the STH is the more complex maneuver because the driver is making it without positive guidance and
must adjust speed, path, and direction. For additional guidance, see the TRB Access Management
Manual starting on p. 155 and also ITE’s Transportation and Land Development starting on p. 6-32.
Desirable
The greater of the downstream functional length of intersection for the side road (see FDM 11-25
2.2.1) or the upstream functional length for the proposed driveway
Minimum
The downstream functional length of intersection for the side road (see FDM 11-25-2.2.1)
if the above corner clearances aren’t possible, apply the conditions of requirement 3 from the section,
“Corner clearances on STH’s”.
2.5.1 Corner Clearance for non-STH roads
WisDOT’s main concern with driveways on non-STH side roads is that they do not adversely affect the STH
roadway (see Figure 2.12). However, WisDOT may not have the same degree of control on non-STH side roads
as it does on the STH, and may need to work with the local jurisdiction to achieve adequate corner clearances.
46
47
(11)Access Management Manual. Transportation Research Board, 2003. (12) Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006. Page 37
Figure 2.12 Inadequate Corner Clearance on Sideroad48
2.6 Intersection Vertical Alignment
See pp.582 and 279-282 of the 2004 GDHS49.
If possible and practical, avoid grades in excess of 3% within the intersection area and on the portion of
approaches where vehicles are required to stop because this complicates intersection design. Desirably, grades
will be flatter than the maximum values allowed (see FDM 11-10-5.4.1 and Attachment 5.3).
On the OSOW Freight Network, check the roadway profile to avoid abrupt grade transitions that may affect
OSOW-ST vehicles with low ground clearance. OSOW-ST vehicles with very low ground clearance can hang up
on the roadway crown or the rollover between a superelevated section and a side road profile at intersections.
Additionally, on the OSOW Freight Network, some loads on OSOW-ST vehicles are susceptible to torsion or
twisting forces that can exceed the torsional shear capacity of a blade, beam, or concrete member. If possible,
design the vertical alignment and cross slopes in the intersection area to help avoid excessive shear forces
created by torsion forces as the OSOW-ST Vehicle maneuvers the intersection.
Avoid locating intersections just beyond the crest of vertical curves.
2.7 Intersection Sight Distance
For information about intersection sight distance, refer to FDM 11-10-5.
2.8 Angle of Intersection50
It is preferable for intersecting streets to meet at an angle as close to 90°as possible. On the OSOW Freight
Network, it is preferable for roadways to intersect at an angle as close to 90° as possible, thus reducing the
impact of those vehicles with a large turning radius.
It may be necessary to shift the intersection and to realign part of the sideroad in order to improve the angle of
intersection. This usually requires inserting a horizontal curve on the sideroad in close proximity to the
intersection. See FDM 11-10-5.1.1.4, “Sight Distance on a Stop Sign Controlled Approach” and FDM 11-10
5.2.2, “Horizontal Curve on a Stop Sign Controlled Approach”.
48
Adapted from (12) Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006., Figure 6-20 on p. 6-30. © 2012 Institute of Transportation Engineers, 1627 Eye Street, NW, Suite 600, Washington, DC 20006 USA, www.ite.org. Used by permission.
49
50
(15) Intersection Angle Geometry and the Driver's Field of View. In Transportation Research Record 1612: Highway Geometric Design Issues TRB, National Research Council, 1998, pp.10-16., (16) Highway Design Handbook for Older Drivers and Pedestrians. FHWA-RD-01-103. Federal Highway
Administration Turner-Fairbank Highway Research Center, 2001.
http://www.tfhrc.gov/humanfac/01103/coverfront.htm.
(17) Guidelines and Recommendations to Accommodate Older Drivers and Pedestrians. FHWA-RD-01-051.
Federal Highway Administration, 2001. http://www.tfhrc.gov/humanfac/01105/cover.htm.
(18) Older Driver Highway Design Handbook [ch.1 - Intersections (At-Grade)]. FHWA-RD-97-135. Federal
Highway Administration, 1998. http://www.tfhrc.gov/safety/pubs/97135/index.htm#intro.
Page 38
2.8.1 Angle of Intersection for New Intersections
The following applies to new intersections on all projects
2.8.1.1 Intersection on Tangent or on Outside of Curve:
- Desirable: between 75° and 105°
- Minimum: 70°
- Maximum: 110°
2.8.1.2 Intersection on Inside of Curve
Table 2.8 Angle of Intersection for Intersection on Inside of Curve
Road
High Speed and Transitional
Low Speed
Radius
(ft)
Desirable angle
Minimum
angle
Maximum
angle
>6000
between 75° and 105°
70°
110°
4000-6000
75°
105°
<4000
80°
100°
>3000
70°
110°
2000-3000
75°
105°
<2000
80°
100°
2.8.2 Angle of Intersection for Existing Intersections on New Construction and Reconstruction Projects
2.8.2.1 Intersection on Tangent or on Outside of Curve
Improve the intersection angle using the guidelines for NEW intersections if the existing intersection meets any
of the following conditions:
- The existing angle is less than minimum or greater than maximum angle for NEW intersections and
the angle is contributing to intersection crashes, or
- The existing angle is less than 65°or greater than 115°.
2.8.2.2 Intersection on inside of curve
- The existing angle is less than the minimum angle for new construction by 5° or more, or
- The existing angle is greater than the maximum angle for new construction by 5° or more.
2.8.3 Angle of Intersection for Existing Intersections on 3R Projects
2.8.3.1 Intersection on Tangent or on Outside of Curve
- The existing angle is less than 60°or greater than 120°.
2.8.3.2 Intersection on Inside of Curve
- The existing angle is less than the minimum angle for new construction by 10° or more, or
- The existing angle is greater than the maximum angle for new construction by 10° or more.
Page 39
2.9 Intersections on Curves
Intersections on curves of any facility are problematic and are discouraged for the following reasons:
- Drivers have more difficulty judging the speed of vehicles approaching on a curve than on a tangent.
- Superelevation complicates the intersection geometry.
- More right-of-way may be required to ensure adequate intersection sight distance (ISD), particularly on
the inside of curves where the line of sight for intersection sight distance may be a considerable
distance outside the roadway.
- Intersections on the inside of a curve require drivers on the side road to turn their heads more to see
approaching traffic. This can be difficult for some drivers, including older drivers.
If an intersection must be on a curve, then try to use a flatter radius curve and to make the intersection as close
to radial as possible. For example, on high speed roads, using a curve that requires a superelevation of 3% or
less will make it easier to match into the side road profile and to transition the cross slope on auxiliary lanes. It
will also keep the ISD line of sight closer to the roadway. A radial intersection in combination with a flat radius
will reduce the amount drivers have to turn their heads to see approaching traffic.
Intersections on curves of high-speed (posted speed greater than 55 mph) expressways require additional
design considerations. Crash history shows that there is no difference in whether the side road intersection
approaches the expressway from the outside or the inside of the curve. Providing more than the minimum
intersection sight distance at these intersections appears to have no impact on the number or severity of
crashes. If there appears to be no alternative to designing an intersection on a curve then provide a wide
median. If a wide median for intersections on curves is not possible then it is important to restrict intersection
movement by closing the median or at least not allowing side road traffic to turn left onto the expressway.
2.10 References
2004.
2. ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon Department of
Transportation, 2008. ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf.
Accessed 8-6-2010.
7. Florida Intersection Design Guide. Florida DOT, 2007. http://www.dot.state.fl.us/rddesign/FIDG
Manual/FIDG2007.pdf. Accessed 8-6-2010.
8. MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design.
Massachusetts Department of Transportation - Highway Division, 2006.
http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf. Accessed 7-27-2010.
9. ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002.
http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. Accessed 1-8-2010.
10. NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, Albany, NY, 2006.
https://www.nysdot.gov/portal/page/portal/divisions/engineering/design/dqab/hdm/hdm-repository/chapt_05.pdf.
Accessed 8-6-2010.
11. TRB Committee on Access Management (ed.) (ed.). Access Management Manual. Transportation
Research Board, Washington, DC, 2003. 51
12. Stover, V. G. and F. J. Koepke. Transportation and Land Development, 2nd edition. Institute of
Transportation Engineers, Washington, DC, 2006. 52
13. Bonneson, J. A. and M. D. Fontaine. NCHRP Report 457: Engineering Study Guide for Evaluating
Intersection Improvements. TRB, National Research Council, Washington, DC, 2001.
51
[Aug 17, 2004 email from Javy Awan, Director of Publications, Transportation Research Board] TRB
references are reproduced with permission of the Transportation Research Board, From Access Management
Manual, Transportation Research Board, National Research Council, Washington, D.C., 2003
52
[Dec 5, 2012 email and attached letter from Zach Pleasant, Information Services Director, ITE] The Institute
of Transportation Engineers grants permission to use Figures 6-19, P6-27 and 6-20, P6-30 from Transportation
and Land Development, 2nd Edition for the Wisconsin DOT’s Facilities Development Manual (FDM 11-25).
Please know that this is a one-time, one-use agreement, and any other use of this material or any other
resource of ITE must be requested and approved in writing.
Please acknowledge our copyright by publishing: © 2012 Institute of Transportation Engineers, 1627 Eye
Street, NW, Suite 600, Washington, DC 20006 USA, www.ite.org. Used by permission.
Page 40
http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf.
14. Manual on Uniform Traffic Control Devices. Federal Highway Administration, Washington, DC, 2009.
15. Gattis, J. L. and S. T. Low. Intersection Angle Geometry and the Driver's Field of View. In Transportation
Research Record 1612: Highway Geometric Design Issues. TRB, National Research Council, Washington, DC,
1998, pp. 10-16.
16. Staplin, L. K., K. Lococo, S. R. Byington, and Scientex Corporation. Highway Design Handbook for Older
Drivers and Pedestrians. FHWA-RD-01-103. Federal Highway Administration Turner-Fairbank Highway
Research Center, McClean, VA, May, 2001. http://www.tfhrc.gov/humanfac/01103/coverfront.htm. Accessed 4
2-2009.
17. Staplin, L., K. Lococo, S. Byington, D. Harkey, and FHWA. Guidelines and Recommendations to
Accommodate Older Drivers and Pedestrians. FHWA-RD-01-051. Federal Highway Administration, McLean,
VA, May, 2001. http://www.tfhrc.gov/humanfac/01105/cover.htm. Accessed 3-16-2006.
18. Staplin, L., K. Lococo, and S. Byington. Older Driver Highway Design Handbook [ch.1 - Intersections (AtGrade)]. FHWA-RD-97-135. Federal Highway Administration, Washington, DC, Jan., 1998.
http://www.tfhrc.gov/safety/pubs/97135/index.htm#intro. Accessed 2-3-2009.
LIST OF ATTACHMENTS
Attachment 2.1
Attachment 2.2
Exhibit 2.1
WisDOT Vehicle Inventory of Oversized Overweight (OSOW) Vehicles
Taper Length Criteria
WisDOT Interim Policy on Checking Criteria for OSOW-ST and OSOW-MT Vehicles at Intersections
FDM 11-25-3 Intersection Control Evaluation
October 22, 2012
3.1 Intersection Control Evaluation (ICE)
There are increasing types of intersections that, combined with traffic control, may be considered for addressing
traffic delay and safety concerns. To select the appropriate intersection configuration and traffic control, the
region shall perform an Intersection Control Evaluation (ICE) at all intersections on the State Trunk Highway
(STH) that are identified as potentially benefiting from an alternative traffic control or intersection type.
The purpose of the ICE worksheet is to document the analysis (i.e. technical & financial) that assists the Region
in determining a recommended alternative. The goal is to select the optimal control, lane configuration, and type
of intersection based on an objective analysis of the existing conditions and future needs. An ICE worksheet
shall be prepared by, or under the supervision of, a professional engineer, registered in Wisconsin, with
experience in traffic engineering operations.
Typically, intersection improvement projects are developed as a portion of a much larger project or as a safety
or capacity driven project at a specific location. For smaller projects, the identified need to evaluate alternative
traffic control or intersection types is usually the major component of the project and the ICE worksheet will have
a major impact in the development process. In contrast, as part of a larger project, intersection control
treatments may be a much smaller component and other decisions in the project development process will have
more impact on the ICE. Therefore, it is important to emphasize that the ICE must occur as early as practical in
the process so that the project proceeds smoothly.
3.1.1 Types of Projects
Projects designed and constructed with federal or state funding must comply with the ICE process when
considering intersection traffic control as discussed above. These include:
- Improvement Projects (3R, 4R)
- Majors
- Highway Safety Improvement Program (HSIP)
- Traffic Impact Analysis (TIA)
- Safe Routes To School (SRTS)
- Congestion Mitigation and Air Quality (CMAQ)
- Local projects
The ICE process is the same for projects identified by WisDOT, counties, municipalities, or local units of
government that are interested in receiving federal or state funds on any of the projects identified above.
Page 41
3.1.2 Guidance and Criteria for Intersection Control Types
1. Stop Sign Control
An intersection may have One-Way or Two-Way Stop Control (OWSC or TWSC)53, or it may have AllWay Stop Control (AWSC). The one-way or two-way stop control is most common and requires traffic
to stop on the minor road connection(s) to a major highway. Typically, one-way or two-way stop
control is the existing traffic control alternative in the ICE study.
Consider All-Way Stop Control if warrants are met and AWSC is shown to improve the safety of the
intersection. Refer to Traffic Guidelines Manual in section 13-26-5 found at
(https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/tgm/index.shtm) for criteria that
combines WisDOT and MUTCD considerations.
2. Signal Control
Consider signal control if certain traffic warrants are met as discussed in the MUTCD - Section 4C and
the Traffic Signal Design Manual (TSDM) Chapter 2
(https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/tsdm/ch02.shtm). Also, see the
TSDM for design, capacity, and operational guidance for signal control.
3. Roundabout Control
The modern roundabout is considered as a traffic control alternative when the minimum vehicular
volumes that warrant all-way stop control or a traffic signal are met. There may also be situations
where it may be appropriate to consider a roundabout where an intersection may have unique safety
or geometric concerns.
Multilane Roundabouts are typically safer than conventional alternatives, particularly with respect to
injury and fatal crashes but, this does not mean they are without safety challenges. One of these
challenges is getting drivers to select and stay in their proper lanes as they navigate the roundabout.
Some drivers take the fastest path through roundabouts by entering in the right lane, crossing to the
center lane midway through the circulatory roadway, and then crossing back into the right lane at the
exit, generally during lower traffic conditions. This driver behavior creates a risk of increased sideswipe
crashes. Another challenge for both safety and operational efficiency is that drivers sometimes have
difficulty interpreting lane-control arrows and signage in the roundabout context. Even with these
concerns, engineers have begun to introduce three-lane roundabouts where design year peak hour
traffic cannot be accommodated in one or two circulating lanes. The use of three-lanes in any part of
the circulatory roadway raises the concern that low comprehension and compliance could be more
significant when compared to two-lane roundabouts. Therefore, whenever the preferred traffic control
alternative for a particular intersection or interchange is a three-lane roundabout the Bureau of Traffic
Operations (BTO) State Traffic Engineer, in conjunction with the Bureau of Project Development
(BPD) Roadway Standards Chief, shall approve the roundabout as the preferred traffic control
alternative. If the roundabout alternative includes a spiral design, this needs to be indicated in the ICE
Worksheet.
Careful consideration and coordination among regional traffic engineers and designers as well as local
entities must be part of the project development process when considering a roundabout. Refer to
section 3.3 of NCHRP Report 672 (http://www.trb.org/Main/Blurbs/164470.aspx) for considerations in
selecting a roundabout as an alternative. Refer to FDM 11-26 for design and operational guidance on
roundabouts.
4. Reduced-Conflict Intersections
Traffic Engineers and designers have additional options in intersection/interchange design that
combined with traffic control may be appropriate for a given situation. Examples include, but are not
limited to, J-turn intersections, continuous flow intersections (CFI), jughandle intersections, echelon
interchanges, single point diamond interchanges (SPI), diverging diamond interchanges (DDIs), and
others. These designs may have advantages over traditional intersection types, depending on the
existing and future safety and operational concerns.
Some of these intersection types are adaptable for only one type of intersection control. It is still
possible to compare alternative intersection control types but intersection geometry may be different
for each control type. The ICE could also compare different intersection types with the same
intersection control. For some reduced-conflict intersection/interchange, microsimulation may be
53
One-Way Stop Control (OWSC) applies to Tee-Intersections; Two-way Stop Control (TWSC) applies to 4
legged intersections
Page 42
necessary to perform the operational analysis. Refer to FDM 11-25-3.7 for a description of approved
microsimulation software’s and their appropriate use.
The Department recognizes the lack of design and analysis standards for reduced-conflict
intersections, but this should not discourage the selection of these types of designs as the preferred
alternative. The expected future demands on many facilities, as well as the benefits reduced conflict
intersections may bring for a particular location, could favor the implementation of these intersection
types. Therefore, whenever the preferred traffic control alternative for a particular intersection or
interchange is a reduced-conflict intersection/interchange the Bureau of Traffic Operations (BTO)
State Traffic Engineer, in conjunction with the Bureau of Project Development (BPD) Roadway
Standards Chief shall approve the reduced-conflict intersection as the preferred traffic control
alternative.
3.2 ICE Process
The ICE process is conducted in two distinct phases. The first phase, Scoping, is usually done early in the
project development process prior to life cycle 11. The purpose of the scoping phase is to prepare a
memorandum that recommends traffic control alternatives for further evaluation in the Alternative Selection ICE
worksheet. The second phase, Alternative Selection, involves a more detailed evaluation of the alternatives and
is documented in the ICE worksheet to assist the Region in selecting a traffic control, lane configuration and
intersection type for the studied intersection.
The Region shall prepare the scoping memorandum during the planning phase (refer to FDM 11-25-3.2.1) and
approve or prepare the Alternative Selection ICE worksheet (refer to FDM 11-25-3.2.2).The region shall submit
the scoping memorandum to BTO for review prior to Region supervisor approval. Also, include the scoping
memorandum as an attachment to the ICE worksheet. All approvals shall occur once public input is taken into
consideration, as close to the 30 percent design as possible. BTO will provide comments to the region within 20
working days.
3.2.1 Scoping
As part of the scoping process, the Region shall summarize the findings of the analysis in a memorandum that
should include the following sections (described below):
- Crash Diagrams
- Signal & Multi-Way Stop Warrants
- Operational Analysis
- Feasibility of Alternatives
- Conclusion and Recommended Alternatives for future consideration
Crash Diagrams
It is important to provide crash diagrams rather than just aggregate crashes because individual crash types
and their approximate location must be known in order to choose the appropriate safety countermeasures.
Crash records for the most recent five years available should be obtained from the Wisconsin Traffic
Operations and Safety Laboratory, WisTransPortal Project site
(http://transportal.cee.wisc.edu/services/crash-data/).
Signal & All-Way Stop Warrants
All-way stop control warrants are discussed in the Traffic Guidelines Manual within section 13-26-5 found at
(https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/tgm/index.shtm) and signal control warrants
are discussed in the MUTCD Section 4C and the TSDM Chapter 2
(https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/tsdm/ch02.shtm). All warrants shall be
evaluated for traffic volume data collected on a typical weekday (i.e. Tuesday through Thursday) for most
locations. Installation of a traffic signal can be considered when warranted within 5 years of construction.
MUTCD - 4B.03 discusses advantages and disadvantages of Traffic Control Signals; MUTCD - 4B.04.
discusses Alternatives to Traffic Control Signals.
Operational Analysis
A capacity analysis must be performed for existing traffic control with forecasted traffic volumes for the
design year. The capacity analysis shall be performed using the 2010 Highway Capacity Manual (HCM)
Methodology (e.g. Highway Capacity Software, Synchro). Consult with the central office or regional
forecasting team for design year traffic projections. Evaluate the existing and expected pedestrian and
bicycle traffic. Discuss the need for pedestrian and bicycle facilities at the intersection.
Page 43
Feasibility of Alternatives
Briefly discuss the potential environmental impacts, right-of-way impacts, safety implication, site deficiencies,
the major expected costs for each alternative, coordination with local government that may be required and
any other important consideration that may be unique to the project or location. Consider all of these when
establishing a budget during the scoping process. To ensure all alternatives are viable based on the
established project budget the expected higher cost alternative shall be used for budgeting purposes.
Conclusions
Briefly discuss the viable alternatives recommended for further consideration in the Alternative Selection
portions of the ICE Worksheet (Attachment 3.1). Document the findings in the project file and share the data
and memorandum with WisDOT’s project development team.
The required data necessary to perform the analysis recommended for the scoping memorandum as described
herein may not be available or the Region may not be able to collect it in time. In this case the Region shall
document in the scoping memorandum the process followed to determine the viable alternatives recommended
for further consideration in the Alternative Selection portions of the ICE worksheet taking into consideration the
recommended process discussed in this section.
The scoping memorandum will also serve to identify intersections that do not require further evaluation of traffic
control alternatives in the Alternative Selection ICE worksheet. For example:
- Intersections identified as requiring minor improvements such as additional turn lanes or stop control
on the minor road
- Intersections that don’t meet signal warrants for current or design year traffic volumes
- The existing traffic control or intersection type does not present capacity or safety concerns for the
current or design year traffic at the studied intersection
- Intersection is part of a signal corridor and an alternative traffic control is expected to disrupt traffic
flow.
- The intersection requires railroad preemption as determined by WisDOT Railroad’s and Harbors
Section (RHS) and BTO
If the region requires further assistance in determining if an intersection requires further evaluation in the
Alternative Selection ICE worksheet consult with BTO.
3.2.2 Alternative Selection
Even if an intersection meets warrants for traffic control, that treatment may not be justified. The alternative
selection process requires engineering judgment. Whether an intersection justifies a particular type of
intersection control, lane configuration or intersection type is based upon a number of factors. The ICE
worksheet shall document those factors as a minimum to support a traffic control alternative. A report can be
submitted if desired as an attachment to the worksheet format. Refer to appendices to determine the minimum
data and exhibit requirements to document the findings.
The worksheet form represented in Table 3.1 shall be used to prepare the ICE report. The form in Word
processing macro-enabled template format (DOTM) format is available at Attachment 3.1.
The factors and the required information to document the alternative selection process are shown in Table 3.1
below.
Page 44
Table 3.1 Intersection Control Evaluation - Alternative Selection
Factor
Description
Request or develop crash diagrams as indicated for the scoping phase and evaluate for trends.
Safety
Explain what type and percent of crashes will be reduced by each alternative. (Refer to the Federal
Highway Administration (FHWA) safety program website
(http://safety.fhwa.dot.gov/tools/crf/resources/#cmfc) For information on Crash Reduction Factors.)
Provide an overview of access near the intersection and side road traffic impacts for each
alternative.
Request or develop the capacity analysis for existing control as described for scoping.
Request or develop the warrant analysis as described for scoping.
Perform a capacity analysis for the selected alternatives with design year peak hour traffic volumes.
Include delay, LOS and volume to capacity ration (v/c) for each movement and overall for each peak
hour.
Document the 95th percentile queues for each movement and each peak hour. Identify any queue
impacts on adjacent intersections and driveways.
Operational
Analysis
On routes parallel to a freeway, consider the capacity of the intersection to accommodate 5 percent
to 20 percent diverted traffic due to incidents on the freeway.
Document railroad crossings within 1000 feet of the intersection and discuss if mitigation measures
are needed.
Include a preliminary layout for each alternative in the appendix. Accommodate the design vehicle
and check vehicles, when applicable, in the design elements.
Perform capacity analysis using the Highway Capacity Manual Methodology (HCM 2010). The most
current version of approved software’s (FDM 11-5-3.7) that support the HCM methodology shall be
used.
List the type of land use and amount of right-of-way acreage impacted.
Right of Way
Impacts
List the number of relocations by land use category, if any.
List the access restrictions, if any.
Estimate the anticipated right-of-way and real estate costs for each alternative.
Calculate estimated costs for each alternative. Include right of way costs.
Document the cost estimate process by attaching a cost estimate table that includes the main items
that account for the estimated cost of each alternative.
Costs
Document how the right of way acquisition costs were estimated.
Consider if there are significant operations and maintenance costs.
Use conceptual drawing as the basis for the cost estimate.
List any concerns that the traffic control alternatives may present.
Practical
Feasibility
Identify major impacts on businesses, parking availability, real estate and utilities.
Describe the use of the intersection as part of a diversion route and the implications this may have
on the design (e.g. alternative selection, design vehicle, lane configuration).
Page 45
Factor
Description
Describe the need for pedestrian and bicycle facilities. Review comprehensive land use plans or
other planning documents for the inclusion of facilities.
Pedestrians and
Bicycles
Identify nearby pedestrian generators, bike routes, transit stops and if the intersection is on a Safe
Route to School
State if and what facilities are proposed, within, or near the project limits.
Ensure that the American with Disabilities Act (ADA) rules and regulations are met for pedestrians
and bicycle facilities.
Identify nearby OSOW generators.
Evaluate and consider the following if either intersecting road is on the OSOW freight network, is a
significant diversion route or near a freight origin or destination:
- Vertical and horizontal clearance shall account for the OSOW vehicle path (e.g. path to avoid
low clearance of monotubes)
OSOW Freight
Network
- Need for additional paved areas, removable signing or gated connection needed to
accommodate the path of the OSOW vehicle.
- Geometric features of the intersection to account for the OSOW vehicle path.
- Grading for the circulatory roadway of a roundabout for the lower clearance of OSOW vehicles.
- Design of a roundabout’s central island (e.g. shape, apron) to account for the path of an OSOW
vehicle.
Environmental
Impacts
Recommendation
Describe the type (i.e. historical, archeological, wetlands or hazardous material) and amount of
environmental acreage affected by each alternative.
Discuss each alternative and make a recommendation as close to the 30 percent design as possible.
3.2.3 Appendices to Attach
The completed worksheet report shall include supporting data, diagrams, and software input/output reports that
support the findings of the study. The following is the minimum data required as part of the ICE.
Existing Geometrics
- Document the existing geometrics of the intersection being considered for improvement.
- Provide an exhibit with an aerial view of the intersection that highlights the existing geometrics, traffic
control, right of way limits, speed limits, the location of schools or other significant land uses near the
intersection and all adjacent driveways.
- Discuss geographic features that may influence the selected alternative (e.g. severe grades, wetlands,
parkland).
- Provide a map showing the intersection in relation to parallel roadways and its relationship to other
access points along the corridor.
Crash Data
- Obtain crash data for the most recent five years from the WisTransPortal site (http://transportal.cee.wisc.edu/services/crash-data/)). Traffic Volumes
- Show the most recent 12-hour turning movement counts available.
- Provide future turning movement volumes for the AM and PM peak hours using a WisDOT provided
forecast or pre-approved growth rates.
- Provide pedestrian and bicycle volumes by approach, if applicable.
Operational Analysis
- Provide warrant analysis worksheets (electronic copies must be available upon request)
- Provide software inputs/outputs (electronic copies must be available upon request)
Costs & Right of Way Impacts
Page 46
- Provide itemized tables documenting the major cost items including right of way cost estimates.
- Document how right of way costs were obtained.
Proposed Geometrics & Traffic Control Alternatives
- Include a preliminary layout of the intersection showing the proposed geometrics for each traffic
control alternative. Show the relationship of the proposed designs to the existing geometrics and the
proposed changes to adjacent driveways.
- Account for the design vehicle and OSOW vehicles in the preliminary intersection layout, if applicable.
Include truck turning template exhibits.
LIST OF ATTACHMENTS
Attachment 3.1
Attachment 3.1 Intersection Control Evaluation Worksheet
FDM 11-25-5 Left Turn Lanes March 4, 2013
5.1 Introduction
A left-turn lane at intersections where left turns are frequent is always desirable from a safety and capacity
standpoint because exclusive left turn lanes are the safest and most effective way to separate left turning traffic
from through traffic.54 A left-turn bay can significantly improve operations and safety at an intersection by
effectively separating those vehicles that are slowing or stopping to turn from those vehicles in through traffic
lanes. This minimizes turn-related crashes and unnecessary delay to through vehicles. 55
5.2 Warranting Criteria
Exclusive left-turn lanes are provided in order to enhance the safety and to facilitate the movement of through
traffic. The primary factors to consider when determining the need for an exclusive left-turn lane are the left-turn
traffic volume, opposing traffic volume, crash history and experience. A capacity analysis is generally used to
determine turn lane requirements at signalized urban intersections. Additional factors to consider include:
- Median width,
- Available right of way,
- Roadway geometry (e.g., shifting of adjacent travel lanes),
- Impacts to other roadway features (e.g., bike accommodations, terrace, and sidewalk),
- Construction and right of way costs, and
- Design classes of the intersecting roadways.
As a general policy, provide exclusive left-turn lanes at the following locations (if a left turn or u-turn is permitted
at that location):
- All median openings on rural divided highways56 and on urban transitional and high-speed divided
highways57
- At median openings on urban low-speed roadways unless left-turn PHV<20 vph or sideroad/driveway
AADT<400 vpd58
- All intersections on a 2-lane community bypass59
54
MNDOT Road Design Manual ch. 5 (19) Rural Intersections - Turn lanes - Left-Turn Bypass Lanes. In MNDOT Road Design Manual ch. 5: At-Grade Intersections Minnesota DOT, 2000, sect. 5-4.01.04, pp.5-4(2). http://www.dot.state.mn.us/design/rdm/english/5e.pdf.
55
See Bonneson & Fontaine in NCHRP Report 457 (13) NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements. TRB, National Research Council, 2001. http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf., pp. 21-22) 56
(12) Transportation and Land Development, 2nd edition. Institute of Transportation Engineers, 2006., p.5-47, “Warrants for Left-Turn Bays” 57
(20) Minimum Required Turn Lane Storage Lengths & Tapers For Left & Right Turn Lanes At Signalized & Non-signalized Intersections. (DRAFT). Wisconsin DOT, 2010.
58
(20) Minimum Required Turn Lane Storage Lengths & Tapers For Left & Right Turn Lanes At Signalized & Non-signalized Intersections. (DRAFT). Wisconsin DOT, 2010.
59
(21) Review of Wisconsin Bypass Road Design Practices February 13-14, 2006. (Final). Federal Highway Administration Resource Center, 2006. http://www.dot.wisconsin.gov/library/publications/docs/wis-bypass
report.pdf.: pp.5-6, “Left Turn Lanes – Part 1: Page 47
- Intersections meeting the warrants of Table 5.1
- Signalized intersections
- To replace TWLTLs at non-signalized intersections/driveways where the left turn volume exceeds100
vph60
Generally, consider providing an exclusive left-turn lane if the construction year AADT on the main road exceeds
4,000 and the side road AADT exceeds 400. Left turn lanes for OSOW movements on OSOW routes should be
provided independent of the AADT guidance, depending on frequency of load.
Left turn lanes in the middle of the highway have a strong proven safety benefit at intersections, whether they
are signalized or unsignalized. Left turn lanes should be a standard at all intersections on bypass roads. Left
turn lanes are not “bypass lanes” installed to the right of the through lane at an intersection; rather left turn lanes
are positioned to the left of the high speed through traffic lane.”
Also, Appendix B, p. i, “Geometric Design – Intersections:
1. Left turn lanes with positive offset on the bypass at all at-grade intersections to enhance left turn safety””
60
(22) Rationale for Median Type Recommendations. Kentucky Transportation Cabinet, 2008.
http://www.planning.kytc.ky.gov/congestion/medians/Median%20Type%20Guidelines.pdf.
Page 48
Table 5.1 Operational Warrants for Left-Turn Lanes at Intersections on Two-Lane Highways 61
Advancing volume to warrant a left-turn lane
(veh/hr)
Opposing
Volume
(veh/hr)
with
5 percent
left turns
with
10 percent
left turns
with
20 percent
left turns
with
30 percent
left turns
40-mph Operating Speed
800
330
240
180
160
600
410
305
225
200
400
510
380
275
245
200
640
470
350
305
100
720
515
390
340
800
280
210
165
135
600
350
260
195
170
400
430
320
240
210
200
550
400
300
270
100
615
445
335
295
800
230
170
125
115
600
290
210
160
140
400
365
270
200
175
200
450
330
250
215
100
505
370
275
240
5.3 Design Criteria
See FDM 11-25-2.1 for guidance on Intersection Design Vehicles and Intersection Check Vehicles (including
OSOW Vehicles).
The assumed speed of a vehicle making a minimum radius left turn is 10-15 mph.62
Develop Intersection designs, including the location and shape of the median nose and median opening, by
using design vehicle turning templates and an appropriate control radius. Design the intersection so that the
Design Vehicle(s) for the turning movement(s) stays in lane (see Table 2.1). Larger vehicles may encroach on
other lanes as shown in Figure 2.2 and Table 2.1.
Design movements to allow vehicles to turn with a smooth continuous radius. Simultaneous opposing left turns
must be able to complete their turns with a clearance between them as they pass each other of 10 feet desirable
/ 3 feet minimum for opposing single left turn lanes (see FDM 11-25-5.4.3.1 for guidance on multiple left turn
lanes). 63
61
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., p. 685, Exh. 9
75, “Guide for Left Turning Lanes on Two-Lane Highways” 62
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., chapter 9, p.690
63
Desirable Minimum clearance per (9) ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002. http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf., sect. 36-3.03, “Left Turn Lane Designs” Page 49
Restrict on-street parking near the intersection if needed to aid in truck turning movements.
5.3.1 Widths
The width of a left-turn lane is desirably the same as the width of the through lane. Curb adjacent to the left-turn
lane is offset to at least the width of the gutter. Provide a turn-lane width of 12 ft on high-speed and transitional
rural and suburban arterial highways. Desirably, fully develop the median width upstream from a left-turn lane
taper before introducing the taper (i.e., fully shadow the left -turn lane).
Narrower turn lanes are often necessary on urban arterials because of restricted right-of-way and median
widths. The minimum and desirable widths for non-slotted left-turn lanes are shown in Table 5.2 below.
The desired minimum separator width at a signalized intersection is 8 feet face to face; the absolute minimum
width is 6 feet face to face. This width is required for signal and sign/structure placement, and pedestrian refuge
Table 5.2 Median, Separator, and Turn Lane Widths for non-slotted Left Turn Lanes on Low-Speed Urban Arterials (Minimum Widths) Highly
Developed Area
Left-turn lane Width
(to gutter flange line)
Outlying Area
Minimum
10 ft
Desirable
11-12 ft
The greater of 6 feet
The greater of 8 feet
OR
OR
Fixed object width (e.g.,
sign or signal head) + 2 feet
on each side.
Fixed object width (e.g., sign
or signal head) + 2-feet on
each side.
Minimum
Separator Width (f/c
f/c )* *
Desirable
Total Median Width between opposing
traffic lanes where cross traffic storage is
NOT required.
10 feet or greater
but not less than
Fixed object width (e.g., sign or signal head) + 2 feet on each
side.
Separator width (f/c – f/c*) + gutter width on each side of
separator + left-turn lane width.
Low Speed Urban Roadways
Desirable: the greater of f/c – f/c* width + gutter width on
each side + left-turn lane width, or 30 feet.
Total Median Width between opposing
traffic lanes where cross traffic storage IS
required.
Minimum: the greater of f/c – f/c* width + gutter width on
each side + left-turn lane width, or 24 feet.
Transitional and High Speed Urban Roadways
the greater of f/c – f/c* width + gutter width on each side +
left-turn lane width, or 30 feet.
* f/c-f/c width is the face of curb to face of curb distance between the curb adjacent to the left-turn lane
and the curb adjacent to the opposing traffic lane.
See FDM 11-20-1 under “Medians” for further guidance on median widths.
5.3.2 Median End Treatment
A typical intersection does not have a continuous physical edge of traveled way delineating the left-turn path.
Instead, the beginning and end of the left-turn path are delineated by:
1. The centerline of an undivided crossroad or the median edge of a divided crossroad, and
www.transportation.org., p.9.138
Page 50
2. The curved median end
Under these circumstances, a simple curve for the minimum assumed edge of left turn - known as the control
radius - is satisfactory. The larger the control radius, the better it will accommodate a given design vehicle, but
the resulting layout will have a greater length of median opening and greater paved areas than a minimum
radius. These may result in erratic maneuvering by small vehicles, which may interfere with other traffic. On the
other hand, a smaller control radius will require wider pavement on the receiving leg to accommodate larger
vehicles.
The following control radii can be used for minimum practical design of median ends:
- 40 ft accommodates P vehicles and occasional SU vehicles with some swinging wide;
- 50 ft accommodates SU-30 vehicles and occasional SU-40 and WB-40 vehicles with some swinging
wide;
- 60-ft is usually appropriate for right-angle urban intersections (see Attachments 5.1 and 5.2);
- 75 ft accommodates SU-40, WB-40 and and WB-62 vehicles with minor swinging wide at the end of
the turn.
- 80 ft is the minimum for rural high-speed 4-lane divided highways (see Attachment 5.4)
- 130 ft accommodates WB-62 vehicles and occasional WB-65 vehicles with minor swinging wide at the
end of a turn.64
For a median width of 10 ft or more, the bullet nose is superior to the semicircular end and is the preferred
design. A bullet nose is designed to closely fit the path of a turning vehicle and results in less intersection
pavement and a shorter median opening than the semicircular shape. The bullet nose is formed by two
symmetrical portions of control radius arcs (see R3 - R6 in Attachment 5.1 - 5.3). These arcs need to be large
enough to accommodate the turning path of the design vehicle. Assume that the inner wheel of each design
vehicle clears the median edge and centerline of the crossroad by 2 ft at the beginning and end of the turn
without encroachment on adjacent lanes.
On the OSOW Freight Network, use the vehicle inventory of OSOW check vehicles, Attachment 2.1 that may
require alternative intersection geometrics The OSOW Freight Network map is available at the following link,
http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf Alternative nose configurations may be warranted
that allow passage of OSOW vehicles while providing direction to turning vehicles.
Median refuge increases safety for pedestrians and bicyclists crossing a street. A median cut-through is the
recommended design for accommodating pedestrians/bicyclists - especially at unsignalized intersection. The
face-face median width for pedestrian/bicyclist refuge is desirably 8-feet or greater and minimally 6-feet. See
FDM 11-46 for additional guidance on pedestrian accommodations and crossings.
5.3.3 Length
See FDM 11-25-2.3 for guidance on calculating the length of a left turn bay. The length of a median left-turn
lane must be adequate for storage or speed change of left-turning vehicles and the entering taper.
Coordinate with the region traffic engineer's staff in determining the required storage length at signalized
intersections. Consider using traffic control devices with left-turn indicators when the number of left-turning
vehicles exceeds 100 per hour. For additional information, see pp 713-723 of the 2004 GDHS65 and the
Highway Capacity Manual 66
Attachment 5.1 and Attachment 5.2 provide guidance on the length and design of turn lanes for urban highways
and streets.
Attachment 5.4 illustrates a left-turn lane on a typical rural expressway and includes a table that relates the
length of a left-turn lane to the type of rural at-grade intersection into which the traffic is turning.
5.4 Special Designs
5.4.1 Slotted Left-Turn Lanes
A problem with left-turn lanes is the inability of drivers in opposing left-turn bays to see past each other to detect
oncoming traffic and pick an adequate gap to complete their maneuver. Although it is desirable to provide a
64
(23) A Policy on Geometric Design of Highways and Streets 2011, 6th edition. AASHTO, 2011. www.transportation.org., section 9.8.2
65
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. 66
(24) Highway Capacity Manual 2010, 5th edition. Transportation Research Board of the National Academies, 2010.
Page 51
positive offset, as shown in Attachment 5.3 this may not be possible at all locations. Desirably, keep the turning
lane as far to the left as practical on wider medians, thus creating a slotted or channelized left-turn lane, as
shown in Figure 5.1.
One possible consequence of poor alignments is that protected left turn arrows will need to be prematurely
added for low volume left turn movements because of crashes resulting from the poor visibility. This will
increase the delay at the intersection.
The total width of a left-turn island is defined as the distance between the right edge of the turn lane and the
median edge of the travel lane.
If the f/c – f/c width of a left-turn island would be less than 4 feet, then install a flush left-turn island of contrasting
pavement or color to delineate the turning lane from the through lane. Otherwise, install a raised left-turn island
and make the lateral offset between the curb face of the left-turn island and the adjacent through lane equal to
the offset from the curb face of the median to the same adjacent through lane. See Figure 5.1.
Figure 5.1 Urban Slotted Left Turn Lane with Left-Turn Island
The width of the channelized turn lane is 14-feet desirable / 12-feet minimum between the gutter flag of the left
turn island and the gutter flag of median separating the left turn lane from the opposing travel lanes (see Figure
5.1). The desirable f/c/ - f/c width is18-feet (using a 14-ft lane and 2-foot gutter on both sides), which provides
some potential for passing a stalled vehicle67. The minimum f/c/ - f/c width is 16-feet - except, 14-feet may
considered under the following conditions:
- Sloping face curb is used (if appropriate) on both sides, or
- Trucks are prohibited on the cross street, or
- Current and projected Traffic counts show a small number of SU-trucks (less than 10/week total) and
WB-trucks (less than 0.5/week total) making the turn
However, if the intersection is on the OSOW Freight Network, this width may need to be increased to
accommodate OSOW vehicle turning movements.
An offset and slotted left-turn design is illustrated on Attachment 5.3. For additional guidance, see pp 723-724
of the GDHS68.
5.4.2 Two-Way Left-Turn Lane (TWLTL)
Two-way left-turn lanes (TWLTLs) consist of a traffic lane in the median area, 14-16 feet in clear width,
delineated by pavement marking strips. The lane serves as a separation for opposing lanes of travel, an
acceleration lane for vehicles turning left to enter the street from midblock driveways, and can be utilized as a
detour route for maintenance work in adjacent lanes. It also allows easier and safer emergency vehicle
movement, particularly during peak-hour periods.
67
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. “Design Widths
of Pavements for Turning Roadways” - Case IIA (tangent) can be interpreted as 18-ft between two vertical-face curbs.
68
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. Page 52
TWLTLs are intended for use by vehicles traveling in either direction for deceleration and refuge while making a
midblock left-turn maneuver. Use of two-way left-turn lanes for passing maneuvers is prohibited and must be
signed appropriately.
In general, only use TWLTLs in an urban setting where operating speeds are relatively low and where there are
no more than two through lanes in each direction. Consider installing a two-way left-turn lane (TWLTL) in
existing commercial or residential areas where the existing roadway is undivided (flush median) and where there
is a combination of traffic congestion and numerous left-turn maneuvers, coupled with rear-end accidents.
Provide median refuge at intersections, particularly unsignalized intersections, for pedestrian crossing. Urban or
suburban arterials and collectors are common candidates for a TWLTL. A TWLTL mid-block treatment is less
desirable on major arterials and arterials with access management priorities.
Use the following design criteria for TWLTLs69:
- Posted speed: Only use on roads with posted speeds <=45 mph
- TWLTL widths: 14.0-ft Desirable; 12.0-ft Minimum; 16.0-ft Maximum
- Design year AADT:
- 3-Lane TWLTL: between 8,000 and 17,500 vpd
- 5-Lane TWLTL: 24,000 vpd maximum
- 7-Lane TWLTL: NOT ALLOWED
- Length of TWLTL: The length of the TWLTL should have sufficient length to operate properly at the
posted speed. Site conditions and the types of intersection treatments will also influence the length of
the TWLTL. Use the following guidelines:
- Posted speed of 30 mph or less: 500-feet minimum uninterrupted length
- Posted speed of greater than 30 mph: 1000-feet minimum uninterrupted length
- Railroad Crossings: Do not extend a TWLTL across a highway/railroad grade crossing. Terminate the
TWLTL 150 ft to 200 ft in advance of the crossing and provide a raised-curb median adjacent to the
railroad. Coordinate with the Region railroad coordinator.
- Intersection Treatment:
- At signalized intersections and at non-signalized intersections/driveways with left-turning turning
volumes > 100vph, convert a TWLTL to an exclusive left-turn lane (see FDM 11-25-2.3 for
guidance on turn bay length). Use a raised median at intersections and driveways with a high
concentration of left turning vehicles and at other locations as needed for pedestrian and bicycle
refuge.
- If turning volumes to a non-signalized minor street/driveway are low, it is not necessary to
convert the TWLTL to an exclusive left-turn lane. However, pedestrians and bicyclists may still
need median refuge.
- Operational/Safety Factors: For traffic to move safely through intersections, drivers need to be able to
see stop signs, traffic signals, and oncoming traffic in time to react accordingly. Do not locate a TWLTL
where there is substandard stopping sight distance. Provide decision sight distance, where practical,
in advance of stop signs, traffic signals, and roundabouts. Appropriate design speed intersection sight
distance shall be provided for the drivers of vehicles that are stopped, waiting to cross or enter a
through roadway.
- Marking and Signing: Mark and sign TWLTLs in accordance with the Manual on Uniform Traffic
Control Devices to identify the lane and regulate its proper use. Additional delineation is possible by
either using a different type of pavement material with contrasting color or texture, or a mountable
raised median. See SDD 15C10, "Raised Pavement Markers" and MUTCD Figure 3-5 for typical
details of marking for two-way left-turn channelization. Two-way left-turn lanes are also discussed in
the GDHS70 on pp 474-478
5.4.2.1 Conversion from 4-Lane Undivided to 3-lane TWLTL (“Road Diet”)
Consider converting a four-lane facility to a 3-lane TWLTL - commonly referred to as a “Road Diet” - if the
following conditions exist:
- High accident rates involving left turning movements, sideswipes, rear-ends, or crossing traffic
- The need for traffic calming (Lowering the average through traffic speeds and reducing weaving)
- Pedestrian and bicyclist safety issues
69
70
(25) Two Way Left Turn Lane (TWLTL). Wisconsin DOT, 2007.
- The existing four-lane facility actually operates similar to a 3-lane facility. The inside lanes operate as
the left turn lane and the outside lanes operate as the through lane.
- Projected traffic volumes do not show a drastic increase
Converting a four-lane undivided section to a three-lane cross section may result in less right of way impacts,
less environmental impacts and less costs than converting to a wider TWLTL or raised median cross section.
The conversion from four to three lanes may also allow the use of wider or designated bike lanes.
Roadways with stop and go traffic such as school buses and delivery trucks or where slow moving heavy
vehicles such as long trucks and farm machinery will result in increased through traffic delays. An increased
delay for access from side roads may also result with the conversion to three-lanes. A design year ADT of
15,000 - 17,50071 is typically the maximum capacity for a three-lane TWLTL cross section, but check for
adequate Level of Service (LOS) (see FDM 11-5-3).
5.4.3 Multiple Left Turn Lanes
Use multiple left-turn lanes at signalized intersections where traffic volumes exceed the capacity of a single leftturn lane. Fully protected signal phasing is required for multiple left turns (refer to TSDM 3-4-1 for guidance on
left turn phasing). Multiple left turn lanes increase capacity for left turning movements and usually improve
overall intersection delay and level of service by allowing a shorter cycle length and reallocation of green time to
other movements. However, multiple left turns add to the complexity of the driving task. In addition, because
multiple left turns increase exposure for cyclists and pedestrians, adequate clearance times for bicycles and
pedestrians is critical.
Multiple left turn lanes are usually NOT appropriate where72.
- A high number of vehicle-pedestrian conflicts may occur.
- Left-turning vehicles do not queue evenly among the left turn lanes because of downstream conditions
(e.g., a high potential for downstream weaving may exist).
- Channelization markings within the intersection may become obscured or confusing
- There is insufficient right-of-way to provide adequate turning maneuver space for the design vehicle
5.4.3.1 Design Considerations for Multiple Left Turn lanes
Consider dual left turn lanes at any signalized intersection where left turn demand exceeds 300 vehicles per
hour or if the storage length exceeds 300 feet; consider triple left turns where left turn demand exceed 600
vehicles per hour73: Determine the actual need by performing a signalized intersection capacity analysis.
Turn lane widths for multiple turn lanes are 12-ft desirable and 11-ft minimum.
Provide adequate throat width on the intersection leg receiving the multiple left turns to compensate for turning
vehicles offtracking and for the relative difficulty of side-by-side left turns. On the other hand, avoid excessive
pavement width, because this can mislead drivers. An Intersection where the turning angle is greater than 90
degrees may require a wider throat width than an Intersection where the turning angle is 90-degerees or less74.
Provide a separation between vehicles turning side-by-side of 4-feet desirable / 3-feet minimum (see Figure
5.3). Provide a clearance between simultaneous opposing left- turns as they pass each other of 10 feet
desirable / 3-feet minimum. It may be necessary to offset opposing approaches to avoid conflicts in turning
paths. If opposing left-turns have inadequate clearance between them then provide separate protective signal
71
(26) Geometric Design of Lanes - Continuous Two-Way Left-Turn Lanes (TWLTLs). In IADOT Design Manual
ch. 6: Geometric Design Iowa DOT, 2001, sect. 6C-6, pp.1-4. http://www.iowadot.gov/design/dmanual/06c
06.pdf.,(27) Facility Selection / Two - Way Left - Turn Lanes. In MODOT Engineering Policy Guide ch. 200: Geometrics Missouri DOT, 2012, sect. 232.3. http://epg.modot.org/index.php?title=232.3_Two_-_Way_Left_
_Turn_Lanes.
72
(28) Criteria for the Geometric Design of Triple Left-Turn Lanes. ITE Journal, vol. 64, no. 12, 1994, pp.27-33.
http://turnlanes.net/files/criteria_for_the_geometric_design_of_triple_left-turn_lanes.pdf.,(29) Individual Movement Treatments / Multiple Left-Turn Lanes. In FHWA-HRT-04-091: Signalized Intersections: Informational Guide Federal Highway Administration Turner-Fairbank Research Center, 2004, ch. 12.1.2, pp.318-319. http://www.tfhrc.gov/safety/pubs/04091/04091.pdf.
(30) Triple Left Turn Lanes At Signalized Intersections. BC131 (FINAL). Florida DOT, 2002. http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_TE/FDOT_BC131rpt.pdf. 73
Same references as in previous footnote
74
(31) NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, 2011. https://www.dot.ny.gov/divisions/engineering/design/dqab/hdm/hdm-repository/chapt_5_final.pdf., sec. 5.9.8.2 B, p.5-125
Page 54
phases.
Use lane line extensions to delineate the turning path through an intersection in order to reduce the potential for
sideswipe collisions and to increase the efficiency of left-turn operations. This is particularly important where
less than desirable clearances are used. Determine lane line (or guideline) and width requirements by plotting
the swept paths of the selected design vehicles. There should be no conditions that obscure, or result in,
confusing pavement markings within the intersection.
Check all turning paths of multiple left turn lanes with truck turning templates allowing 2-ft.between the tire path
and edge of each lane.
Provide adequate signing and marking to make the intended operation clear to every road user. Each turn lane
should be marked with turn arrows and "ONLY" legends as appropriate.
Provide a raised median island on the receiving leg of the intersection to provide drivers on the inside lane with
a visual point of reference to guide the vehicle through the left-turn maneuver.
Because of the added width, signal-timing intervals for bicycle and pedestrian movements require special
attention,
5.4.3.1.1 Dual Left Turn Lanes
For details on dual left turn lanes, see Figure 5.4 and Table 5.3.
The receiving roadway needs to carry two through lanes a sufficient distance to allow the effective utilization of
both lanes (As a minimum, use the desirable values from the “Tangent Prior to Merge” column in Table A.2.2,
from Attachment 2.2).
Assume that the Design Vehicle from Table 2.1 will turn from the outside lane of the dual left turn lanes.
Desirably, the inside vehicle should be a SU but, as a minimum, the other vehicle can be a passenger car, 75 if
any or all of the following conditions are present:
- Cross street volume is minimal (< 400 ADT) and route is unlikely to be used as a detour route for a
nearby higher volume roadway
Table 5.3 shows throat width guidelines for dual lane left turn lanes where the left turning vehicles have a
turning angle of 90-degerees or less.
Figure 5.2 Outside and Inside Lanes for Dual Left Turn Lanes
75
ILDOT Bureau of Design and Environment Manual 2002 (9) ILDOT Bureau of Design and Environment
Manual ch. 36: Intersections. Illinois DOT, 2002.
http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. sect. 36-3.05(b), “Dual Turn Lanes /
Design”
Page 55
Figure 5.3 Dual Left Turn Lane with Throat Widening on Departure Leg - Design Vehicle & Single Unit
Vehicle Turning Together
Figure 5.4 Dual Left Turn Lanes76
76
Adapted from (9) ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002.
http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. , Figure 36-3U.
Page 56
Table 5.3 Expanded Throat Width (W) Guidelines for Dual Left Turn Lanes77
Expanded Throat Width (W)
(feet)
‘SU’ truck or Passenger
Car
‘WB’ truck
75
33 + gutter width(s)
100
150
200
Control Radius (R)
(feet)
Intersection
Design Vehicle =
5.4.3.1.2 Triple Left Turn Lanes
Consider triple left turn lanes only if meeting the following conditions78:
- An operational analysis of the intersection shows that a triple left turn lane would correct a situation in
which the overall capacity of the intersection is seriously deficient, and that no other geometric or
signal modifications would correct the deficiency. Take into account the effects of adjacent
intersections, including:
- Traffic backup from a downstream signal on the receiving roadway
- Relative turning movement distribution at a downstream intersection that would compromise the
ability of the receiving lanes to store the left turning vehicles
- The receiving roadway also accommodates heavy volumes from other approaches.
- Upstream features that would make it difficult to distribute approaching left turning vehicles over
the three left turn lanes (e.g. a heavy single lane exit ramp from a freeway).
- Triple left turn lanes would not cause a safety problem or aggravate an existing safety problem including bicycle and pedestrian safety.
- The signal-timing plan must be able to provide adequate pedestrian clearance intervals for all phases.
Typically, design triple left turn lanes using the Design Vehicle from Table 2.1 in both the outside and middle
lanes, and an SU vehicle in the inside lane. Desirably, design triple left turn lanes using a WB-65 vehicle (WB
67 if near a freeway) in both the outside and middle lanes, and an SU vehicle in the inside lane. As a minimum,
design triple lane turns using an SU vehicle and two P vehicles turning simultaneously with a minimum 4 feet
separation between the swept paths of the vehicles. The SU vehicle should be able to turn in all lanes.
Triple left turn configurations featuring three exclusive left turn bays (Type A) are preferable to either two
exclusive left turn bays plus an exclusive left turn trap lane (Type B), or two exclusive left turn bays plus an
optional through-left lane (Type C)79.
Although three continuous downstream receiving lanes are desirable in order to avoid a lane drop, the receiving
roadway needs to carry three through lanes a sufficient distance to allow the effective utilization of those lanes and at least two continuous downstream lanes exist beyond that point. As a minimum, use the desirable values
from the “Tangent Prior to Merge” column in Table A.2.2, from FDM 11-25-2, Attachment 2.2.
77
Adapted from (9) ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002. http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf., Figure 36-3T,and (32) OHDOT
Location & Design Manual, Vol.1, Roadway Design ch.400: Intersection Design. Ohio DOT, 2006. http://www.dot.state.oh.us/roadwayengineering/standards/Publications/LDM/2006-07-21/400_jul06.pdf., Figure 401-11E
78
(30) Triple Left Turn Lanes At Signalized Intersections. BC131 (FINAL). Florida DOT, 2002. http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_TE/FDOT_BC131rpt.pdf., p6-7 to 6-9 / 132-134pdf
(33) Development of Guidelines for Triple Left and Dual Right-Turn Lanes: Technical Report. FHWA/TX-11/0
6112-1. Texas Transportation Institute, Texas A&M University, 2011. http://tti.tamu.edu/documents/0-6112
1.pdf., pp.6-5 to 6-9 / 121-125pdf
79
(28) Criteria for the Geometric Design of Triple Left-Turn Lanes. ITE Journal, vol. 64, no. 12, 1994, pp.27-33.
http://turnlanes.net/files/criteria_for_the_geometric_design_of_triple_left-turn_lanes.pdf.
Page 57
5.4.4 Shared left-turn/thru lanes at Signalized Intersections
Shared left-turn/thru lanes are not desirable at signalized intersections. Only use shared left-turn/thru lanes on
minor low-speed streets or on intersection legs where it is physically impossible to provide separate lanes. If
used, monitor their crash history, especially along principal roads.
5.5 Tee Intersection Bypass Lane
A Tee intersection bypass lane (also known as a “SHOULDER BYPASS AT THREE-WAY (T) INTERSECTION”
and as “LEFT TURN BYPASS LANE”) allows a through vehicle to bypass a left-turning vehicle that is stopped in
the traffic lane. See SDD 9A1a for a detail.
A Tee intersection bypass lane is not as safe as an exclusive left-turn lanes because left turning motorists need
to stop or slow down in the thru travel lane. This makes them vulnerable to rear end collisions by inattentive
following motorists. However, a Tee intersection bypass lane is preferable to no left-turn treatment at all and can
improve the efficiency of traffic operations.
Use a Tee intersection bypass lane at the following locations on non-community bypass 2-lane roads:
- Type A intersections if a left-turn lane is not warranted, or if the construction of a warranted left-turn
lane is not technically feasible, leaving no left-turn treatment as the only other alternative,
- Non-Type A intersections if the construction of a warranted left-turn lane is not technically feasible,
leaving no left-turn treatment as the only other alternative,
- Consider at non-Type A intersections where a left-turn lane is not warranted.
Do not use a Tee intersection bypass lane at a four-legged intersection.
Use exclusive left turn lanes with positive offsets at all intersections on a 2-lane community bypass. Do not use
Tee intersection bypass lanes.80
5.6 References
2004.
9. ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002.
12. Stover, V. G. and F. J. Koepke. Transportation and Land Development, 2nd edition. Institute of
Transportation Engineers, Washington, DC, 2006.
14. Manual on Uniform Traffic Control Devices. Federal Highway Administration, Washington, DC, 2009.
15. Gattis, J. L. and S. T. Low. Intersection Angle Geometry and the Driver's Field of View. In Transportation
Research Record 1612: Highway Geometric Design Issues. TRB, National Research Council, Washington, DC,
1998, pp. 10-16.
16. Staplin, L. K., K. Lococo, S. R. Byington, and Scientex Corporation. Highway Design Handbook for Older
Drivers and Pedestrians. FHWA-RD-01-103. Federal Highway Administration Turner-Fairbank Highway
Research Center, McClean, VA, May, 2001. http://www.tfhrc.gov/humanfac/01103/coverfront.htm. Accessed 4
2-2009.
17. Staplin, L., K. Lococo, S. Byington, D. Harkey, and FHWA. Guidelines and Recommendations to
18. Staplin, L., K. Lococo, and S. Byington. Older Driver Highway Design Handbook [ch.1 - Intersections (AtGrade)]. FHWA-RD-97-135. Federal Highway Administration, Washington, DC, Jan., 1998.
19. Rural Intersections - Turn lanes - Left-Turn Bypass Lanes. In MNDOT Road Design Manual ch. 5: At-Grade
Intersections. Minnesota DOT, 2000, sect. 5-4.01.04, pp. 5-4(2).
http://www.dot.state.mn.us/design/rdm/english/5e.pdf. Accessed 2-3-2009.
80
(21) Review of Wisconsin Bypass Road Design Practices February 13-14, 2006. (Final). Federal Highway
Administration Resource Center, 2006. http://www.dot.wisconsin.gov/library/publications/docs/wis-bypass
report.pdf.
Page 58
20. Baumann, A. J. Minimum Required Turn Lane Storage Lengths & Tapers For Left & Right Turn Lanes At
Signalized & Non-signalized Intersections. (DRAFT). Wisconsin DOT, Feb. 25, 2010 current revision (created
Dec 2, 2008; previously revised: Nov 4, 2009).
21. FHWA Resource Center Safety and Design Team. Review of Wisconsin Bypass Road Design Practices
February 13-14, 2006. (Final). Federal Highway Administration Resource Center, Olympia Fields, IL, 2006.
http://www.dot.wisconsin.gov/library/publications/docs/wis-bypass-report.pdf. Accessed 4-2-2009.
22. Rationale for Median Type Recommendations. Kentucky Transportation Cabinet, 2008.
http://www.planning.kytc.ky.gov/congestion/medians/Median%20Type%20Guidelines.pdf. Accessed 5-3-2012.
2011. www.transportation.org.
24. Highway Capacity Manual 2010, 5th edition. Transportation Research Board of the National Academies,
Washington, DC, 2010.
25. Revello, B. Two Way Left Turn Lane (TWLTL). Wisconsin DOT, Nov. 26, 2007.
26. Geometric Design of Lanes - Continuous Two-Way Left-Turn Lanes (TWLTLs). In IADOT Design Manual
ch. 6: Geometric Design. Iowa DOT, 2001, sect. 6C-6, pp. 1-4. http://www.iowadot.gov/design/dmanual/06c
06.pdf. Accessed 5-3-2012.
27. Facility Selection / Two - Way Left - Turn Lanes. In MODOT Engineering Policy Guide ch. 200: Geometrics.
Missouri DOT, 2012, sect. 232.3. http://epg.modot.org/index.php?title=232.3_Two_-_Way_Left_-_Turn_Lanes.
Accessed 5-3-2012.
28. Ackeret, K. W. Criteria for the Geometric Design of Triple Left-Turn Lanes. ITE Journal, vol. 64, no. 12,
1994 December, pp. 27-33. http://turnlanes.net/files/criteria_for_the_geometric_design_of_triple_left
turn_lanes.pdf. Accessed 7-24-2012.
29. Rodegerdts, L. A., B. Nevers, B. Robinson, J. Ringert, P. Koonce, J. Bansen, T. Nguyen, J. McGill, D.
Stewart, J. Suggett, T. Neuman, N. Antonucci, K. Hardy, and K. Courage. Individual Movement Treatments /
Multiple Left-Turn Lanes. In FHWA-HRT-04-091: Signalized Intersections: Informational Guide. Federal
Highway Administration Turner-Fairbank Research Center, McLean, VA, 2004, ch. 12.1.2, pp. 318-319.
http://www.tfhrc.gov/safety/pubs/04091/04091.pdf. Accessed 8-6-2007.
30. Courage, K., A. Gan, B. Stephens, M. Willis, and University of Florida Transportation Research Center.
Triple Left Turn Lanes At Signalized Intersections. BC131 (FINAL). Florida DOT, Tallahassee, FL, Dec., 2002.
http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_TE/FDOT_BC131rpt.pdf. Accessed 7-30
2012.
31. NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, Albany, NY, 2011.
https://www.dot.ny.gov/divisions/engineering/design/dqab/hdm/hdm-repository/chapt_5_final.pdf. Accessed 7
30-2012.
32. OHDOT Location & Design Manual, Vol.1, Roadway Design ch.400: Intersection Design. Ohio DOT, 2006.
http://www.dot.state.oh.us/roadwayengineering/standards/Publications/LDM/2006-07-21/400_jul06.pdf.
33. Cooner, S. A., S. E. Ranft, Y. K. Rathod, Y. Qi, L. Yu, Y. Wang, and S. Chen. Development of Guidelines
for Triple Left and Dual Right-Turn Lanes: Technical Report. FHWA/TX-11/0-6112-1. Texas Transportation
Institute, Texas A&M University, College Station, TX, Jan., 2011 Published July 2011.
http://tti.tamu.edu/documents/0-6112-1.pdf. Accessed 7-24-2012.
LIST OF ATTACHMENTS
Attachment 5.1
Urban Median Opening and Intersection Guidelines
Attachment 5.2
Median Openings and Left Turn Lanes in Urban Roadways
Attachment 5.3
Details for Slotted Left Turn Lanes and Median Opening Openings at Urban
Intersections
Attachment 5.4
Median Opening with Left Turn Lane on Rural High-Speed 4-Lane Divided Highway
FDM 11-25-10 Right-Turn Lanes
March 4, 2013
10.1 Introduction
These guidelines apply to right-turn lanes at intersections without channelizing islands. See FDM 11-25-15 for
Page 59
guidance about channelized right-turn lanes.
A right-turn bay can significantly improve operations and safety at the intersection because it effectively
separates those vehicles that are slowing or stopping to turn from those vehicles in the through traffic lanes.
This separation minimizes turn-related collisions (e.g., angle, rear-end, and same-direction-sideswipe) and
unnecessary delay to through vehicles.81
The selection of a right turn radius requires consideration of design speed, types of turning vehicles, type of
intersection by location (rural, urban or suburban), pedestrian needs and whether the through highway is divided
or undivided.
The assumed speed of a vehicle making a right turn at an intersection designed for minimum-radius turns is less
than 10 mph.82
When providing a designated right turn lane, continue the bicycle accommodation adjacent to the turn lane and
thru the intersection (as shown in SDD 15C29). This is particularly important at signalized intersection and
intersections with pork chop islands. See FDM 11-46-15 for additional guidance. The Intersection Design
Vehicle (see FDM 11-25-2.1 and Table 2.1) does not encroach into a contiguous bike lane between a right-turn
lane and a travel lane. Check the swept path of the Intersection Check Vehicle(s) (e.g., a WB-65) to see if it is
possible to avoid encroaching into the bike lane without significantly disrupting traffic or going outside of the
roadway. Otherwise, consider:
- accepting infrequent bike lane encroachments but consider a warning sign that right turning large
trucks pull left before turning.
- If bike lane encroachment is frequent enough to be potentially dangerous, consider:
- parking restrictions and/or a larger curb radius
- mark as a shared bike/right-turn lane instead of a separate bike lane and right-turn lane
- re-design to reduce or eliminate the conflict
10.2 Intersections in Rural and Developing Areas
Refer to Attachment 1.1 for guidance about right turn lanes on rural high-speed highways.
10.2.1 Storage Length
The right turn lane lengths in the standard rural intersection designs (discussed in Attachemnt 1.1) are for
deceleration of turning vehicles. Where cross road traffic volumes are high, additional length may be needed to
accommodate vehicle storage. Storage requirements should also be evaluated where signals are added to the
intersection. See FDM 11-25-2.2 for guidance on queue storage requirements.
The length of turn lane required for vehicle storage should be determined in cooperation with the region traffic
engineer's staff based on a length of 25 feet per vehicle stored. If the intersection is on the OSOW Freight
Network, depending on frequency of load, it may be appropriate to consider additional length for OSOW
vehicles.
10.3 Two-Way Stop-Controlled Intersections on Urban Low Speed and Transitional Roads
Check with traffic operations on the need for right turn lanes. Accommodate transit, pedestrian and bicyclists
roadway users.
Use the charts in Figure 10.1 as an aid in determining whether to add a right-turn bay on the major road at a
two-way stop-controlled intersection.
See FDM 11-25-10.4 below for guidance on signalized intersections.
81
See Bonneson & Fontaine in NCHRP Report 457 (13) NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements. TRB, National Research Council, 2001. http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf., pp. 22-23) 82
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., p.583, “Minimum edge of Traveled Way Designs” Page 60
Figure 10.1 Guidelines for a Major-road Right-turn Bay at Urban Two-way Stop-controlled Intersections83
83
(13) NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements. TRB, National
Research Council, 2001. http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf., Figure 2.6, p.23 - see also
Page 61
10.3.1 Corner Curb Radius
Central Business District (CBD) streets are typically undivided and often operate as one-way roadways. A
minimum corner radius of 10 feet may be adequate for these streets especially where there is pedestrian activity
and little truck traffic. Progressively larger corner radii are required depending on the functional classification of
the intersecting street; at least 15 feet for locals, 20 feet for collectors and 25 feet for arterials. Large trucks can
be accommodated with these radii if encroachment into the opposing traffic lane can be allowed and parking
can be held back from the corner by at least two spaces. See Chapter 9 of 2004 AASHTO GDHS sections,
“Effect of Curb Radii on Pedestrians” and “Corner Radii into Local Urban Streets”.84
Where there are a significant number of trucks turning and encroachment into the opposing lanes cannot be
allowed, the corner should be designed using an appropriate turning template or three centered compound
curves. See 2004 AASHTO GDHS Exhibit 9-42, "Typical Designs for Turning Roadways". Pedestrian, bicycle
and transit accommodations and signal locations need to be included in the design accordingly .
Where truck volumes are not significant, the right turn radius can be as small as 10 feet in downtown streets and
25 feet at intersections with arterial streets. Intersections that handle large numbers of turning trucks require a
minimum corner radius of 30 feet to turn onto a four lane divided highway where the semitrailer can encroach
into the median lane. Truck drivers will use the median lane when necessary which is allowed under state law
for large trucks. A larger radius should be provided where possible. However, a radius greater than 45 feet
should not be used because it can cause substantial problems with the location of stop signs, traffic signals,
pedestrian push buttons, and crosswalk locations. A large radius also causes crosswalks to be extra long which
results in more pedestrian exposure and visibility problems.
The corner radius can be shorter where the intersection is on a street where parking is permitted. However,
future growth in traffic volumes may demand that the parking lane be converted to a traffic lane. If this is
foreseeable, a large radius should be provided.
For additional guidance, see 2004 AASHTO GDHS sections, ”Minimum Edge of Traveled Way Design” and
“Design For Specific Conditions (Right Angle Turns”85.
10.3.2 Lane Width
The width of a non-channelized right turn lane should generally be the same as the width of the through lane.
For guidance on the use of narrower lanes, see Table 5.2. The desirable width for channelized right turn lanes is
discussed in FDM 11-25-15.
10.3.3 Lane Length
See FDM 11-25-2.3.
10.4 Signalized Intersection Considerations
Consider providing exclusive right turn lanes for all approaches at a signalized intersection. A right turn lane
provides refuge for safe deceleration outside a high speed through lane and provides storage for right-turning
vehicles to assist in optimizing traffic signal phasing.
Improperly designed right turn radii most likely will result in traffic signal knockdowns. A flat corner curb radius
(i.e.,>70 feet) creates a traffic signal design problem when locating the near right traffic signal. The preferred
solution is to design a small pork chop island (minimum of 150 square feet) to place the traffic signal and lighting
bases, pull boxes, pedestrian pushbuttons, and pedestrian walkways. The island also facilitates channelization
of the right turn movement (see FDM 11-25-15 for guidance on channelized right turns).
10.4.1 Dual Right Turn Lanes
Dual right-turn lanes have typically been installed at signalized intersections and at roundabout right-turn bypass
lanes on urban arterial roadways and interchange ramps. Determine the actual need by performing a signalized
intersection capacity analysis. Use dual right-turn lanes only if necessary because they are particularly difficult
for bicyclists and pedestrians86. Dual right-turn lanes at signalized intersections are required to be signalcontrolled. See FDM 11-26 for guidance on dual right-turn lanes at roundabout right-turn bypasses.
interactive spreadsheet included in on-line version, “http://onlinepubs.trb.org/onlinepubs/nchrp/esg/figure 2
6.xls” (NCHRP references from “NCHRP Report 457” are reproduced with permission of the TRB through the
National Academy of Sciences (NAS))
84
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004., pp.614-621
85
86
(33) Development of Guidelines for Triple Left and Dual Right-Turn Lanes: Technical Report. FHWA/TX-11/0
6112-1. Texas Transportation Institute, Texas A&M University, 2011. http://tti.tamu.edu/documents/0-6112
1.pdf., p.6-2
Page 62
There are generally two reasons for using dual right-turn lanes87:
1. To accommodate high right turn volumes and provide enhanced capacity at intersections where a
single right-turn lane is not adequate and a free-flow right turn lane is not advisable
2. To mitigate weaving traffic conflicts, e.g. drivers who are making a left-turn at the next downstream
intersection would make their turn from the left-hand lane of the dual right-turn lanes
Research has shown that a well-designed dual right-turn lane does not cause significantly higher crash
frequency or severity compared to a single right-turn lane88 and will usually improve the operations of
intersections. The additional deceleration and storage space helps prevent spillover into adjacent through lanes.
Right-turning traffic requires less green time, and this time thus can be allocated to other movements. However,
because of the added width, signal-timing intervals for bicycle and pedestrian movements require special
attention,
Consider using dual right-turn lanes at an intersection with a single right-turn lane - and where the receiving leg
has at least two lanes - if one of the following conditions exists89:
- The right-turn volume is greater than 500 vph;
- There is not sufficient space to provide the necessary length of a single turn lane because of restrictive
site conditions (e.g., closely spaced intersections),
- The required length of a single turn lane becomes excessive (usually about 300-ft or greater)
- The volume to capacity ratio for a single right-turn lane is greater than or equal to 0.90, or LOS is
worse than D
- Right-turn green time and green time from an overlap are not sufficient to handle the right-turn volume
There are two (2) types of dual right-turn lane configurations:
1. Shared dual right-turn lanes: the right hand lane (i.e., the curb lane) is an exclusive right-turn lane and
the left-hand lane is a shared right-turn/thru lane
2. Exclusive dual right-turn lanes: both the right-hand lane and the left-hand lane are exclusive right-turn
lanes
Exclusive dual right-turn lanes are generally preferable because they provide more capacity enhancement.
Exclusive dual right-turn lanes also allow for placement of a bicycle lane between the through lane and the rightturn lanes.90
The shared right-turn/thru lane has lower lane utilization than an exclusive right-turn lane because thru vehicles
block right-turning vehicles during protected right-turn phases; the lower lane utilization may result in the need
for longer storage in the curb right-turn lane. In addition, shared dual right-turn lanes, do not allow for placement
of a bicycle lane between the shared right-turn/thru lane and the exclusive right-turn lane (or between the
exclusive thru lane and the shared right-turn/thru lane). However, shared dual right-turn lanes are preferred
where:
- More flexibility is needed to use an optional lane
- Less impacts on the adjacent through movement is desired
- Right-of-way for providing an additional turn lane is restricted
By statute91, Right turns on red (RTOR), when permitted, are only allowed from the right-hand (i.e., curb) lane of
exclusive dual right-turn lanes Pull the curb lane out beyond the left-hand turn lane so drivers in the curb lane
have a clear unobstructed view of approaching traffic.. Consider prohibiting right-turn-on-red (RTOR) from a
dual right-turn lane if one or more of following conditions exist:92
- Insufficient sight distance
- Frequent presence of pedestrians
87
(34) Development of Warrants for Installation of Dual Right-Turn Lanes at Signalized Intersections.
SWUTC/12/161141-1. Texas Transportation Institute, Texas A&M University, 2012.
http://d2dtl5nnlpfr0r.cloudfront.net/swutc.tamu.edu/publications/technicalreports/161141-1.pdf., p.91-95 / 107
111pdf
88
Reference (34), p.18 / 34pdf
89
90
Reference (34), pp34-35, 94 / 50-51, 110pdf
91
Section 346.37(1)(c)3, Wis. Stats
92
Page 63
-
-
-
-
Use of split phase
Significant U-turns from right-hand cross-street
High crash history
High-speed road, onto which subject RTOR vehicles turns
Inadequate capacity of receiving lanes
There are some potential issues with dual right-turn lanes, including93:
- Sideswipes between turning vehicles are a possibility at double turn lanes. This is especially an issue
if the turn radius is tight and large vehicles are likely to be using the turn lanes. Delineation of turn
paths should help address this issue.
- Impaired intersection sight distance (ISD) for drivers in the right-hand turn lane due to vehicles in the
left-hand lane obstructing their view of on-coming traffic
- Right-of-way acquisition may be expensive.
- Possible access restrictions to adjacent properties
- Dual right-turn lanes make crosswalks longer, which can affect minimum cycle time, increase
pedestrian exposure, and precipitate long pedestrian clearance intervals that may or may not work
with coordination timing plans.
- Pedestrian movement may also be less safe because a vehicle in the curb lane whose driver is
yielding to a pedestrian can block sight lines for drivers in the left-hand turn lane
Design considerations for dual right-turn lanes include94:
- Check all turning paths of dual right-turn lanes with truck turning templates allowing 2-ft.between the
tire path and edge of each lane.
- Provide a separation between vehicles turning side-by-side of 4-feet desirable / 3-feet minimum.
- Turn lane widths for dual right-turn lanes are 12-ft desirable and 11-ft minimum.
- The minimum width of channelized roadway for dual right-turn lanes is 30-feet, not including gutters.
- Determine the length of dual right-turn lanes as discussed in FDM 11-25-2.
- WisDOT’s practice95 is to assume that the Intersection Design Vehicle (see FDM 11-25-2, Table 2.1)
turns from the left-hand lane of the dual right turn lanes (see Figure 10.2). However, there may be
locations where it is appropriate to assume that the Intersection Design Vehicle turns from the righthand lane (for example, a significant number of the vehicles are making a right turn at a close by
downstream intersection or driveway). Desirably, the vehicle in the other lane (typically, the right-hand
lane) should be a SU truck but, as a minimum, can be a passenger car96, if any or all of the following
conditions are present:
- Cross street volume is minimal (< 400 ADT) and route is unlikely to be used as a detour route
for a nearby higher volume roadway
Figure 10.2 Dual Right-Turn Lanes
93
Reference (34), pp5, 41-42 / 21, 57-58pdf
Reference (34), pp5-14 / 21-30pdf
95
per interim TSDM 3-3-4, July 2009
96
ILDOT Bureau of Design and Environment Manual 2002 (9) ILDOT Bureau of Design and Environment
Manual ch. 36: Intersections. Illinois DOT, 2002.
http://www.dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/chap36.pdf. sect. 36-3.05(b), “Dual Turn Lanes /
Design”
94
Page 64
- Dual right-turn lanes require sufficient turn radii to allow a smooth turn from both turn lanes, but not so
large as to encourage excess speed.
- Provide adequate throat width on the intersection leg receiving the dual right turns to compensate for
turning vehicles offtracking and for the relative difficulty of side-by-side right turns. Provide throat
widening comparable to that used for dual left-turn lanes (see FDM 11-25-5.4.3.1.1, including Table
5.3 and Figure 5.4). Consider how throat widening will affect the traffic approaching from the other
side. Make sure that the through lanes line up relatively well to ensure a smooth flow of traffic through
the intersection.
- The receiving roadway needs to carry two through lanes a sufficient distance to allow the effective
utilization of both lanes (As a minimum, use the desirable values from the “Tangent Prior to Merge”
column in Table A.2.2, from FDM 11-25 Attachment 2.2) - but not less than 150-ft.
- Truck traffic utilization is an issue when designing dual right-turn lanes. Similar to a roundabout, if
designed too wide to accommodate truck traffic, then traffic may create a "third turn lane", especially
during snowy conditions.
- Avoid installing dual right-turn lanes near access points (e.g., from gas stations, parking lots, or other
traffic generators).
- For closely spaced intersections, if a downstream intersection uses dual right-turn lanes, do not align
the curb right-turn lane with any through lane at the upstream intersection.
- Provide adequate signing and marking to make the intended operation clear to every road user. Each
turn lane should be marked with turn arrows and "ONLY" legends as appropriate. Use lane line
extensions to delineate the turning path through an intersection in order to reduce the potential for
sideswipe collisions and to increase the efficiency of right-turn operations. Determine lane line (or
guideline) and width requirements by plotting the swept paths of the selected design vehicles. There
should be no conditions that obscure, or result in, confusing pavement markings within the
intersection.
10.5 References
2004.
97
33. Cooner, S. A., S. E. Ranft, Y. K. Rathod, Y. Qi, L. Yu, Y. Wang, and S. Chen. Development of Guidelines
for Triple Left and Dual Right-Turn Lanes: Technical Report. FHWA/TX-11/0-6112-1. Texas Transportation
Institute, Texas A&M University, College Station, TX, Jan., 2011 Published July 2011.
97
[Dec 3, 2012 email from Ellen Chafee, Editor, CRP-TRB] The TRB through the National Academy of Sciences
(NAS) grants permission to use the material listed below from Maze et al. (2010) NCHRP Report 650:Median
Intersection Design for Rural High-Speed Divided Highways and J. A. Bonneson and M. D. Fontaine (2001)
NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements in a proposed revision
to Chapter 11, Section 25 of Wisconsin DOT’s Facilities Development Manual (FDM 11-25).
Permission is also granted for any subsequent versions of the Work, including versions made for use with blind
or physically handicapped persons, and all foreign-language translations of the Work prepared for distribution
throughout the world.
Permission is given with the understanding that inclusion of the material will not be used to imply Transportation
Research Board, AASHTO, Federal Highway Administration, Transit Development Corporation, Federal Transit
Administration, Federal Aviation Administration, or Federal Motor Carriers Safety Administration endorsement of
a particular product, method, or practice.
Permission is also provided on condition that appropriate acknowledgment will be given as to the source
material.
Page 65
34. Yi, Q., C. Xiaoming, D. Li, and Center for Transportation Training and Research - Texas Southern
University. Development of Warrants for Installation of Dual Right-Turn Lanes at Signalized Intersections.
SWUTC/12/161141-1. Texas Transportation Institute, Texas A&M University, College Station, TX, Apr., 2012.
http://d2dtl5nnlpfr0r.cloudfront.net/swutc.tamu.edu/publications/technicalreports/161141-1.pdf. Accessed 1-9
2013.
FDM 11-25-15 Turning Roadways (Channelized Right)
March 4, 2013
15.1 Criteria
At intersections with a considerable number of turning movements, especially by trucks, and where it is
desirable to maintain a turning speed for passenger vehicles of roughly 15 mph (25 km/h) or greater, a separate
turning roadway or channelized right-turning lane should be provided between intersection legs. Check the
turning movements of OSOW vehicles if needed (see FDM 11-25-2, Table 2.2). Verify that OSOW vehicles are
not prohibited from turning at the intersection where needed.
The term "turning roadways" also applies to ramps and ramp terminals, particularly at the crossroad. Refer to
FDM 11-30 Attachment 1.3, 1.4, 1.5, and 1.6 for geometrics at ramp terminals.
15.2 Speed And Curvature
The speed maintained on the free flow segment of turning roadways is governed by the radius of curve and
superelevation (see FDM 11-10-5 for superelevation guidance). "Free-Flow Turning Roadways at Intersections"
are discussed on pages 639-649, GDHS.
Compound curves should be used at the downstream connection with the departure leg of the intersection to
avoid vehicle encroachment onto the curb or shoulders. Three-centered compound curves for vehicles of
different design classification are shown in Exhibit 9-20, pages 584-591, GDHS98. It is desirable that the right
turn radius be kept as small as possible to avoid excess speed, while still accommodating the Intersection
Design Vehicle.
15.3 Design Guides
The width of turning roadways should accommodate the design class of vehicle that is anticipated. For turning
lanes that are longer than 50 feet, provisions should be made to pass a stalled vehicle in the turning lane. The
design width of pavement for turning roadways is shown in Exhibit 3-51, page 220, GDHS99 with 15 feet as a
minimum plus the gutter width. "Turning Roadways with Corner Islands" are discussed on pages 634-639
GDHS.
Channelized right turns should be brought in as close to perpendicular as possible for vision to the left. Right
turn lanes separated by islands having intersecting angles less than 60 degrees with the cross street require the
driver to look back over their left shoulder to view oncoming traffic, which is particularly difficult for older drivers.
Design right turn islands in urban/suburban areas with the right-turn lane at an angle as close to 90-degrees as
possible, based on the guidance in FDM 11-25-2.7, “Angle of Intersection”, and as shown in Figure 15.1.
Figure 15.1 Intersection Angle for Channelized Right Turn
The taper on the approach to the turn is dependent upon design speed, but 20:1 is typical with 10:1 as a
98
99
Page 66
practical minimum for most urban streets. Use this taper on the receiving leg of the turning roadway as well.
Consider the type of controls to use for channelized right turns. Typically, it is preferred to use a less restrictive
method and increase the degree of control as volumes, safety, and geometric conditions dictate. Refer to TSDM
3-4-2 for guidance on control of channelized right turns at signalized intersections.
Provide offsets to raised curb islands as described in FDM 11-25-25.2.1.
15.4 References
2004.
FDM 11-25-20 Median Openings
March 4, 2013
20.1 Introduction
Median openings, whether they are located at major intersections or serve traffic generators between
intersections, all tend to interrupt through traffic flow. On arterial streets, it is highly desirable to maintain a free
flow of traffic without interruptions.
Median openings accommodate left-turn movements and/or u-turn movements from the highway; and/or also
accommodate cross traffic movements and/or left-turn movements from a side road or driveway. A full median
opening allows all movements. A directional median opening (Figure 20.1 and Figure 20.2) allows some but not
all movements - but has fewer conflict points - and has been found to reduce crash rates.
Provide a pedestrian crossing where the side road has sidewalks on one or both sides of the street and the
through street has sidewalk on the opposite side. This condition establishes a legal crosswalk whether the
crosswalk is pavement marked or not per ss340.01 (10) (b).Also providing median refuge for pedestrian and
share-use path crossings may influence median nose design. See FDM 11-25-5 and FDM 11-46-10.
Figure 20.1 Directional Median Opening100
100
Adapted from (11)Access Management Manual. Transportation Research Board, 2003., Figure 11-4 on
p.207. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the
Transportation Research Board.
Page 67
Figure 20.2 Separator Overlap for Directional Median Opening101
If there is sufficient space, providing unsignalized directional openings between signalized intersections
facilitates access to abutting properties and reduces U-turns / left turns at the signalized intersections. The
operations of the adjacent signalized intersections are of greater importance than the midblock opening(s). The
midblock opening(s) must not compromise the design or operations of the signalized intersections.
Figure 20.3 Examples of Directional Median Openings between Signalized Intersections102
See Table 20.1 below. Also, see AASHTO GDHS, pp.689-712103 for additional guidance on median openings.
20.2 U-Turns
Median openings for U-turns may be appropriate at some locations, such as:
- In advance of some signalized intersections,
- Downstream from intersections where side road traffic thru movement is not allowed
101
Adapted from (11)Access Management Manual. Transportation Research Board, 2003., Figure 11-7, p. 209. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the Transportation Research Board.
102
Adapted from (11)Access Management Manual. Transportation Research Board, 2003., Figure 11-5, p. 208. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the Transportation Research Board.
103
Figure 20.4 Directional Median Openings for U-turns104
(a) Downstream from Signalized Intersection
(b) Upstream from Signalized Intersection
See Table 20.1 below. Also, see AASHTO GDHS, pp.709-712105 for additional guidance, including minimum
widths of medians required to accommodate U-turns. Also, see chapter 11 of the TRB Access Management
Manual.
20.3 Length of Opening
Use the control radii for vehicles making a left-turn or making a U-turn to determine the length of a median
opening. The minimum median opening length is 40-feet. See AASHTO GDHS, Exhibits 9-76 to 9-83106. Also,
see FDM 11-25-5.3 for guidance on control radii and median end design.
20.4 Spacing
Provide median openings only at locations that are safe for all allowed movements. Also, provide adequate
spacing for traffic weaving in order to preserve traffic flow and for safe lane changes and turns.
At a signalized intersection, do not provide a median opening that crosses a left turn lane or left turn storage.
The functional area of an intersection is the critical area where motorists are responding to the intersection,
decelerating, and maneuvering into the appropriate lane to stop or complete a turn. Access connections too
close to intersections can cause serious traffic conflicts that impair the function of the affected facilities. Drivers
need sufficient time to address one potential set of conflicts before facing another.
Ideally, do not place a median opening for a public access intersection (street or alley) or a private access
intersection (driveway or private road) within the upstream functional area of another intersection. A median
opening within the limits of an exclusive left-turn bay or within the downstream functional area of an intersection
is especially un-desirable because it violates driver expectancy and can have a negative effect on the safety,
operation and capacity of an intersection.
See Table 20.1 for guidance on evaluating existing and proposed median openings.
104
(11)Access Management Manual. Transportation Research Board, 2003., Figure 11-8, p.210. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the Transportation Research Board.
105
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. 106
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004.. Page 69
Table 20.1 Median openings – allowable locations (applicable to STH and connecting highways)
Median Opening
location
Relative to
Functional Area of
intersection [B]
Conditions and Requirements for Median Openings [A]
Existing opening
(includes formerly undivided roadways on
which a new median is added)
New Opening
Inside downstream
functional area of
intersection
for approach lanes
and/or for opposing
lanes
* CLOSE median openings at these locations. Do
not consider leaving these open under any
circumstances; and
DO NOT ALLOW
upstream of (d1)
AND
Outside of Functional
Areas for adjacent
intersection
* Construct a separate turn bay for the opening if
the opening is inside the left-turn bay for the
downstream median opening; and
* The proposed location meets applicable
access, intersection and median opening spacing
standards; and
* Make any modifications necessary to correct
safety, design and/or operational deficiencies.
* Sufficient sight distance, turning geometry,
storage and deceleration can be provided for all
allowed movements; and
* Evaluate and improve the operation and
geometry of adjacent intersection to
accommodate any increase in turning movements
resulting from closing the median opening.
* The projected design year level of service is D
or better for all allowed movements; and
* Either a full or a directional median opening is
allowed. The opening and its associated turn
bay(s) must be separate from the left-turn bay(s)
for the adjacent median opening(s).
Inside (d1)
* The proposed location meets applicable
* Construct a separate turn bay if the opening is
inside the left-turn bay for the downstream median access, intersection and median opening spacing
standards; and
opening ; and
* Do not allow additional movements at a
directional median opening.
* Close or restrict a full median opening to a
directional median opening if level of service is
worse than D or if there is a crash problem
associated with the opening; and
* Close or restrict a directional median opening if
level of service is worse than D or if there is a
crash problem associated with the opening; and
* Sufficient sight distance, turning geometry,
storage and deceleration can be provided for all
allowed movements; and
*The opening and its associated turn bay(s) must
be separate from the left-turn turn bay(s) for the
adjacent median opening(s); and
* The projected design year level of service is D
or better for all allowed movements; and
* Make any modifications necessary to correct
safety, design and/or operational deficiencies.
* Do not allow left-outs and thru movements from
side roads / driveways and left-ins from the
opposite-direction mainline unless meeting the
conditions in Note [D] below.
Inside (d2)
It meets the conditions in Note [C] below.
DO NOT ALLOW
Inside (d3)
* CLOSE median openings at these locations. Do
not consider leaving these open under any
circumstances; and
Inside (d4)
Inside Mainline
design hour queue
* Evaluate and improve the operation and
geometry of adjacent intersection to
accommodate any increase in turning movements
resulting from closing the median opening.
Notes
A Evaluate each opening for both directions of travel, and for both peak and non-peak conditions. If movement restrictions
or prohibitions are ineffective or impractical then close median opening.
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B The upstream functional lengths for the thru lane(s), left turn bay, and right turn bay determine the boundary for the
upstream functional area of intersection.
Upstream functional length of intersection elements d1, d2, d3, and d4 are independent of turn bay elements although,
ideally, they correlate as shown in FDM 11-25-2, Figure 2.9. However, this correlation is not always possible, or may
change because of changes in traffic at an intersection.
C Consider allowing an existing opening to remain only if there is a written request from a municipality to do so. This
request must contain acceptable documentation for all of the following:
- Design alternative for closing the existing opening that evaluates alternate accesses, operations and safety
and includes a good faith comparison showing that keeping the existing median opening is the preferred
alternative.
- Minimum of the most recent available 3-year crash history showing that there is not a crash problem
associated with the existing median opening. Crashes that might be associated with the opening can occur at
the opening but also up to several hundred feet from the opening in both directions of travel. Examples
include:
- Crashes involving vehicles in the approach thru lane forced to decelerate or stop because of spillback
from the median opening.
- Crashes involving vehicles from a side road or driveway that are making left-out or thru movements.
- Crashes involving vehicles from the opposing lane that are making left-in or u-turn movements.
- Crashes involving vehicles unable to clear the intersection because of queuing in the oppositedirection mainline at the median opening.
- Crashes involving vehicles from the turn lane that are making left-in movements.
- The existing opening is not within the design storage queue of either the turn lane or the mainline.
- Evaluation and proposed improvements of adjacent intersections capability to accommodate increased
turning movements resulting from restrictions at the existing median opening.
- There is adequate storage and deceleration length available for same-direction left-in movement at the
existing median opening.
- Prohibit left-outs and thru movements from side roads / driveways and left-ins from the opposite-direction
mainline unless meeting the conditions in Note [D] below.
- The design hour level of service for any of the allowed movements at the existing opening is C or better, and
the design hour level of service for the intersection is C or better.
- The municipality agrees to close the opening if:
- the design hour level of service deteriorates to D or worse, or
- there is a crash problem, or
- the design storage queue for either the turn lane and/or the same-direction mainline extends into the
median opening
D Movements that are prohibited during certain times of day can be allowed during other times of day if all of the following
conditions are met:
- Sufficient median width and turn bay length must be available for all allowed movements.
- Levels of service for all allowed movements shall be C or better.
- Signing that prohibits these movements at all other times shall be installed.
20.5 References
2004.
11. TRB Committee on Access Management (ed.) (ed.). Access Management Manual. Transportation
Research Board, Washington, DC, 2003. 107
107
[Dec 6, 2012 email from Phyllis Barber, Transportation Research Board Publications Office / Javy Awan,
Director of Publications, Transportation Research Board] The Transportation Research Board grants permission
to the Wisconsin DOT to reproduce 4 figures from the TRB Access Management Manual, in a proposed revision
to Chapter 11, Section 25 of Wisconsin DOT’s Facilities Development Manual, as identified in your request of
December 3, 2012, subject to the following conditions:
1. Please credit as follows:
From Access Management Manual, Figure 11-4, p. 207; Figure 11-5, p. 208; Figure 11-7, p. 209; and Figure 11
8, p. 210. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the Transportation Research Board.
Page 71
FDM 11-25-25 Channelization March 4, 2013
25.1 General
Traffic can be channelized by using various combinations of islands, pavement markings, rumble strips,
contrasting pavement, traffic signals, etc. The design guides for providing left- and right-turn lanes (FDM 11-25
5 and FDM 11-25-10) are also methods of channelizing traffic.
25.2 Islands
This discussion assumes that islands are raised by using curb and gutter. The use of islands for directing traffic
should be held to a practical minimum, as they in themselves can present problems, especially for winter
maintenance activities. The desirable minimum size for islands is 150 square feet; the minimum size is
100 square feet. The approach end of the island should provide sufficient warning to identify the island's
existence. This can be accomplished by using a raised delineator (non-rigid) or a rumble strip. To prevent
damage to snowplows or errant vehicles, a mountable curb should be constructed on the approach nose.
Islands may also need to provide for pedestrian crossing. The crossing area needs to be unobstructed with a
flat, level surface.
Minimize channelization islands, raised islands and other raised features that may inhibit turning movements of
OSOW vehicles on the OSOW Freight Network.
25.2.1 Offsets
The approach nose of a curbed island needs to be conspicuous to approaching drivers - and definitely clear of
the vehicle path, both visually and physically, so that drivers will not shy away from the island. Where possible,
offset median islands 8 feet from the travel lane and transition to a normal curb offset, - typically 2 feet. The
transition length is dependent on the design speed.
Offsets from the edge of thru travel lane to the face of a curbed channelizing island for a turning roadways are
as follows (these offsets include a continuation of the width provided for on-street bicycle accommodation (see
SDD 15C29)):
Low speed urban roadways (posted speed of 40 mph or less)
- If the offset to curb face from the outside edge of approach travel lane is <=2-ft then offset the
approach nose of the right turn channelizing island by 4-ft from the outside edge of the travel
lane and taper down to a 2-ft offset at the departure nose.
- If the offset to curb face from the outside edge of approach travel lane is >2-feet then offset the
approach nose of the right turn channelizing island by an additional 2-ft from the outside edge of
the travel lane and taper down to the normal offset at the departure nose.
Transitional and high speed urban roadways (posted speed of 45 mph or greater)
- If the offset to curb face from the outside edge of approach travel lane is <=6-ft then offset the
approach nose of the right turn channelizing island by 8-ft from the outside edge of the travel
lane and taper down to a 6-ft offset at the departure nose.
- If the offset to curb face from the outside edge of approach travel lane is >6-feet then offset the
approach nose of the right turn channelizing island by an additional 2-ft from the outside edge of
the travel lane and taper down to the normal offset at the departure nose.
Rural roadways
- If the outside finished shoulder width is <=6-ft then offset the approach nose of the right turn
channelizing island by 8-ft from the outside edge of the travel lane and taper down to a 6-ft
offset at the departure nose.
- If the outside finished shoulder width is >6-feet then offset the approach nose of the right turn
channelizing island by an additional 2-ft from the outside edge of the travel lane and taper down
to the normal shoulder width at the departure nose.
Offset the edge of a channelized turning roadway by 2-3 feet from the face of a curbed channelizing island at
the approach nose and continue this offset to the departure nose.
25.2.2 Signalized Intersection Considerations
As discussed in FDM 11-25-10, right-turn pork chop islands are typically provided for delineation, pedestrian
refuge, and traffic signal placement at intersections with flat radii. Revisions to a signalized intersection will
2. None of this material may be presented to imply endorsement by TRB of a product, method, practice, or
policy.
Page 72
typically be needed at some point in the future. Therefore, the construction of islands is very important.
Monolithic concrete islands are not desirable because installing a pull box or base would require removing
concrete.
25.3 Pavement Markings
Painted islands should not be offset from the through lane except where the lane width is insufficient. For
additional discussion, refer to pages 621-639 of GDHS108.
25.4 References
2004.
FDM 11-25-30 Curb Ramps
October 5, 2011
This portion of the FDM has been transferred to FDM 11-46-10.
FDM 11-25-35 Auxiliary Lanes
March 4, 2013
35.1 Auxiliary Lanes
An auxiliary lane is defined as the portion of roadway adjoining the traveled way such as turning lanes, storage
for turning, weaving, or the added lane between two interchange ramp areas, and other purposes
supplementary to through-traffic movement.
Truck climbing lanes and passing lanes are not considered auxiliary lanes. For more information on truck
climbing lanes and passing lanes see FDM 11-15-10.
35.2 Acceleration Lanes
For design details of acceleration lanes refer to FDM 11-30-1. Acceleration lanes may also be used at nonsignalized intersections with turning roadways, particularly for right-turning vehicles entering an arterial. In some
cases, a length of the parking lane may become the acceleration lane. For details relating to a tapered or a
parallel type of acceleration lane, refer to pages 688-689, GDHS109.
35.3 Bus Stops
Bus transit is an integral part of the operation of many urban streets and highways. The existing operating
policies and the future transit needs of communities should be given design consideration where applicable,
particularly where bus movements caused by bus stops will affect intersection capacity.
A bus stop area, landing pad or platform is the portion of roadway designated for transit users to facilitate
boarding and alighting110. A bus stop connects to an intersection corner, sidewalks, or paths by an accessible
route. Connections directly to the roadway are not permitted because roadways are not pedestrian facilities. The
minimum requirements for a bus stop site are:
- A firm, stable surface with a 2% cross slope;
- A minimum clear length of 96 inches, measured from the curb or vehicle roadway edge;
- A minimum clear width of 60 inches, measured parallel to the vehicle roadway;
- A bus stop area, landing pad or platform must meet ADA design standards.
Other transit facilities that should be considered for buses are bus passenger shelters, park-and-ride lots, and
turnouts (separate loading lane). The decision to include bus turnouts should be based on the volume and
turning movements of both the bus traffic and through traffic, the distance between bus stops, and right-of-way
limitations. The design features for turnouts should be based on the size and turning radius of the bus.
Generally, turning radii should be such that buses can remain in the outer lane during the full turn. For a more
108
(1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. (1) A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, 2004. 110
(35) Toolkit for the Assessment of Bus Stop Accessibility and Safety. Easter Seals Project ACTION, 2006. http://www.transitaccessproject.org/InternalDocs/TransitFacilities/06BSTK_Complete_Toolkit.pdf;http://www.nel
sonnygaard.com/Documents/Articles/Complete_Toolkit-new.pdf. Note: This is listed as a reference by FHWA at http://safety.fhwa.dot.gov/ped_bike/ped_transit/ped_transguide/ch1.cfm, “Pedestrian Safety Guide for Transit Agencies” 109
Page 73
complete discussion of bus considerations, see pages 367-373, GDHS111.
FDM 11-25-40 Railroad Crossings
March 4, 2013
40.1 General
If there is a railroad crossing on a project, include the region railroad coordinator early in project scoping and
thereafter during project design.
FDM 17-60-5 establishes railroad grade crossing design criteria. FDM 17-40-5 explains factors to consider when
evaluating the potential need for a grade separation structure. All signing, marking, signals, and gate
installations shall conform to the Manual on Uniform Traffic Control Devices, FHWA, 2000 and the Wisconsin
Supplement. Additional information can be found on pages 731-739, GDHS112
Sight distance triangles should be provided for vehicles approaching a crossing, but a separate sight distance
triangle must be provided for vehicles such as buses and trucks, which are required to stop. Stopped vehicles
need additional sight distance to proceed safely across a railroad crossing. An additional lane should be
considered for stopped vehicles, particularly on multi-lane highways.
On new construction, reconstruction and pavement replacement projects being designed with Civil 3D software
and using a 3D model check the 5-axle expandable-deck lowboy (DST Lowboy) OSOW-ST vehicle at railroad
crossings on the OSOW Freight Network (FN) to ensure sufficient vehicle body clearance so that vehicles can
cross the tracks without “hanging up”. See FDM 11-25-1.4 for information on the OSOW Freight Network (FN).
See FDM 11-25-2.1.1 and Attachment 2.1 for information on OSOW vehicles.
40.2 References
2004.
FDM 11-25-45 Frontage Roads
March 4, 2013
A service road (also commonly referred to as a frontage or backage road) is a public or private street or road
that runs generally parallel to but is separated from the major roadway by a physical barrier. Its primary function
is to provide access to the abutting properties. Service roads are also referred to commonly as frontage or
backage roads.
A frontage road is a service road between the right-of-way of the major roadway and the front building setback
line. It provides access to properties while separating them from the principal roadway. Frontage roads will
“front” on the major roadway.
A backage road is a service road that is separated from the major roadway by intervening land uses. The
arterial abuts the rear lot line and buildings may face the backage road. Buildings on backage roads face away
from the major roadway.
Freeway/expressway interchange areas that have frontage road access to the crossroad outside the ramps are
addressed in FDM 11-5-5.
Service roads provide the following benefits:
- Effectively control access to the through lanes on the arterial street,
- Provide access to adjoining property,
- Separate local traffic from through traffic, and
- Permit circulation of local traffic adjacent to the arterial.
From an operational and safety standpoint, one-way service roads on each side of an arterial may be preferred
to two-way service roads.
Maximize the separation distance between the service road/crossroad intersection and the arterial/crossroad
intersection to ensure sufficient storage for traffic on the crossroad between the service road and the arterial. At
some time the arterial/crossroad intersection may be signalized or include a roundabout. Provide adequate
storage for queued vehicles.
The absolute minimum separation distance (Dimension A in Figure 45.1) is 150 feet in a tightly constrained
urban environment with low crossroad traffic volumes. This is the shortest length for placing signs and other
111
112
Page 74
traffic control devices. Greater distances are needed to provide adequate vehicle storage and to separate
operation of the two intersections. Spacing of at least 300-feet, preferably more, in urban areas enables turning
movements to be made from the arterial lanes onto the service road without seriously disrupting arterial traffic.
High crossroad traffic volumes with high service road volumes will typically justify a greater separation distance.
This may be achieved by taking the service road around an existing or proposed development as shown in the
“bulbed separation” area, in effect developing a backage road for that portion of the otherwise frontage road. A
greater separation than those shown in Figure 45.1 may be needed if signalization is required. The
recommended separation distance between signals is about 1,300 feet, unless the signals are coordinated like
the close spacing between interchange ramps. The separation between properly designed roundabouts may be
300 feet or less in tight situations.
Away from the arterial intersection consider the distance separating the service road travel lanes from the
arterial travel lanes, distance “B” on the bulbed separation” side of Figure 45.1. Headlight glare, driver confusion
about the location of an approaching vehicle and errant vehicles are safety concerns that suggest keeping that
distance as wide as practical. In tight built-up urban areas, this distance may be as low as 45 feet. In situations
that present a safety concern, glare fence or other protective shielding may be required between the service
road and the arterial.
Min. Distance A113 (stop control)
Crossroad Design year
AADT
Distance (ft)
Urban
Rural
< 100
150
300
100 – 1,000
300
300
> 1,000
600
600
113
(36) NCHRP Report 420: Impacts of Access Management Techniques. TRB, National Research Council,
1999. http://www.accessmanagement.info/pdf/420NCHRP.pdf., pp.121-127
Page 75
Distance B
Urban
Rural
Desirable
85 ft
115 ft
Minimum
45 ft
85 ft
Greater distances may be warranted where noise barriers,
berms or landscaping are located along the arterial.
Distance ‘B’ for a backage road does not necessarily equal
Distance ‘A’ along the crossroad.
Figure 45.1 Frontage Road Offset Guidelines
45.2 References
2004.
36. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques.
TRB, National Research Council, Washington, DC, 1999.
http://www.accessmanagement.info/pdf/420NCHRP.pdf.
Page 76
FDM 11-25-50 Master Reference List114 115 116 117 118
March 4, 2013
1. [Ref 350] A Policy on Geometric Design of Highways and Streets 2004, 5th edition. AASHTO, Washington,
DC, 2004.
2. [Ref 721] ORDOT Highway Design Manual ch. 9.0: Intersection and Interchange Design. Oregon
Department of Transportation, 2008.
ftp://ftp.odot.state.or.us/techserv/roadway/web_drawings/HDM/Rev_E_2003Chp09.pdf. Accessed 8-6-2010.
3. [Ref 698] Maze, T. H., J. L. Hochstein, R. R. Souleyrette, CTRE - Iowa State University, H. Preston, and R.
Storm. NCHRP Report 650: Median Intersection Design for Rural High-Speed Divided Highways. Transportation
Research Board of the National Academies, Washington, DC, 2010.
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_650.pdf. Accessed 5-18-2010.
4. [Ref 455] Harwood, D. W., M. T. Pietrucha, M. D. Wooldridge, R. E. Brydia, and K. Fitzpatrick. NCHRP
Report 375: Median Intersection Design. TRB, National Research Council, Washington, DC, 1995.
5. [Ref 294] Potts, I. B., D. W. Harwood, D. J. Torbic, K. R. Richard, J. S. Gluck, H. S. Levinson, P. M.
Garvey, and R. S. Ghebrial. NCHRP Report 524: Safety of U-Turns at Unsignalized Median Openings.
114
[Dec 3, 2012 email from Ellen Chafee, Editor, CRP-TRB] The TRB through the National Academy of
Sciences (NAS) grants permission to use the material listed below from Maze et al. (2010) NCHRP Report
650:Median Intersection Design for Rural High-Speed Divided Highways and J. A. Bonneson and M. D.
Fontaine (2001) NCHRP Report 457: Engineering Study Guide for Evaluating Intersection Improvements in a
proposed revision to Chapter 11, Section 25 of Wisconsin DOT’s Facilities Development Manual (FDM 11-25).
Permission is also granted for any subsequent versions of the Work, including versions made for use with blind
or physically handicapped persons, and all foreign-language translations of the Work prepared for distribution
throughout the world.
Permission is given with the understanding that inclusion of the material will not be used to imply Transportation
Research Board, AASHTO, Federal Highway Administration, Transit Development Corporation, Federal Transit
Administration, Federal Aviation Administration, or Federal Motor Carriers Safety Administration endorsement of
a particular product, method, or practice.
Permission is also provided on condition that appropriate acknowledgment will be given as to the source
material.
115
[Aug 17, 2004 email from Javy Awan, Director of Publications, Transportation Research Board] TRB
references are reproduced with permission of the Transportation Research Board, From Access Management
Manual, Transportation Research Board, National Research Council, Washington, D.C., 2003
116
[Dec 6, 2012 email from Phyllis Barber, Transportation Research Board Publications Office / Javy Awan,
Director of Publications, Transportation Research Board] The Transportation Research Board grants permission
to the Wisconsin DOT to reproduce 4 figures from the TRB Access Management Manual, in a proposed revision
to Chapter 11, Section 25 of Wisconsin DOT’s Facilities Development Manual, as identified in your request of
December 3, 2012, subject to the following conditions:
1. Please credit as follows:
From Access Management Manual, Figure 11-4, p. 207; Figure 11-5, p. 208; Figure 11-7, p. 209; and Figure 11
8, p. 210. Copyright, National Academy of Sciences, Washington, D.C., 2003. Reproduced with permission of the Transportation Research Board.
2. None of this material may be presented to imply endorsement by TRB of a product, method, practice, or policy.
117
[Dec 5, 2012 email and attached letter from Zach Pleasant, Information Services Director, ITE] The Institute of Transportation Engineers grants permission to use Figures 6-19, P6-27 and 6-20, P6-30 from Transportation and Land Development, 2nd Edition for the Wisconsin DOT’s Facilities Development Manual (FDM 11-25).
Please know that this is a one-time, one-use agreement, and any other use of this material or any other resource of ITE must be requested and approved in writing. Please acknowledge our copyright by publishing: © 2012 Institute of Transportation Engineers, 1627 Eye Street, NW, Suite 600, Washington, DC 20006 USA, www.ite.org. Used by permission.
118
See footnote after reference no. 3 above [Dec 3, 2012 email from Ellen Chafee, Editor, CRP-TRB] Page 77
Transportation Research Board of the National Academies, Washington, DC, 2004.
http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rpt_524.pdf.
6. [Ref 442] Eyler, D. Innovative Intersection Designs. (DRAFT PowerPoint presentation for 2009
ACEC/WisDOT Transportation Improvement Conference). SRF Consulting Group, Inc., Plymouth, MN, Feb. 25,
2009.
7. [Ref 719] Florida Intersection Design Guide. Florida DOT, 2007. http://www.dot.state.fl.us/rddesign/FIDG
Manual/FIDG2007.pdf. Accessed 8-6-2010.
8. [Ref 723] MADOT Highway Department Project Development & Design Guide ch. 6: Intersection Design.
Massachusetts Department of Transportation - Highway Division, 2006.
http://www.mhd.state.ma.us/downloads/designGuide/CH_6_a.pdf. Accessed 7-27-2010.
9. [Ref 659] ILDOT Bureau of Design and Environment Manual ch. 36: Intersections. Illinois DOT, 2002.
10. [Ref 239] NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, Albany, NY, 2006.
https://www.nysdot.gov/portal/page/portal/divisions/engineering/design/dqab/hdm/hdm-repository/chapt_05.pdf.
Accessed 8-6-2010.
11. [Ref 228] TRB Committee on Access Management (ed.) (ed.). Access Management Manual.
Transportation Research Board, Washington, DC, 2003.
12. [Ref 647] Stover, V. G. and F. J. Koepke. Transportation and Land Development, 2nd edition. Institute of
Transportation Engineers, Washington, DC, 2006.
13. [Ref 648] Bonneson, J. A. and M. D. Fontaine. NCHRP Report 457: Engineering Study Guide for
Evaluating Intersection Improvements. TRB, National Research Council, Washington, DC, 2001.
14. [Ref 718] Manual on Uniform Traffic Control Devices. Federal Highway Administration, Washington, DC,
2009.
15. [Ref 406] Gattis, J. L. and S. T. Low. Intersection Angle Geometry and the Driver's Field of View. In
Transportation Research Record 1612: Highway Geometric Design Issues. TRB, National Research Council,
Washington, DC, 1998, pp. 10-16.
16. [Ref 510] Staplin, L. K., K. Lococo, S. R. Byington, and Scientex Corporation. Highway Design Handbook
for Older Drivers and Pedestrians. FHWA-RD-01-103. Federal Highway Administration Turner-Fairbank
Highway Research Center, McClean, VA, May, 2001. http://www.tfhrc.gov/humanfac/01103/coverfront.htm.
Accessed 4-2-2009.
17. [Ref 41] Staplin, L., K. Lococo, S. Byington, D. Harkey, and FHWA. Guidelines and Recommendations to
18. [Ref 409] Staplin, L., K. Lococo, and S. Byington. Older Driver Highway Design Handbook [ch.1 Intersections (At-Grade)]. FHWA-RD-97-135. Federal Highway Administration, Washington, DC, Jan., 1998.
19. [Ref 405] Rural Intersections - Turn lanes - Left-Turn Bypass Lanes. In MNDOT Road Design Manual ch. 5:
At-Grade Intersections. Minnesota DOT, 2000, sect. 5-4.01.04, pp. 5-4(2).
http://www.dot.state.mn.us/design/rdm/english/5e.pdf. Accessed 2-3-2009.
20. [Ref 807] Baumann, A. J. Minimum Required Turn Lane Storage Lengths & Tapers For Left & Right Turn
Lanes At Signalized & Non-signalized Intersections. (DRAFT). Wisconsin DOT, Feb. 25, 2010 current revision
(created Dec 2, 2008; previously revised: Nov 4, 2009).
21. [Ref 312] FHWA Resource Center Safety and Design Team. Review of Wisconsin Bypass Road Design
Practices February 13-14, 2006. (Final). Federal Highway Administration Resource Center, Olympia Fields, IL,
2006. http://www.dot.wisconsin.gov/library/publications/docs/wis-bypass-report.pdf. Accessed 4-2-2009.
22. [Ref 368] Rationale for Median Type Recommendations. Kentucky Transportation Cabinet, 2008.
http://www.planning.kytc.ky.gov/congestion/medians/Median%20Type%20Guidelines.pdf. Accessed 5-3-2012.
23. [Ref 804] A Policy on Geometric Design of Highways and Streets 2011, 6th edition. AASHTO, Washington,
DC, 2011. www.transportation.org.
24. [Ref 624] Highway Capacity Manual 2010, 5th edition. Transportation Research Board of the National
Academies, Washington, DC, 2010.
Page 78
25. [Ref 808] Revello, B. Two Way Left Turn Lane (TWLTL). Wisconsin DOT, Nov. 26, 2007.
26. [Ref 815] Geometric Design of Lanes - Continuous Two-Way Left-Turn Lanes (TWLTLs). In IADOT Design
Manual ch. 6: Geometric Design. Iowa DOT, 2001, sect. 6C-6, pp. 1-4.
http://www.iowadot.gov/design/dmanual/06c-06.pdf. Accessed 5-3-2012.
27. [Ref 814] Facility Selection / Two - Way Left - Turn Lanes. In MODOT Engineering Policy Guide ch. 200:
Geometrics. Missouri DOT, 2012, sect. 232.3. http://epg.modot.org/index.php?title=232.3_Two_-_Way_Left_
_Turn_Lanes. Accessed 5-3-2012.
28. [Ref 825] Ackeret, K. W. Criteria for the Geometric Design of Triple Left-Turn Lanes. ITE Journal, vol. 64,
no. 12, 1994 December, pp. 27-33. http://turnlanes.net/files/criteria_for_the_geometric_design_of_triple_left
turn_lanes.pdf. Accessed 7-24-2012.
29. [Ref 824] Rodegerdts, L. A., B. Nevers, B. Robinson, J. Ringert, P. Koonce, J. Bansen, T. Nguyen, J.
McGill, D. Stewart, J. Suggett, T. Neuman, N. Antonucci, K. Hardy, and K. Courage. Individual Movement
Treatments / Multiple Left-Turn Lanes. In FHWA-HRT-04-091: Signalized Intersections: Informational Guide.
Federal Highway Administration Turner-Fairbank Research Center, McLean, VA, 2004, ch. 12.1.2, pp. 318-319.
http://www.tfhrc.gov/safety/pubs/04091/04091.pdf. Accessed 8-6-2007.
30. [Ref 827] Courage, K., A. Gan, B. Stephens, M. Willis, and University of Florida Transportation Research
Center. Triple Left Turn Lanes At Signalized Intersections. BC131 (FINAL). Florida DOT, Tallahassee, FL, Dec.,
2002. http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_TE/FDOT_BC131rpt.pdf. Accessed
7-30-2012.
31. [Ref 828] NYSDOT Highway Design Manual ch. 5: Basic Design. New York State DOT, Albany, NY, 2011.
https://www.dot.ny.gov/divisions/engineering/design/dqab/hdm/hdm-repository/chapt_5_final.pdf. Accessed 7
30-2012.
32. [Ref 339] OHDOT Location & Design Manual, Vol.1, Roadway Design ch.400: Intersection Design. Ohio
DOT, 2006. http://www.dot.state.oh.us/roadwayengineering/standards/Publications/LDM/2006-07
21/400_jul06.pdf.
33. [Ref 823] Cooner, S. A., S. E. Ranft, Y. K. Rathod, Y. Qi, L. Yu, Y. Wang, and S. Chen. Development of
Guidelines for Triple Left and Dual Right-Turn Lanes: Technical Report. FHWA/TX-11/0-6112-1. Texas
Transportation Institute, Texas A&M University, College Station, TX, Jan., 2011 Published July 2011.
34. [Ref 833] Yi, Q., C. Xiaoming, D. Li, and Center for Transportation Training and Research - Texas Southern
University. Development of Warrants for Installation of Dual Right-Turn Lanes at Signalized Intersections.
SWUTC/12/161141-1. Texas Transportation Institute, Texas A&M University, College Station, TX, Apr., 2012.
http://d2dtl5nnlpfr0r.cloudfront.net/swutc.tamu.edu/publications/technicalreports/161141-1.pdf. Accessed 1-9
2013.
35. [Ref 829] Weiner, R., and Nelson\Nygaard Consulting Associates Inc. Toolkit for the Assessment of Bus
Stop Accessibility and Safety. Easter Seals Project ACTION, Washington, DC, 2006.
http://www.transitaccessproject.org/InternalDocs/TransitFacilities/06BSTK_Complete_Toolkit.pdf;http://www.nel
sonnygaard.com/Documents/Articles/Complete_Toolkit-new.pdf. Accessed 8-16-2012.
36. [Ref 495] Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management
Techniques. TRB, National Research Council, Washington, DC, 1999.
http://www.accessmanagement.info/pdf/420NCHRP.pdf.
Page 79
Chapter 11
Section 26
FDM 11-26-1 General
Design
Roundabouts
March 4, 2013
1.1 General
This section and its sub-sections are comprised of roundabout design guidelines developed through research and
experience. Much of the prescribed guidance has been proven through application, evaluation and refinement - a
truly continuous improvement process.
The Department has updated previous versions of this guide to account for changes in national roundabout
guidelines made possible through research, namely NCHRP 572 - Roundabouts in the United States, 2006 and
NCHRP 672, Roundabouts: An Informational Guide, Second Edition. The NCHRP guidelines and research are
heavily relied upon in this chapter. Where appropriate and justified by local experience, exceptions for use by
the Wisconsin Department of Transportation are noted. Where both references are cited but differences exist,
the Facilities Development Manual guidance shall govern.
The modern roundabout is a subset of many types of circular intersections. The term modern roundabout and
roundabout are used interchangeably throughout this document. The roundabout is a one-way circular
intersection where circulating traffic is given priority over entering traffic and where entry speeds are low relative
to older unconventional circular intersections. The term “modern roundabout” is used in the United States to
differentiate roundabouts from the older and often large diameter non-conforming traffic circles, rotaries or very
small traffic calming circles used on residential streets.
Traffic circles fell out of favor in this country by the mid 1950’s because they encountered safety and operational
problems as traffic volumes increased beyond their operational thresholds. However, substantial progress has
been achieved in the subsequent design of circular intersections, and the modern roundabout should not be
confused with the traffic circles of the past.
Roundabouts may be considered for a wide range of intersection types including but not limited to freeway
interchange ramp terminals, state route intersections, and state route/local route intersections. Roundabouts
generally process high volume left turns more efficiently than all-way stop control or traffic signals, and will
process a wide range of side road volumes. Roundabouts can improve safety by simplifying traffic movements,
reducing vehicle speeds, and providing a clearer indication of the driver’s right of way compared to other forms
of intersection control. The required intersection sight distance is greatly reduced from what is required for a
signalized intersection due to the reduced intersection speeds.
The modern roundabout is defined by three basic principles:
1. Yield-at-Entry - Vehicles approaching the roundabout must wait for a gap in the circulating flow, or
yield, before entering the circle.
2. Deflection - Traffic entering the roundabout is directed or channeled to the right with a curved entry
path into the circulating roadway.
3. Geometric Curvature - The radius of the circular road and the angles of entry are designed to slow the
speed of vehicles.
The following is a list of locations where a roundabout may be feasible:
1. Intersections with a high-crash rate or a higher severity of crashes
2. High-speed rural intersections
3. Freeway ramp terminals
4. Transitions in functional class or desired speed change (including rural to urban transitions)
5. Existing intersections that are failing
6. Alternative intersection types are expensive
7. Aesthetics is an objective
8. Intersections of dissimilar functional class (arterial-arterial, arterial-collector, arterial-local, collectorcollector, collector-access)
9. Four-leg intersections with entering volumes less than 5,000 vph or approximately 50,000 ADT
10. Three-leg intersections of any volume
11. Intersection of two signalized progressive corridors where turn proportions are heavy (random arrival
is better than off-cycle arrival)
12. Closely spaced intersections where signal progression cannot be achieved
Page 1
FDM 11-26 Roundabouts
13. Locations where future access will be added to the intersection
14. Replacement of all-way stops
15. Intersections near schools
16. Other intersections where safety is a major concern, such as HSIP Funds
FHWA and AASHTO have made intersection safety a high priority. The objective is to improve the safety and
operation of intersections. When compared to signalized intersections, studies by the Insurance Institute for
Highway Safety [1] show that roundabouts typically reduce overall delay and congestion, increase capacity, and
improve safety. For example, right-angle collisions are a prominent cause of death at signalized intersections.
Studies by the Insurance Institute for Highway Safety show that signalized intersections converted to
roundabouts experienced on average: 75% fewer injury crashes, 90% fewer fatality crashes, and fewer crashes
overall.
While roundabouts are still fairly new in Wisconsin, their safety benefits have been studied with equally
encouraging results. In a study of roundabout collision history, prepared by the University of Wisconsin Traffic
Operations and Safety Laboratory [4], local researchers analyzed 24 roundabouts that were built in Wisconsin in
2007 or before. Three years of before and after crash data were gathered as well as geometric and volume
data. An Empirical Bayes (E-B) analysis was used to examine the safety benefits for total crashes and injury
crashes. A simple before-and-after crash analysis was also completed to analyze specific types of injury
crashes for each roundabout. The E-B analysis was performed using Safety Performance Functions (SPFs)
from both the Highway Safety Manual (HSM) and Wisconsin specific data. The results from both values were
very similar adding strength to the numbers. Using the HSM SPFs, researchers found mixed results for total
crash frequency but a significant decrease in crash severity. Nationally, 35% reduction was observed for all
crashes as noted in NCHRP Report 572 while Wisconsin roundabouts showed a 9% decrease across the 24
roundabouts. Wisconsin roundabouts had a decrease of 52% for fatal and injury crashes. Roundabouts
nationwide are also experiencing a significant decrease in severe crashes.
When looking at predictor variables, the speed limit of the approaches did not show a significant impact on the
safety of the roundabout. While multilane roundabouts seem to be safer than single lane roundabouts when
looking at fatal and injury crashes, single lane roundabouts saw a larger decrease in total crashes. Two-way
stop controlled conversions had the highest safety benefit as compared to All-way stop controlled and
signalized.
According to FHWA, some or all of the following safety benefits can be realized with proper roundabout design
and implementation:
- Provide more time for entering drivers to judge, adjust speed for, and enter a gap in circulating traffic,
allowing for safer merges
- Reduce the size of sight triangles needed for users to see one another
- Increase the likelihood of drivers yielding to pedestrians (compared to an uncontrolled crossing)
- Provide more time for all users to detect and correct for their mistakes or mistakes of others
- Make crashes less frequent and less severe, including crashes involving pedestrians and bicyclists
- Make the intersection safer for novice users
Critical to the acceptance of the roundabout intersection concept is overcoming the internal and external
skepticism of its advantages and value compared to stop controlled or signalized intersections. Meet with local
officials and adjoining property owners early in the process to address potential political or economic impacts.
Designers should also coordinate presentation materials with region staff as well as the Bureau of Project
Development in an effort to present a consistent unified approach for roundabout implementation throughout the
State.
1.2 Modern Roundabout vs. Other Circular Intersections
On the surface, modern roundabouts, old traffic circles and rotaries look similar; however, there are subtle
differences that distinguish the two intersection concepts. The fundamental difference is their differing design
philosophies. Modern roundabouts control and maintain low speeds for entering and circulating traffic. This is
achieved by small diameters and low-speed entry geometry. By contrast, traffic circle geometry encourages
high-speed merging and weaving, made possible by larger diameters and large high-speed entry radii. Modern
roundabouts control vehicle speed by geometric design elements that allow only slow speeds therefore creating
safer driving conditions. The common characteristics distinguishing a modern roundabout from a traffic circle or
a rotary type intersection are summarized in Table 1.1.
Page 2
Table 1.1 Distinguishing Characteristics of Modern Roundabouts
Feature
Modern Roundabout
Traffic Circle or Rotary
Control at Entry
Yield at all entries. The circulatory roadway
has no control
Stop, signal, or give priority to entering
vehicle. Circulating vehicles yield to
entering traffic.
Operational
Characteristics
Vehicles are sorted by destination at the
approach. Weaving within the circulatory
roadway is minimized. Using proper lane
line markings, lane changes are strongly
discouraged in the circulatory roadway.
Weaving is unavoidable and weaving
sections are provided to accommodate
conflicting movements
Deflection
Large entry angle helps to create entry
deflection to control speed through the
roundabout
Entry angle likely to be reduced to allow
higher speed at entry
Speed
Maintain relatively low circulating speeds
(<25 mph)
Higher circulating speeds allowed (>25
mph)
Circle Diameter
Smaller diameters improve safety
Larger diameters allowed. Small diameter
circle sometimes used for traffic calming
Pedestrian Crossing
No pedestrian activity on central island
Some large traffic circles allow pedestrian
crossing to and from the central island
Splitter Island
Required
Optional
Parking
No parking on the circulatory roadway or in
close proximity of the yield line
On large traffic circles, occasional parking
permitted within circulating roadway
A roundabout can provide a possible solution for locations that experience high crash rates or crash trends by
reducing the number of conflict points where the paths of opposing vehicles intersect. For example, over half of
the crashes at conventional intersections occur when a driver either; misjudges the distance or speed of
approaching vehicles while making a left turn, or violates a red light or stop sign resulting in a right angle
collision. Such crashes would be eliminated with a roundabout, where left turns and crossing movements are
prohibited. Furthermore, collisions at roundabouts involve low speeds and low angles of impact, and therefore,
are less likely to result in serious injury for all road users. Crash evaluation is an important process to complete
for any intersection improvement alternative. Crash evaluation will consist of reviewing individual crash records
and will typically include factors such as location, date, type of crash, time of day, age of driver, weather
conditions, severity of crash, and other important information to assess the problem(s), patterns and potential
improvement need.
When considering methods to increase the capacity of an intersection, a roundabout can be an alternative to
stop or signal controlled intersections. With conventional signal controls, only alternating streams of vehicles are
permitted to proceed through an intersection at one time, which means a loss of capacity when the intersection
clears between phases. In contrast, the only restriction on entering a roundabout is the availability of a gap in
the circulating flow. The reduced speeds within the roundabout will allow the approaching driver to safely select
a gap that is relatively small. By allowing vehicles to enter simultaneously from multiple approaches using short
headways, a possible advantage in capacity can be achieved with a roundabout. This advantage becomes more
prominent when the volumes of left or right turning movements are relatively high.
By constructing a pair of roundabouts at ramp terminal intersections, capacity improvements to the interchange
can be accomplished without the cost of widening the structure to carry additional lanes over or under a
freeway, or expressway (see FDM 11-30-1 and NCHRP Report 672, Chapter 6.10 for more information on
interchanges).
Roundabouts can produce operational improvements in locations where the space available for queuing is
limited. Roadways are often widened to create storage for vehicles waiting at red lights, but the reduced delays
and continuous flows at roundabouts allow the use of fewer lanes between intersections. One possible
application can be found at diamond interchanges, where high left turn volumes can cause signals to fail.
Conventional forms of traffic control are often less efficient at intersections with a difficult skew angle, significant
offset, odd number of approaches, or close spacing to other intersections. Roundabouts may be a good fit for
such intersections, because they do not require signal phasing. The ability of a roundabout to accommodate
high turning volumes, make them especially effective at “Y” or “T” junctions. Roundabouts may also be useful in
eliminating a pair of closely spaced intersections by combining them to form a multi-legged roundabout.
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Intersection sight distance for roundabouts is about half what it is for other intersection treatments because of reduced intersection speeds.
Another possible application is where access is controlled with raised medians. Roundabouts would facilitate left turns and U-turns to access properties on the opposite side of the highway. 1.3 Advantages and Disadvantages
Table 1.2 lists advantages and disadvantages of roundabouts versus other intersection alternatives.
Table 1.2 Advantages and Disadvantages of Roundabouts vs. Other Alternatives.
Category
Safety
Advantages
Reduced number of conflict points compared to
other non-circular intersections. Left-turn
conflicts are removed.
Elimination of high angles of conflict and high
operational speeds; fewer and less severe
accidents.
Reduction in conflicting speeds passing
through the intersection.
Reduced decision making at point of entry.
Long splitter islands and other geometric
features provide good advanced warning of the
intersection.
Disadvantages
Crashes may temporarily increase due to improper
driver education.
During emergencies, signalized intersections can
preempt control.
Multilane roundabouts present more difficulties for
pedestrians with blindness or low vision due to
challenges in detecting gaps and determining that
vehicles have yielded at crosswalks.
May reduce the number of available gaps for midblock
unsignalized intersections and driveways
Raised level of consciousness for drivers.
Facilitate U-turns that can substitute for more
difficult midblock left turns.
Operations
Traffic yields, nonstop, continuous traffic flow.
Generally higher capacities experienced.
Can reduce the number of lanes required
between intersections, including bridges
between interchange ramp terminals.
During off-peak hours, signal timing can create
undue delay at signalized intersections.
Cost
Pedestrians &
Bicyclists
No maintenance of signals (heads, loop
detectors, controllers).
Coordinated signal systems can increase capacity of
the network.
As queues develop, drivers accept smaller gaps, which
may increase crashes.
Equal priority for all approaches can reduce the
progression for high volume approaches.
Cannot provide explicit priority to specific users (e.g.,
trains, emergency vehicles, transit, pedestrians) unless
supplemental traffic control devices are provided.
Central island landscaping maintenance.
Illumination cost.
Lower accident rate and severity; reduced
accident costs.
May have significant right of way impacts
Splitter islands provide pedestrian refuge and
shorter one-directional traffic crossing.
Pedestrians only need to consider one direction
of traffic at a time.
Pedestrians, especially handicapped may experience
increased delay in securing acceptable gaps to cross.
Pedestrians with vision impairments may have the most
trouble establishing safe opportunities to cross.
Low speed conditions improve bicycle and
pedestrian safety.
Longer travel path.
Bicycle ramps could be confused for pedestrian ramps.
Depending on their skills and level of comfort,
bicyclists have the option to take a lane to
negotiate through a roundabout.
Environmental
Reduced starts and stops; reduced air
pollution.
Possible impacts to natural and cultural resources due
to potentially greater spatial requirements at the
intersection.
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Category
OSOW Freight
Network
Aesthetics
Advantages
Disadvantages
Reduction of potential obstacles at
intersections (traffic signals, signing, median
islands).
The geometric design may be challenging to allow the
navigation of OSOW vehicles.
Provide attractive entries or centerpieces to
communities.
May create a safety hazard if hard objects are placed in
the central island directly facing the entries.
Additional right-of-way and paved areas may be
needed to accommodate OSOW vehicles.
Used in tourist or shopping areas to separate
commercial uses from residential areas.
Provide opportunity for landscaping and/or
gateway to enhance the community.
1.4 Defining Physical Features
The defining features of a roundabout are shown in, Figure 1.1, Figure 1.2 and described in Table 1.3.
Figure 1.1 Single-lane Roundabout Features
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Figure 1.2 Multilane Roundabout Features
Page 6
Table 1.3 Roundabout Features
Feature
Description
Central island
The raised area in the center of a roundabout around which traffic circulates. The central island
does not necessarily need to be circular in shape.
Splitter island
A raised curb island (special situations may be painted) area on an approach used to separate
entering from exiting traffic, deflect and slow entering traffic, and to provide refuge for
pedestrians crossing the road in two stages.
Circulatory roadway
(counter clockwise
circulation)
The curved path used by vehicles to travel in a counterclockwise fashion around the central
island. The width of the circulatory roadway is typically 1.0 to 1.2 times the width of the widest
entry width.
Truck Apron
The traversable portion of the central island adjacent to the circulatory roadway. It is required to
accommodate snow plows and the wheel off-tracking of large trucks, and OSOW vehicles. It is
usually paved with a contrasting color (red) to delineate the apron from the normal vehicle path.
Yield Line
A point of demarcation separating traffic approaching the roundabout from the traffic already in
the circulating roadway. The yield point is usually defined by a thick, (typically 18-inch wide),
dotted edge line pavement marking.
Accessible pedestrian
crossings
Provide accessible pedestrian crossings at all roundabouts. The crossing location is set back
from the yield line, typically one car length. The splitter island is cut to allow pedestrians,
wheelchairs, strollers, and bicycles to pass through.
Bicycle treatments
Bicycle treatments at roundabouts provide bicyclists the option of traveling through the
roundabout either by riding in the travel lane as a vehicle, or by exiting the roadway and using
the crosswalk as a pedestrian, or as a cyclist using the shared-use path, depending on the
bicyclist’s level of comfort. Bicycle exit ramps should generally leave the roadway within a 25 to
35 degree angle range. Bicycle entrance ramps should generally enter the roadway within a 25
to 35 degree angle range. The entrance and exit ramps should be located approximately 50-150
feet from the circulating traffic to allow the bicyclist an opportunity to transition onto a path away
from the circulatory roadway.
Landscaping buffer
Landscaping buffers are provided at most roundabouts to separate vehicular and pedestrian
traffic and to encourage pedestrians to cross only at the designated crossing locations.
Landscaping buffers can also significantly improve the aesthetics of the intersection as long as
they are placed outside the required sight limits.
Shared-use path
Pathway for pedestrians to walk. In the urban environment it is common to provide a shared-use
path at the perimeter of the roundabout to accommodate pedestrians and bicyclists.
1.5 Roundabout Categories
Roundabouts are categorized by size and environment. The following is a list of basic categories explained in
FHWA, Roundabouts: An Informational Guide [3]. (FHWA Roundabout Guide) There may be situations where
categories are not applicable. The planning process and final design methodologies for roundabouts are to be
based on “principles” versus strict rules or one-size fits all standards. For example there are no categories for
transitional areas and the final design will depend on various factors.
1.5.1 Single-Lane Roundabout
1.5.1.1 Urban Single-Lane Roundabouts
This type of roundabout is characterized as having a single-lane entry at all legs and one circulatory lane. The
roundabout design is focused on achieving consistent entering and circulating vehicle speeds. The geometric
design includes raised splitter islands, a non-traversable central island, and may include an apron surrounding
the non-traversable part of the central island to accommodate long trucks. The minimum inscribed diameter to
accommodate a WB-65 is 120 feet. Where long trucks are anticipated, verify that the circulating roadway width
and the truck apron can accommodate off-tracking of a WB-65 design vehicle. A truck apron is included to allow
the semi tractor to stay in the circulating roadway while the trailer off-tracks onto the apron. If the roundabout is
located on the OSOW Freight Network, verify that the roundabout geometry, splitter islands, truck apron, and
off-tracking can accommodate the appropriate OSOW check vehicle.
1.5.1.2 Rural Single-Lane Roundabouts
Rural single-lane roundabouts generally have high speeds on the approach roadway in the range of 45 to 55
Page 7
mph. They require supplementary geometric and traffic control device treatments on the approach roadway to
encourage drivers to slow to an appropriate speed before entering the roundabout. Such treatments include
raised and extended splitter islands, a non-traversable central island, and adequate horizontal deflection. Rural
roundabouts may have larger diameters than urban roundabouts which may allow slightly higher speeds at the
entries, on the circulatory roadway, and at the exits. This is permissible if few pedestrians are expected at these
intersections, currently and in the future.
Rural roundabouts which may one day become part of an urbanized area should be designed as urban
roundabouts, with slower speeds and pedestrian accommodations. In the interim, design them with
supplementary approach and entry features to achieve safe speed reduction.
1.5.2 Multilane Roundabouts
1.5.2.1 Urban Multilane Roundabouts
Urban multilane roundabouts are roundabouts in urban areas that have at least one approach leg with two or
more entry lanes. These require wider circulatory roadways to accommodate more than one vehicle traveling
side by side. Again, it is important that the vehicular speeds be consistent throughout the roundabout. The
geometric design includes raised splitter islands, a non-traversable central island, and appropriate horizontal
deflection, and may include an apron surrounding the non-traversable part of the central island to accommodate
long trucks. A truck apron should be included to allow the semi tractor to stay in the inner lane and the trailer to
off-track onto the apron. When long trucks are anticipated, or if the roundabout is located on the OSOW Freight
Network, verify that the roundabout geometry, splitter islands, truck apron, and off-tracking can accommodate
the appropriate OSOW check vehicle.
1.5.2.2 Rural Multilane Roundabouts
Rural multilane roundabouts have speed characteristics similar to rural single-lane roundabouts with approach
speeds in the range of 45 to 55 mph. They differ in having two or more entry lanes, or entries flared from one or
more lanes, on one or more approaches. Consequently, many of the characteristics and design features of rural
multilane roundabouts mirror those of their urban counterparts. The main design differences are designs with
higher entry speeds, larger diameters, and recommended supplementary approach treatments. Design rural
roundabouts that may one day become part of an urbanized area for slower speeds, with design details that fully
accommodate pedestrians and bicyclists. In the interim, design them with approach and entry features to
achieve safe speed reduction. A truck apron should be included to allow the semi tractor to stay in the inner lane
and the trailer to off-track onto the apron. When long trucks are anticipated, or if the roundabout is located on
the OSOW Freight Network, verify that the roundabout geometry, splitter islands, truck apron, and off-tracking
can accommodate the appropriate OSOW check vehicle.
1.5.3 Combination Roundabouts
Combination roundabouts are roundabouts that combine single and multilane entries. This combination usually
occurs when roads of different approach volumes intersect roads of two different classifications; a State Trunk
Highway (STH) with a local road. These roundabouts are commonly found in suburbanized locations, but can
also be found in rural locations.
1.6 Through Highway Declaration (ss 340.01(67) and 349.07)
By statutory authority, a signal, roundabout, or stop sign installation on a STH requires an approval process.
Guidance on “Through Highway Declarations” is provided in the Traffic Guidelines Manual (TGM) 13-1. This
requirement applies to new or modified traffic control installations on a STH. Regardless of the type of traffic
control proposed, associated “through highway declarations” need to be developed and are maintained by the
Regional Traffic staff.
1.7 Speed Zone Declarations (ss 346.57 and 349.11)
Also by statutory authority, speed zone declarations are required when the traffic on a STH is required to reduce
speed as a result of a regulatory speed sign installation. Guidance on “Speed Limits” is provided in the TGM 13
5. If speed reductions are required in advance of an intersection traffic control device, develop a declaration
based on an engineering study coordinated with Region Traffic staff.
1.8 References
[1] Insurance Institute for Highway Safety publications, May 13, 2000; July 28, 2001; November 19, 2005;
www.iihs.org
[2] Insurance Institute for Highway Safety publications, March, 2001; July 17, 2011;
www.iihs.org/research/qanda/roundabouts.html
[3] NCHRP 672, Roundabouts: An Informational Guide, Second Edition, December 2010,
http://www.trb.org/Publications/Blurbs/164470.aspx
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[4] Comprehensive Evaluation of Wisconsin Roundabouts, Volume 2: Traffic Safety, Wisconsin Traffic
Operations and Safety (TOPS) Laboratory, Department of Civil and Environmental Engineering, University of
Wisconsin-Madison, September 2011
FDM 11-26-5 Design Process and Qualifications
March 4, 2013
5.1 Roundabout Design Process and Qualifications
Due to modern roundabouts’ status as a relatively new and unique design form as well as the inherent
complexity of their geometric and operational aspects, WisDOT has developed a roundabout design process
which requires a qualified designer participate in each roundabout design.
This section describes the 3-stage design process and the critical design elements. A qualified designer must be
involved with each stage of the process. In addition, this procedure describes the various roles the qualified
designer may take in completing a roundabout design.
5.2 Roundabout Designer Requirements
A qualified designer must meet the skills, knowledge and experience level determined appropriate by the
Wisconsin Department of Transportation for roundabout design. A list of qualified designers for each of the
following 3 levels of roundabout complexity is available from the Division of Transportation Systems
Development, Bureau of Project Development.
1. Level 1 Roundabout - The design complexity at this level is limited to roundabouts where all legs (not
to exceed 4 legs) are single lane entries without bypass lanes. A Level 1 designer must have an
understanding of roundabout design with high confidence in designing truck aprons, developing a
design with appropriate values for the six geometric parameters, design for appropriate fastest speed
paths, design for truck turning paths, have the ability to properly assess the basic capacity
requirements of single lane roundabouts from traffic turning movements using the approved analysis
software per FDM 11-26-20. The Level 1 qualified designer shall inform the Region when the
roundabout design exceeds the complexity stated above for a Level 1.
2. Level 2 Roundabout - The design complexity at this level is limited to roundabouts where legs are dual
lane entries or less and may have bypass lanes. A Level 2 designer must be proficient in roundabout
design with ability to design truck aprons, developing a design with appropriate values for the six
geometric parameters, design for appropriate fastest speed paths, design for truck turning paths, and
develop special signing and pavement marking needs. The designer will have the ability to properly
run the approved capacity analysis software (see FDM 11-26-20) evaluate alternative lane
configurations and output from the software program. The Level 2 qualified designer shall inform the
Region when the roundabout design exceeds the complexity stated for a Level 2. See discussion
below about dual lane roundabouts in close proximity and the potential for Level 3 involvement.
3. Level 3 Roundabout - The design complexity at this level involves all roundabout designs to include 3
or 4-lane entries, or has closely spaced roundabouts where the operations of one may have an impact
on the operations, signing and/or marking of another. See discussion below about dual lane
roundabouts in close proximity and the potential for Level 3 involvement. A Level 3 designer must
have the skills and knowledge for the most complex roundabout designs.
The Region will use the best traffic data available to select the appropriate qualified designer (Level 1, 2, or 3).
This is typically determined prior to project solicitation by the Project Development Section. The project team will select either a Level 2 or 3 qualified designer if the Region anticipates that the project will include a dual lane roundabout. There are certain situations when it is desirable for the Region to involve a Level 3 qualified design on dual lane roundabout projects. Some examples include situations where:
- There are other multilane roundabouts in close proximity
- Lane assignment and/or lane continuity is difficult to achieve without adding another lane
- Reduction in weaving between roundabouts is desired
- Queue backup into an adjacent multilane roundabout is possible
- Other special needs that have been identified
The Region will discuss the involvement of a Level 3 qualified designer for dual lane roundabout projects to determine if expertise is needed beyond that provided by a Level 2 qualified designer.
WisDOT Regions, consultants, local agencies such as a counties, townships, municipalities, and developers, etc. shall have a qualified designer on staff, or contract with an approved designer, to provide the required sign-
off on the Critical Design Parameters document for roundabout designs, as described below, for both WisDOT and WisDOT oversight projects.
Qualified designers may participate in different ways in order to provide the required sign-off on the Critical
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Design Parameters document.
1. Independently complete the roundabout design. When a WisDOT Region, consultant, local agency
such as a county, township, municipality etc. or a developer has a roundabout on a project they must
have a qualified designer to oversee or complete all aspects of the plans, specifications and estimate
(PS & E) package for the roundabout according to the 3-Stage Design Process described below.
2. Assist and mentor the project team in their completion of the roundabout design. A WisDOT Region,
consultant or local agency such as a county, township, municipality etc. or developer has a
roundabout on the project may prefer to contract for assistance or mentoring from a qualified designer
in the plans preparation process. The qualified designer must directly assist the project team
addressing the critical design elements in the 3-Stage Design Process described below.
3. Independently review the roundabout design prepared by a project team. A WisDOT Region,
consultant, local agency such as a county, township, municipality etc. or developer has a roundabout
on the project and the design is prepared without any assistance from a qualified designer. The
roundabout designer is responsible to contract with one of the qualified designers to review the critical
elements of the design at each stage of the 3-Stage Design Process described below. The information
to be provided to the qualified designer at each stage of plans complete is provided below.
Coordinate the proposed roundabout design with a qualified designer early in the design process. It is better to
allow the qualified designer to be proactive and in a position to suggest modifications rather than to be reactive
and lose design options because the design or commitments on the project are too far along.
The qualified designer’s review comments shall be submitted to the project team and the WisDOT Region at
each Stage. The critical design recommendations from the qualified designer should be identified clearly so the
roundabout design team knows what to modify on the plans. Less critical comments will likely improve the
design more toward optimal and should not be taken lightly. A discussion between the qualified designer, design
team, and Region may be needed to properly address recommendations in the plans or document the dismissal
of the comment(s).
The qualified designer in consultation with WisDOT will determine which elements of the design are critical in
the situation where a dispute may take place. Department personnel are responsible to ensure that the qualified
designer recommendations and comments are properly addressed by the design team.
5.3 Intersection Control Evaluation, Program Level Scoping Phase
For an explanation of the required level of analysis see FDM 11-25-3. The Program Level Scoping phase
typically does not yield the final determination on the selected intersection control. However, there are early
screening criteria some of which are identified in FDM 11-25-3 and typically evaluated during the Program Level
Scoping phase that may eliminate the roundabout from further consideration.
A qualified designer is not required for the Program Level Scoping phase of an Intersection Control Evaluation.
5.4 The 3-Stage Roundabout Design Process
The following information, including Figure 5.1, describes each of the stages of development where it is critical
to have a qualified designer involved in the roundabout design. There may be a project schedule delay or
adverse cost ramifications associated with a roundabout design if each stage of the evaluation is not followed in
sequence.
Page 10
Figure 5.1 WisDOT 3-Stage Design Process
5.4.1 Stage 1, Roundabout Design Process
Prior to 30% plans complete. While the desired type of intersection control may still be undetermined; the
roundabout has been identified as one of the viable alternatives from the Program Level Scoping phase.
Complete Stage 1, requires qualified designer involvement, prior to the 30% plans complete level so the
comments and design adjustments are incorporated and ready with the typical 30% plan review
discussion/meeting conducted by the region. For designs prepared outside the Region, submit Stage 1 plans to
the Region in .dgn format. Generally, it is preferred to have the roundabout design developed far enough to
have an idea of right-of-way needs, raised median locations identified, access, major utilities and other potential
impacts prior to a Public Informational Meeting (PIM) so relatively accurate information can be presented and
discussed with property owners to include Level of Service (LOS), or delay, comparisons with other intersection
control alternatives. It is advisable to include a roundabout expert or other highly experienced roundabout
designer at the initial PIM. At the very least, they should be consulted in the planning process for the initial PIM.
Initial project acceptance and understanding by the project stakeholders and users is key for a smooth project
development process. There may be situations where the design is accurate and detailed enough showing the
proper size and location of the roundabout, LOS, extent of the splitter island curb locations and type of access
along the roadway that a more detailed design could be completed after the PIM.
This is a list of critical elements of design that the qualified designer needs to address at this stage of plans
complete.
1. Determine optimum location of circle with inscribed diameter.
2. Use Traffic Flow Worksheet, FDM 11-26, Attachment 20.1. Completed with existing volumes, design
year volumes for AM and PM peak and midday if a tourist area that may have higher mid-day than AM
or PM peaks.
3. Establish lane configuration(s) and analyze the existing and forecasted traffic turning movements
using the approved analysis software per FDM 11-26-20
4. Complete lane markings and pavement arrows for multilane only.
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5. Complete a highly developed design that shows face of curb locations, crosswalks, splitter islands,
shared-use path, bike ramps, truck apron etc. with appropriate widths.
6. Verify design vehicle movement and required check vehicles (WB-65 minimum, WB-92 (formerly WB
67 Long), farm combine and an 80 foot mobile home on the STH system). (See also the OSOW
vehicle inventory FDM 11-25-1 and route network).
7. Show the fast path with speed calculations for R1 thru R5.
8. Fill out FDM 11-26-5,doc1.
9. Prepare preliminary stopping sight distance for - approach, circulatory roadway, crosswalk and exit,
and the intersection sight distance.
10. Prepare preliminary centerline profile of circulatory and approach roadway.
11. Prepare preliminary typical sections on the mainline roadway.
Prior to 60% plans complete. Complete design revisions recommended by the qualified designer from the
previous 30% design. At this stage a qualified designer is required to complete the design/review of the critical
design elements identified below. Prepare the plans such that the environmental documents may be completed,
DSR approved and plat work may begin. Complete Stage 2, including all qualified designer involvement prior to
the 60% plans complete level so the review comments and design adjustments are incorporated and ready for
the Region in preparing for the typical 60% plan review discussion/meeting. For designs prepared outside the
Region, submit Stage 2 plans to the Region in .dgn format. At this stage the qualified designer shall sign the
Critical Design Parameters document (Attachment 5.1) for attachment to the DSR. One of the primary critical
elements of design at this stage is the vertical control with each leg having vertical profiles, circulating roadway
profile, crown location, slope intercepts, central island grading, drainage consideration with inlet locations, and
spot elevations.
complete.
1. Finalize horizontal design changes implemented
2. Establish roadway profiles on each leg
3. Establish circulating roadway profile
4. Show crown location, cross slopes, spot elevations
6. Consider central island grading design
7. Consider drainage design/inlet locations
8. Show preliminary light standard locations
9. Identify the need for large green and white guide signs, overhead guide signs, or other non-standard
installations
10. Finalize lane pavement marking and lane assignment pavement marking for multilane roundabouts
11. Identify major utility conflicts (i.e. utility conflicts that may result in relocating the circle)
12. Prepare preliminary typical sections
13. Consider preliminary construction staging layout and identify potential staging conflicts, such as
access control, large grade differences between stages, etc. that may impact the design
Prior to 90% plans complete. Finalize the vertical, drainage, pavement marking, signing, lighting, landscaping
plans, work zone traffic control, and utility coordination. In preparation for PS & E complete Stage 3, including all
qualified designer involvement, prior to the 90% plans complete level so the review comments and design
adjustments are incorporated and ready for the region in preparing for the typical 90% plan review
discussion/meeting. This is the final design with construction staging or detour plan.
complete.
1. Complete final plan and profile with any vertical and horizontal control details included for field layout
2. Prepare final signing and pavement marking plan
3. Prepare final landscaping and lighting plan (refer to TGM 11-11-1 for lighting policy)
4. Prepare final construction staging plan.
LIST OF ATTACHMENTS
Attachment 5.1
Roundabout Critical Design Parameters Document
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FDM 11-26-10 User Considerations
March 4, 2013
10.1 Pedestrian and Bicyclist Accommodations
Accommodating non-motorized users is a Department priority. Therefore, give special consideration to locations
where:
- Pedestrian volumes are high
- There is a presence of young, elderly or visually impaired citizens wanting to cross the road
- Pedestrians are experiencing particular difficulty in crossing and being delayed excessively
Also, consider the adjacent land use near the roundabout location, such as schools, playgrounds, hospitals, and
residential neighborhoods. These sites may warrant additional treatments as presented below. Prior to
determining whether bicycles and/or pedestrian concerns will be a factor in the design of the roundabout, the
designer is strongly encouraged to contact the Region or State Bicycle and Pedestrian Coordinator for their
guidance.
10.1.1 Pedestrians
Research conducted in the U.S. and Europe as presented in the NCHRP 672 [1] indicates fewer pedestrian
accidents with less severity occur at roundabout intersections when compared to signalized and unsignalized
intersections with comparable volumes. Design principles need to be applied that provide for slow entries and
exits for pedestrian safety.
Due to relatively low operating speeds of 15 to 20 mph, pedestrian safety is generally better with a roundabout
design than with other intersection types. Table 10.1 lists the advantages and disadvantages of roundabouts as
related to pedestrians.
Table 10.1 Roundabout Advantages and Disadvantages for Pedestrians
Advantages
Disadvantages
Vehicle speed is reduced as compared to other
intersections.
Vehicle traffic is yield controlled so traffic does not
necessarily come to a full stop. Therefore, pedestrians
may be hesitant to use the cross walk at first.
Pedestrians have fewer conflict points than at other
intersections.
May be unsettling to the pedestrian, depending on age,
mobility, visual impairments, and ability to judge gaps in
traffic.
Pedestrians are responsible for judging their crossing
opportunities. This requires more alertness and may be
considered an advantage.
The splitter island gore allows pedestrians to resolve
conflicts with entering and exiting vehicles separately
and simplifies the task of crossing the roadway.
Crossing is often accomplished with less wait than at
signalized intersections.
Pedestrians at first glance may have to adjust to the
operation of a roundabout. Part of this adjustment
includes the crosswalk location, which is behind the first
stopped vehicle or approximately 20 feet from the yield
point.
Choosing the appropriate crossing location for pedestrians is a delicate balance between their safety and
convenience, and operation of the roundabout. Pedestrians want crossing locations as close to the intersection
as possible to minimize out-of-direction travel. The further the crossing is from the roundabout, the more likely
that pedestrians may choose a shorter route that may put them in greater danger. Both crossing location and
crossing distance are important. Minimize crossing distance to reduce exposure to pedestrian-vehicle conflicts.
The continual movement of traffic, and the inability of some pedestrians to judge gaps in an oncoming travel
stream, reduces the perception of safety for pedestrians at roundabouts. This is especially true of children, the
elderly or the disabled. These types of pedestrians generally prefer larger gaps in the traffic stream, and walk at
slower speeds than other pedestrians. In recognition of pedestrians with disabilities, pedestrian crossings at
roundabouts should be designed to comply with Americans with Disabilities Act (ADA) mandated accessibility
standards. Refer to the following guides for further information:
- FDM 11-26-35.5.13, for non-motorized users
- NCHRP 672, Chapter 6, §8.1
- NCHRP 672, Chapter 7, §5.3
- 2009 MUTCD, §3B.18
The “pedestrian hybrid signal” sometimes referred to as the HAWK crosswalk signal may be considered where
there is an identified or demonstrated need to accommodate the visually impaired. Another option to consider is
the Rectangular Rapid Flashing Beacon (RRFB) that is being studied at this time. However, contact the regional
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traffic operations unit and the Bureau of Traffic Operations if either of these pedestrian crossing traffic control
systems are being considered to determine if they are appropriate for the location and if they will be by permit
to the community or by WisDOT. These devices are being studied at the time of this publication and one of the
factors for installation may be the distance the device is set back from the entrance and installed upstream from
the exit. At this time it appears the guidance for set back at the entrance may be around 165 feet or so in
advance of the yield line, and the distance upstream from the theoretical exit (end of the splitter island) may also
be around 165 feet.
10.1.2 Bicyclists
The experience in other countries with bicyclists at roundabouts has been mixed with regard to safety. The
Insurance Institute for Highway Safety reports that roundabouts provide a 10 percent reduction in bicycle
crashes at 24 signalized intersections that were converted to roundabouts in the U.S. Multilane entry
roundabouts may be more problematic than single lane entries.
The complexity of vehicle interactions within a roundabout could leave a cyclist vulnerable, and for this reason,
designated bike lane markings within the circulatory roadway shall not be used [2009 MUTCD, §9C.04].
Effective designs that constrain motorized vehicles to speeds more compatible with bicycle speeds, around 15 20 mph, are much safer for bicyclists.
The operation of a bicycle through a roundabout presents challenges to the bicyclist similar to that of traditional
signalized intersections especially for turning movements. As with pedestrians, one of the difficulties in
accommodating bicyclists is their wide range of skills and comfort levels. While experienced bicyclists may have
no difficulty maneuvering through a roundabout, less experienced bicyclists may have difficulty and discomfort
mixing with vehicles, and may feel safer on a roundabout sidepath.
Design features such as proper entry curvature and entry width help slow traffic entering the roundabout.
Providing a ramp from the roadway to a roundabout sidepath or shared-use path prior to the intersection allows
a bicyclist to exit the roadway and proceed around the intersection safely through the use of crosswalks.
Bicyclists are often less visible and therefore more vulnerable when merging into and diverging from multilane
roundabouts. Therefore, it is recommended that a wider shared-use pedestrian-bicycle path, separate from the
circulatory roadway, be built where bicycle use is expected. While this will likely be more comfortable for the
casual bicyclist, the experienced commuter bicyclist will be slowed down by having to cross as a pedestrian at
the cross walk and may choose to continue to traverse a multilane roundabout as a vehicle. Refer to FDM 11
26-30.5.13 for design guidance.
10.2 Transit, Large Vehicle, Oversize Vehicles and Emergency Vehicle Considerations
10.2.1 Transit
Transit considerations at roundabouts are similar to those for any other intersection configuration. A properly
designed roundabout will readily accommodate buses. For rider comfort, transit vehicles should not have to use
the truck apron.
Bus stops on the far side are preferred and should be constructed with pull-outs. They should be located beyond
the pedestrian crossing to improve visibility of pedestrians to other exiting vehicles. Far-side stops result in the
crosswalk being behind the bus, which provides for better sight lines for vehicles exiting the roundabout to
pedestrians and keeps bus patrons from blocking the progress of the bus when they cross the street.
The use of bus pull-outs has some trade-offs to consider. A positive feature of a bus pullout is that it reduces the
likelihood of queuing behind the bus into the roundabout. A possible negative feature is that a bus pullout may
create sight line challenges for the bus driver to see vehicles approaching from behind when attempting to
merge into traffic. It may also be possible at multilane roundabouts in slow-speed urban environments to include
a bus stop without a bus pullout immediately after the crosswalk, as exiting traffic has an opportunity to pass the
waiting bus. In a traffic-calmed environment, or close to a school, it may be appropriate to locate the bus stop at
a position that prevents other vehicles from passing the bus while it is stopped.
If a bus stop must be located upstream of the roundabout (near side), it should be placed far enough away from
the splitter island, such that a vehicle overtaking the stationary bus has adequate space. If the approach is a
single lane and capacity is not an issue, the bus stop could be placed at the pedestrian crossing. Nearside stops
provide the advantage of having a potentially slower speed environment where vehicles are slowing down,
compared to a far-side location where vehicles may be accelerating upon exiting the roundabout. Nearside
stops are not recommended for entries with more than one lane because vehicles in the lane next to the bus
may not see pedestrians.
The decisions in regards to transit stop location must be coordinated with the local transit authority.
10.2.2 Legal Large Vehicles
Design roundabouts for the largest vehicle that is anticipated to use the roundabout on a regular basis. All
roundabouts on the State Highway system must accommodate a WB-65 design vehicle, which is the largest
Page 14
vehicle allowed on the State Highway system without a permit (legal large vehicle). On the National Highway
System or STH system, multi-trip permitted vehicles must also be accommodated. The additional vehicles which
must be checked for path accommodations include: the WB-92 (formerly WB-67 Long), a farm combine, and an
80-foot mobile home transport. These vehicles must be able to navigate the roundabout while remaining within
the curb lines. Only the expected movements which allow the above multi-trip permitted vehicles to remain on
the NHS or STH systems need to be accommodated. Designing a roundabout for a large legal semi to stay inlane at entry and within the roundabout presents challenges such as the possibility of:
- A larger diameter
- Wider entries
- Wider circulating lanes
- Increased right-of-way needs
- Increases in certain types of crashes
- Other unique design features
In rare cases, roundabouts have been designed with a gated bypass roadway to accommodate turns.
Load shifting may be problematic for the contents of any vehicle while navigating a turning maneuver. Load
shifting is a common concern for liquid or semi-liquid loads where the weight of the load may shift in a manner to
exacerbate overturning. It is not uncommon for a vehicle with a high center of gravity to overturn when
navigating a turn at speeds that exceed the laws of mechanics. A roundabout is designed to minimize loadshifting problems with larger vehicles however speed is major factor related to overturning.. Problems such as
minimal entry deflection may lead to high entry speeds, long tangents leading into tight curves, sharp turns at
exits, excessive cross slopes, and adverse cross slopes have been the principal causes of load shifting. See
FDM 11-26-30.5 for geometric design of roundabouts.
10.2.3 Permitted Oversized Overweight (OSOW) Vehicles
During the preliminary design, check with local officials and the public to determine if there are any special
OSOW vehicles that regularly use the route and refer to the WisDOT OSOW vehicle inventory in FDM 11-25
Attachment 2.1. Coordinate OSOW Freight Network (OSOW FN) and routing activities with the Regional Freight
Operations engineer.
Review the truck guidance provided in FDM 11-25-1.4 and FDM 11-25-2, which includes additional information
related to truck routes, the OSOW Freight Network and intersection design guidance. The Department produced
a map showing designated state and federal truck routes, and the OSOW FN in Wisconsin which is available on
the web, see the link in FDM 11-25-1. This map may experience updates and changes therefore use the most
current on-line version.
It is becoming somewhat common to widen the truck apron along the sides to accommodate OSOW vehicle
through movements. Additional pavement (behind a mountable curb) may also be provided along the right side
of the entries to accommodate wheel off-tracking. Sign posts may also have to be mounted in removable
sleeves to provide additional lateral space for OSOW vehicles (see FDM 11-26-35.1.12).
10.2.4 Emergency Vehicles
Emergency vehicles passing through a roundabout encounter the same problem as other large vehicles and
may require the use of the truck apron. On emergency response routes, compare the delay for the relevant
movements with alternative intersection types and controls.
Roundabouts provide the benefit of lower vehicle speeds, which may make them safer for emergency vehicles
to negotiate than conventional intersections.
The Wisconsin Motorist’s Handbook provides information on what to do when the driver encounters an
emergency vehicle. The driver must yield the right-of-way for emergency vehicles using a siren, air horn or a red
or blue flashing light. The driver in the circulatory roadway should exit the roundabout before pulling over.
Emergency vehicles will typically find the safest and clearest path to get through an intersection. This may
include driving the emergency vehicle, with caution and with lights and siren on, in the opposing lane(s) or
however the operator sees as the most desirable alternative path.
10.3 References
[1] NCHRP 672, Roundabouts: An Informational Guide, Second Edition
FDM 11-26-15 Agency & Public Coordination
March 4, 2013
15.1 Public Meetings
Public meetings provide an excellent opportunity to bring the public into the design process. It is generally
desirable to take the 30% preliminary plans of all feasible alternatives on an equal basis to a public meeting and
Page 15
explain that a roundabout appears to be a reasonable alternative. Inform the public that no preference to any
alternative is indicated at that stage, but that input to all alternatives is being gathered. Try to be as specific as
possible about the real estate impacts, access impacts and anticipated operations (LOS) between the various
alternatives. At this level of design it may be important to let the public know that you do not have all the
answers about the various impacts. Roundabouts are a new form of intersection control that most people are
not familiar with. Set a specific time at each PIM of approximately 10-20 minutes to explain:
- The project time-line
- Source(s) of funding
- Concept of roundabouts
- Why the Department has included the roundabout as an alternative
- Construction duration and possible detours or road closures
- Illustrations of how pedestrians, bicyclists, and vehicles should travel through the roundabout
- Holding an open house and public information “exchange” meetings, and attending village and town
board meetings or local service organizational meetings are good formats for education and
consensus building
After the initial public meeting, a screening evaluation accounting for public support can be completed. Refer to
FDM 11-25-3. At the next public meeting, the preferred alternative can then be presented.
15.2 Public Outreach Resources & Methods
The success or failure of a project can often be attributed to how well the Department included the public in its
development. This can be particularly true when introducing the modern roundabout due to its confusion with
past circular intersections. There are excellent resources to assist the designer in explaining roundabouts to the
public and to help educate drivers (http://www.wisconsinroundabouts.gov).
Typically in the project planning process, alternatives are considered. The alternatives generally include traffic
signal, stop sign, or roundabout control; some of which are familiar to drivers and pedestrians. Presenting a
comparison of traffic operations and safety between alternatives is a good way to introduce roundabouts. It is
essential to inform the public of the planning process that led to the decision favoring a roundabout as the
preferred traffic control. A traceable transparent planning process engenders trust and validates the process of
wise investment in infrastructure. Designers are encouraged to generate project-specific roundabout outreach
materials on their Region’s web site. Coordination of this effort must be through the Central Office (IT)
Coordinator and the Web Site Content Coordinator.
The common dilemmas for most agencies that want to start using roundabouts are:
- Recognized public perception of roundabouts vs. their proven performance
- Driver education: way-finding and lane choice
- Pedestrian perception of safety vs. proven conditions
- Bicyclist education
- Permitted trucking (standard large trucks)
Pitfalls in the initial push for roundabouts can be avoided by developing detailed components of project outreach
resources for internal (local agency) and externally (public outreach) early and continuously. A public
acceptance and education campaign is critical to the successful implementation of roundabouts at the State
level and for local communities. A successful project oriented public outreach campaign involves assembling a
collection of educational and acceptance resources of a general nature. Many of these are readily available
through the department’s website: http://www. wisconsinroundabouts.gov , but some require adaptation to the
project location and context. Examples of the kinds of resources that should be collected and distributed through
various media include:
- Case studies
- Testimonials
- National and Wisconsin-specific statistics
- How-to videos
- Web-cam
- Driver training
- Website
- Brochures
- Talking points/discussion bulletins for legislators and staff to respond to calls
- Vulnerable user training materials
Page 16
A strategy to apply these components requires starting with internal staff (planning, design and maintenance
operations); State legislators; District Attorney, State Patrol; then moving to external stakeholders, e.g. interest
groups, trucking associations and mobility advocacy groups. Finally, once a consensus is reached with internal
and external stakeholders a general public meeting or outreach contact can be arranged.
Prior to any general public outreach, a local officials meeting should be held with local council members, police
and fire services, senior staff, and maintenance operations staff. The general education process is exercised
with this group and the project specific presentation of the engineering study that led to the choice of a
roundabout as an alternative control is made. A consensus must be the goal of the local officials meeting in
order that the subsequent public contact, e.g. open house goes smoothly with upper and lower tier agency
agreement on why the use of a roundabout and how the project will be implemented, including proposed
education for the locally affected.
Preparation for the local project public contact requires development of context specific education and outreach
components. An inventory of resources that have proven effective for local project outreach is as follows:
- Scale model (1:87, 1 inch = 7.25 feet) of the layout accompanied by scale model trucks and cars
- Animation/simulation of the expected operation of the roundabout and possibly a comparison to the
alternative
- Renderings or visualizations
- A project location brochure
- How-to driver, pedestrian and bicycle user resources
- Talking points bulletins for local councilors that give a summary of the planning process, traces the
results of studies and documents funding sources, schedule and staging of construction
15.3 References
[1] National Safety Council. Estimating the Costs of Unintentional Injuries, 2008. National Safety Council
Website.http://www.nsc.org/news_resources/injury_and_death_statistics/Pages/EstimatingtheCostsofUnintentio
nalInjuries.aspx
[2]
Boardman, A., Greenberg, D., Vining, A., and Weimer, D. Cost Benefit Analysis: Concepts and Practice.
Prentice Hall; 3rd Edition, 2005.
[3]
Gómez-Ibáñez, J. A., Tye, W. B., and Winston, C. Essays in Transportation Economics and Policy: A
Handbook in Honor of John R. Mayer. Brookings Institution Press, 1999
FDM 11-26-17 System Considerations March 4, 2013
17.1 System Considerations
Roundabouts may need to fit into a network of intersections with the traffic control functions of a roundabout
supporting the function of nearby intersections and vice versa. Because the design of each roundabout
generally follows the principles of isolated roundabout design, this guidance is at a conceptual and strategic
level and generally complements the planning of isolated roundabouts. In many cases, site-specific issues will
determine the appropriate roundabout design elements. Closely spaced roundabouts are characterized by the
operations of one roundabout having an impact on the operations of an adjacent roundabout and may have
overhead lane signs and spiral designs with additional lanes for lane balance and lane continuity issues that
arise with closely spaced roundabouts in a series.
17.2 Adjacent Intersections and Highway Segments and Coordinated Signal Systems
A strategic level traffic assessment of system conditions of a series of roundabouts analysis is needed to
determine how appropriate it is to locate a roundabout within a coordinated signal network. There may be
situations where an intersection within the coordinated signal system requires a very long cycle which is caused
by high side road traffic or large percentage of turning movements and is dictating operations and reducing the
overall efficiency for the coordinated system. Replacing a signalized intersection with a roundabout may allow
for the system to be split into two systems thus improving the efficiency of both halves while also improving the
efficiency of the entire roadway segment. A traffic analysis is needed to evaluate each specific location.
It is generally undesirable to have a roundabout located near a signalized intersection; however, a corridor
analysis may show the roundabout as a good option. Traffic queues that extend into adjacent intersections need
to be analyzed further.
17.3 Roundabouts in an Arterial Network
In order to understand how roundabouts operate within a roadway system, it is important to understand their
fundamental arrival and departure characteristics and how they may interact with other intersections and
highway features. Lane use and lane balance on an approach can vary from ideal conditions where
roundabouts are in a system and at times closely spaced. Sensitivity testing of alternative lane use patterns and
Page 17
lane designation alternatives in geometric design is necessary. Simulation of traffic patterns using micro
simulation software is recommended for roundabouts being treated as a system.
17.3.1 Planned Network, Access Management
Rather than thinking of roundabouts as an isolated intersection or replacement for signalization, identify likely
network improvements early in the planning process. This is consistent with encouraging public and other
stakeholder interaction to prepare or update local comprehensive or corridor plans with circulation elements.
Project planning and design are likely to be more successful when they are part of a larger local planning
process. Then, land-use and transportation relationships can be identified and future decisions related to both.
Roundabouts may be integral elements in village, town, and city circulation plans with multiple objectives of
improving circulation, safety, pedestrian and bicycle mobility, and access management. Roundabouts rely on
the slowing of vehicles to process traffic efficiently and safely which results in a secondary feature of “calming”
traffic. It can be expected that local studies and plans will be a source of requests for roundabout studies,
projects, and coordination on State arterials. A potential use of arterial roundabouts is to function as gateways or
entries to denser development, such as villages or towns, to indicate to drivers the need to reduce speed for
upcoming conflicts including turning movements and pedestrian crossings.
Retrofit of suburban commercial strip development to accomplish access management objectives of minimizing
conflicts can be a particularly good application for roundabouts. Raised medians are often designed for State
arterials to minimize left turn conflicts; and roundabouts accommodate U-turns. Left-turn exits from driveways
onto an arterial that may currently experience long delays and require two-stage left-turn movements could be
replaced with a simpler right turn, followed by a U-turn at the next roundabout. Again, a package of
improvements with driveway consolidation, reverse frontage, and interconnected parking lots, should be
planned and designed with close local collaboration. Also, a roundabout can provide easy access to corner
properties from all directions.
17.3.2 Platooned Arrivals on Approaches
Vehicles exiting a signalized intersection tend to be grouped into platoons. Platoons, however, tend to disperse
as they move down-stream. Roundabout performance is affected by its proximity to signalized intersections and
the resulting distribution of entering traffic. If a signalized intersection is very close to the roundabout, it causes
vehicles to arrive at the roundabout in closely spaced platoons. The volume of the arriving platoon and the
capacity of the roundabout will dictate the ability of the roundabout to process the platoon. Analyze these
situations carefully to achieve a proper design for the situation. Discuss proposed roundabout locations with the
Regional traffic section staff.
17.3.3 Roundabout Departure Pattern
Traffic leaving a roundabout tends to be more random than for other types of intersection control. Downstream
gaps are shorter but more frequent as compared to a signal. The slower approach and departing speeds along
with the gaps allow for ingress/egress from nearby driveways or side streets. The slowing effects are diminished
as vehicles proceed further downstream. However the gaps created at the roundabout are carried downstream
and vehicles tend to disperse again providing opportunities for side street traffic to enter the main line roadway.
Sometimes traffic on a side street can find it difficult to enter a main street at an un-signalized intersection. This
happens when the side street is located between two signalized intersections and traffic platoons from the
signalized intersections arrive at the side street intersection at approximately the same time. If a roundabout
replaced one of these signalized intersections, then its traffic platoons would be dispersed and it may be easier
for traffic on the side street to enter the main street. Alternatively, when signals are well coordinated they may
provide gaps at nearby intersections and mid-block for opportunities to access the main line.
If a roundabout is used in a network of coordinated signalized intersections, then it may be difficult to maintain
the closely packed platoons required. If a tightly packed platoon approached a roundabout, it could proceed
through the roundabout as long as there was no circulating traffic or traffic upstream from the left. Only one
circulating vehicle would result in the platoon breaking down. Hence, this hybrid use of roundabouts in a
coordinated signalized network needs to be evaluated carefully.
Another circumstance in which a roundabout may be advantageous is as an alternative to signal control at a
critical signalized intersection within a coordinated network. Such intersections are the bottlenecks and usually
determine the required cycle length, or are placed at a signal system boundary to operate in isolated actuated
mode to minimize their effect on the rest of the surrounding system. If a roundabout can be designed to operate
within its capacity, it may allow a lowering of the system cycle length with resultant benefits to delays and
queues at other intersections.
17.4 Closely Spaced Roundabouts
It is sometimes desirable to consider the operation of two or more roundabouts in close proximity to each other.
Closely spaced roundabouts can potentially reduce queues and balance traffic flows. The spacing between any
Page 18
two roundabouts is considered closely spaced if they are less than 1,000 feet from center to center (see FDM
11-26-30.5.13). They also can accommodate a wide range of access, both public and private. In any case, the
expected queue length at each roundabout becomes important. Compute the expected queues for each
approach to check that sufficient queuing space is provided for vehicles between the roundabouts. If there is
insufficient space, then drivers may occasionally queue into the upstream roundabout, potentially causing a
reduction from the desired operations. However, the roundabout pair can be designed to minimize queuing
between the roundabouts by limiting the capacity of the inbound approaches.
Closely spaced roundabouts may improve safety and accessibility to business or residential access or side
streets by slowing the traffic on the major road. Drivers may be reluctant to accelerate to the expected speed on
the arterial if they are also required to slow again for the next close roundabout. This may benefit nearby
residents.
For additional information, see NCHRP 672, §6.9.
17.5 Roundabout Interchange Ramp Terminals
Freeway ramp junctions with arterial roads are potential candidates for roundabout intersection treatment. This
is especially true if the subject interchange typically has a high proportion of left-turn flows from the off-ramps
and to the on-ramps during certain peak periods, combined with limited queue storage space on the bridge
crossing, off-ramps, or arterial approaches. In such circumstances, roundabouts operating within their capacity
are particularly amenable to solving these problems when compared with other forms of intersection control.
OSOW vehicle accommodations need to be evaluated when considering a roundabout ramp terminal at the
junction of two OSOW Freight Network routes.
Occasionally, an OSOW vehicle may have to bypass a bridge by taking the off-ramp and making a through
movement and entering the on-ramp (a.k.a. “ramping”). Design the median island to accommodate the OSOW
through movement. Refer to FDM 11-30-1 for additional guidance on interchange design.
The benefits and costs associated with this type of interchange also follow those for a single roundabout. Some
potential benefits of roundabout interchanges are:
- The queue length on the off-ramps may be less than at a signalized intersection. In almost all cases, if
the roundabout would operate below capacity, the performance of the on-ramp is likely to be better
than if the interchange is signalized.
- The intersection site distance is much less than what it is for other intersection treatments.
- The headway between vehicles leaving the roundabout along the on-ramp is more random than when
signalized intersections are used. This more random ramp traffic allows for smoother merging
behavior onto the freeway and a slightly higher performance at the freeway merge area similar to ramp
metering.
There are no unique design parameters for roundabout interchanges. They are only constrained by the physical
space available to the designer and the configuration selected. Several geometric configurations for ramp
terminals with roundabouts exist:
- The raindrop form, which does not allow for full circulation around the center island, can be useful if
grades are a design issue since they remove a potential cross-slope constraint on the missing
circulatory road segments. However, raindrop shapes lack the operational consistency, because one
entry will not be required to yield to any traffic. Because of this, an undesirable increase in speed may
occur. If an additional road connects to the ramp terminal, the raindrop form should not be used.
- A single-point diamond interchange incorporates a large-diameter roundabout centered either over or
under the freeway. While remaining somewhat compact, this solution may not be cost-effective,
especially for retro-fit locations, as existing overpass structures may not be adequately sized or
oriented.
- Dual roundabouts are the common choice for interchange locations. This design may delay or
eliminate the need for overpass reconstruction, while also allowing for easier future roundabout
expansion. It offers the greatest flexibility in location of the roundabouts while improving ramp
geometry and minimizing the need for retaining walls. It may require additional right of way to be
acquired, as this design typically requires the most space.
17.6 Traffic Signals at Roundabouts
Roundabouts typically are not planned to include metering or signalization. The “pedestrian hybrid beacon”
sometimes referred to as the HAWK crosswalk signal, in addition to Rectangular Rapid Flashing Beacon
(RRFB) are discussed in FDM 11-26-10.1.1.
Page 19
17.7 At-Grade Rail Crossings
Locating any intersection near an at-grade railroad crossing is generally discouraged. However, due to
necessity, intersections are sometimes located near railroad grade crossings. When considering locating a
roundabout within 1000 feet of a railroad, contact the Region Railroad Coordinator early in the process. It is
preferable to cross one of the legs of a roundabout and leaving a desired distance of at least 100 feet from the
center of the track to the yield line. Treatment should follow the recommendations of the MUTCD whenever
possible. Consider allowing the railroad track to pass directly through the circle center of the roundabout rather
than through another portion of the circular roadway if the at-grade crossing is not on one of the legs. Also,
consider the design year traffic on the roadway, the number of trains per day, speed of trains, length of trains,
type of crossing warning devices, and anticipated length of vehicular queues when evaluating the intersection
control needed in close proximity to the railroad.
Refer to FDM 17-1-1 for additional railway information. Expert assistance is required to address rail pre-emption
requirements of roundabouts in close proximity.
17.8 References
[1] NCHRP 672, Roundabouts: An Informational Guide, Second Edition, Chapter 7, Section 6.
FDM 11-26-20 Operations
March 4, 2013
20.1 Operational Analysis References and Methods
The growing number of roundabouts in the United States (US) has led to an increase in national and local
research of roundabout operations and capacity. The National Cooperative Highway Research Program
(NCHRP) published the largest study in the US to date on roundabout operations in the 2007 NCHRP Report
572[1]. This research found that driver behavior is one of the largest variables affecting the performance of US
roundabouts.
The capacity and operations of US roundabouts is more sensitive to the interaction between drivers entering
and circulating the roundabout and the number of entry lanes than the detailed geometric parameters (e.g. lane
width, entry radius, phi angle, and inscribed circle diameter) used in the Australian[2] and UK models [3].
Although important to ensure the safety and efficiency of travel through a roundabout, the fine details of
geometric design are considered secondary and less significant than variations in driver behavior when
analyzing capacity at roundabouts in the US.
The Highway Capacity Manual (HCM) 2010 has published in Chapter 21 analytical procedures for the analysis
of planned and existing roundabouts that are largely based in the findings of the NCHRP report. The methods of
the HCM allow traffic engineers and designers to assess the operational performance of a roundabout, given
information about the demand levels for motor vehicles, pedestrians and bicycles. The following sections
provide guidance on operational analysis for Wisconsin DOT projects considering the installation of a new
roundabout or evaluating the capacity of an existing roundabout.
20.2 Roundabout Operation
A roundabout brings together conflicting traffic streams at reduced speeds, allowing the streams to safely cross
paths, traverse the roundabout, and exit. Modern roundabouts do not have merging or weaving between
conflicting traffic streams. Compactness of circle size and geometric speed control make it possible to establish
priority to circulating traffic. The geometric elements, signage and pavement markings of the roundabout
reinforce the rule of circulating traffic priority and provide guidance to drivers approaching, entering, and
traveling through a roundabout.
The operation of vehicular traffic at a roundabout is determined by gap acceptance (i.e. headway). Vehicles at
each approach look for and accept gaps in circulating traffic. The low speeds of a properly designed roundabout
facilitate this gap acceptance process. The width of the approach roadway, the curvature of the roadway, and
the volume of traffic present on a given approach govern this speed. As drivers approach the yield point, they
must first yield to pedestrians and then to conflicting vehicles in the circulatory roadway. The size of the
inscribed circle affects the radius of the driver's path, which in turn determines the speed at which drivers travel
in the circulatory roadway.
20.2.1 Planning Level Analysis and Space Requirements
The inscribed circle diameter needed for a roundabout is one of the most critical space requirements when
considering impacts to right of way, costs, and design vehicle among others. The following table gives general
inscribed circle diameters and daily service volumes for the different types of roundabouts. The typical daily
service volumes ranges described in Table 20.1 are derived from Exhibit 3-12 in the NCHRP 672 report and are
depended on the left turn percentage of the daily service volume. Three-leg roundabouts may be assumed to
have a capacity that is 75% of the service volumes shown in Exhibit 3-12 of the NCHRP report for a planning
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level analysis. Use Table 20.1 for inscribed circle diameter values to help in the initial steps of considering a
roundabout as a feasible alternative. Diameters will vary and may fall outside these typical ranges in some
situations.
Table 20.1 Typical Inscribed Circle Diameters and Estimated Daily Service Volumes
Roundabout Type
Typical Inscribed Circle
Diameter1
Typical Daily Service Volume2,3
(vpd) 4-leg roundabouts
Single-Lane
120 -160 ft (35 – 50 m)
less than 25,000
Multilane (2-lane entry)
160 - 215 ft (50 – 65 m)
25,000 to 45,000
Multilane (3 lane entry)
215 - 275 ft (65 – 85 m)
45,000 or more
1
For additional guidance based on design vehicle see Exhibit 6-9 Inscribed Circle Diameter Ranges in
NCHRP Report 672
2
Capacities vary substantially depending on entering traffic volumes and turning movements.
3
Consult with Exhibit 3-12, “NCHRP Report 672, Roundabouts: An Informational Guide, Second
Edition” to estimate the ADT for a specific left-turn percentage.
The capacity of each entry to a roundabout is the maximum rate at which vehicles can reasonably be expected
to enter the roundabout during a given time period under prevailing traffic and roadway conditions. An
operational analysis considers entering and circulating traffic flow rates defined for the morning and evening
peak periods for each lane at a roundabout. Analysis of the peak hour period is critical to assess the level of
performance at each approach and the roundabout as a whole.
For a properly designed roundabout, the entry area is the relevant point for capacity analysis. The approach
capacity is the capacity provided at the yield point. This is determined primarily by the interaction between
entering and circulating streams of traffic, the basic number of entry and circulating lanes and to a lesser degree
by the geometric parameters, signage and pavement markings that control entry and circulating speed.
The maximum flow rate that can be accommodated at a roundabout entry depends on two factors: the
circulating flow in the roundabout that conflicts with the entry flow, and the number of entering lanes on the
approach to the circulatory roadway. When the circulating flow is low, drivers at the entry are able to enter the
roundabout without significant delay. The larger gaps in the circulating flow are more useful to the entering
drivers and more than one vehicle may enter each gap. As the circulating flow increases, the size of the gaps in
the circulating flow decreases, thus the rate at which vehicles can enter also decreases. See FDM 11-26-20.5
for guidance on unbalanced flows.
Each approach leg of the roundabout is evaluated individually to determine the number of entering lanes that
are required based upon the conflicting flow rates. The number of lanes within the circulatory roadway is then
based on the number of lanes needed to provide lane continuity. More detailed lane assignments and
refinements to the lane configurations must be determined through a more formal operational analysis as
described later in this section.
On multilane roundabouts, it is important to balance the traffic use of each lane; otherwise some lanes may be
overloaded, while others are underutilized. Also, poorly designed exits may influence driver behavior and cause
lane imbalance and congestion on the opposite leg.
20.3.1 Planning Estimates of Lane Requirements
Where existing and/or projected turning-movement data is available at the planning level, an estimate of the
potential lane configurations can be identified and should be performed prior to detailed operational analysis.
The procedure provided within this section is simplified to aid in the basic sizing considerations and to determine
if multilane entries and circulating lanes may be needed.
The sum of the entering and conflicting traffic volumes can be used to evaluate the number of lanes required on
the entry. If the sum of the entering and conflicting volumes is less than 1,000 vehicles per hour (veh/h), then a
single lane entry can be reasonably assumed to operate within its capacity. A single lane entry may be sufficient
if the sum of entering and circulating volume is between 1,000 and 1,300 veh/h, with either case a more detailed
analysis as outlined in the following sections will be required.
A two lane entry will likely be sufficient for the sum of entering and circulating volume in the range of 1,300 to
1,800 veh/h, but a more detailed capacity evaluation is required to verify number of lanes and lane assignments.
Page 21
20.3.2 Pedestrian Effects on Entry and Exit Capacity
Pedestrians crossing at a marked crosswalk have priority over entering motor vehicles and can have a
significant effect on the entry capacity if sufficient pedestrians are present. In such cases, if the pedestrian
crossing volume and circulating volume are known, the vehicular capacity is multiplied by the entry capacity
adjustment factor (fped) according to the relationship shown in Exhibit 21-18 and 21-20 of HCM 2010 Chapter
21 for single-lane and two-lane roundabouts, respectively.
Note that the effects of conflicting pedestrians on the approach capacity decrease as conflicting vehicular
volumes increase, as entering vehicles become more likely to have to stop regardless of whether pedestrians
are present. Consult the Highway Capacity Manual for additional guidance on the capacity of pedestrian
crossings if the capacity of the crosswalk itself is an issue. A similar effect in capacity may occur at the
roundabout exit where pedestrians cross.
20.4 Operational Analysis Methodology
As is shown in Figure 20.1, the first steps to roundabout analysis and design are to gather traffic data for the
existing intersection and to complete an HCM analysis. A roundabout design will be based on the lane
configuration selected for acceptable operations with design year traffic conditions. A lane configuration for
acceptable operations means that all or most movements operate at LOS C or better with a volume to capacity
ratio lower than one. Level of service D may be acceptable for certain movements and at certain locations at the
discretion of a regional traffic engineer.
In the capacity analysis results of a multi-lane roundabout, consideration should be given to an interim layout
with fewer circulatory lanes (i.e. 15-20 years traffic projection). The interim design should be arranged so as to
be convertible to the ultimate design at a later date (for example by reducing the diameter of the central island).
This approach offers safety and operational advantages during the early years, including reduction in fastestpath maneuvers and a simpler layout that is easier for unfamiliar drivers to navigate. The determination of
whether to construct the interim layout should take into consideration the extent to which drivers in the project
area already have roundabout driving experience, as well as the level of uncertainty in the traffic forecasts
(design-year forecasts often assume full build-out of nearby real estate development projects, but in many cases
those projects are unable to proceed as quickly as the developers or local officials anticipate). In all cases,
utilities should be cleared and real estate should be acquired to accommodate the ultimate design.
Supplemental software analysis tools include a Paramics simulation model; and two deterministic models Sidra
Standard, and Rodel. Paramics and Sidra Standard can be used as specified in. FDM 11-26-20.5. Rodel and
any other tool that designers rely on as design aids can be used to refine the roundabout geometric design once
the basic lane configuration has been established based on the operational analysis with HCM and other
considerations. In addition to the HCM analysis supplemental software’s analysis is also appropriate for
evaluating operations for in-service roundabouts whereby collection of data under capacity conditions can be
used to calibrate the capacity equations. FDM 11-26-20.5 and FDM 11-26-20.6 discuss the application of
supplemental tools in more detail.
Only after the analysis is completed and the preferred lane configuration determined should the detailed design
of the roundabout begin. An overview of the roundabout HCM analysis process is published in Exhibit 21-9 of
the HCM 2010. Detailed descriptions and equations for each step are provided in Chapter 21 of the HCM 2010,
starting on page 21-12. Step-by-step example problems for analyzing a single lane roundabout with a bypass
and a multilane roundabout using HCM procedures are provided on page 21-28 and page 21-33, respectively,
of the HCM 2010.These steps describe how to calculate the capacity, LOS, and queue for a roundabout by
hand. The use of software makes analyzing the operations of a roundabout much quicker. WisDOT’s approved
method for analyzing roundabouts using HCM guidance is diagramed in Figure 20.1 and detailed in the next
sections.
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Figure 20.1 WisDOT Approved Method for Analyzing Roundabouts
20.4.1 Gather Traffic Volumes, Peak Hour Factors, and Truck Percentages
Obtain existing 12-hour traffic counts for the intersection and establish the peak traffic hours for analysis. Also
obtain counts for off peak, midday, or special event times if applicable. Traffic counts shall be no older than the
most recent three years. Note any special lane utilizations or imbalances, especially if the existing intersection is
a roundabout. Calculate the peak hour factor for each peak period. Determine percentages of trucks by
approach. Also include the number and percentage of bicycles and pedestrians, if present.
The existing traffic turning counts shall be sent to the central office traffic forecasting section for development of
design year traffic volumes. Consider intermediate design year forecasts in preparation for sensitivity analysis to
determine staged improvement or capacity expansion, e.g. one to two lane entries or two to three lane entries.
20.4.2 Enter Forecasted Traffic Volumes into Traffic Flow Worksheet
A volume diagram can be developed using Attachment 20.1 to provide existing peak hour turning volumes (AM,
PM, Weekend/Special Event) and design year peak hour turning volumes. Before starting the capacity analysis
the traffic forecasts should be checked for reasonableness by a person who is familiar with the site.
For example, growth rates throughout the intersection should be fairly consistent, unless local factors such as
new development are expected to increase specific movements disproportionately. Similarly, the dominant
movements in the forecasted volume set should be similar to the existing pattern, unless changes in land use or
highway routing are expected. In areas with high commuter traffic, corresponding AM and PM movements
should be compared, for example a high westbound left turn movement in the morning is usually accompanied
by a high northbound right turn movement in the afternoon.
If the intersection is part of a corridor project, the consistency of forecasts along the corridor should also be
reviewed, since the outputs of one intersection are usually the inputs to the next, plus or minus the driveway
traffic in between. Attachment 20.1 provides a format for summarizing the traffic volumes at a 3-leg, 4-leg, or
interchange ramp roundabout.
20.4.3 Determine Number of Entry Lanes and Lane Configuration, Draw Lane Configuration Sketch
Based on planning level capacity requirements determine how many entry lanes a roundabout would require to
serve the traffic demands (see Table 20.1 and FDM 11-26-20.3.2). Determine the entry volumes for each lane of
the roundabout approach. Adjust lane volumes based on observed or estimated lane utilization patterns or
imbalances, if applicable. If no lane utilization patterns are observed, the HCM 2010 default values are 47% of
entry flow in the left lane and 53% of entry flow in the right lane for left/through and left/through/right and
left/through/right-right lane configurations, and 53% in the left lane and 47% in the right lane for left
left/through/right lane configurations.
A lane configuration sketch of the roundabout should accompany the traffic volumes to facilitate the selection of
the number of lanes and the lane assignments. This is a critical step that precedes the roundabout capacity
analysis and the layout process because it affects the geometry. In Figure 20.2, the assessment of lane
assignments for the example traffic flows could include three different options. Unless traffic demand for a given
approach is indicative of the potential need for an exclusive left turn lane, option 1 is preferred for its simplicity of
design and because the configuration can be expected to accommodate both peak and off-peak traffic demand.
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In the example Options 2 and 3 would require spiral geometry and marking treatment for the upstream entry left
turn. Also, Options 2 and 3 imply a single lane exit for lane continuity of the through movement. These
alternatives complicate the design and may influence driver behavior by causing confusion when navigating the
circulatory roadway. Figure 20.3 is an example of the roundabout lane configuration sketch employing Option 1.
Figure 20.2 Lane Configuration Options
Figure 20.3 Lane Configuration Sketch
20.4.4 Analyze Roundabout Lane Configuration
The preliminary lane configuration estimated in FDM 11-26-20.4.3 must be analyzed with HCM procedures
using one of two WisDOT approved analysis tools: Highway Capacity Software (HCS) 2010 (version 6.4 or
newer) and Sidra Intersection (5.1 or newer).
HCS 2010 is limited to no more than four approaches and two entry lanes plus one or more bypass lanes.
Partial right turn bypasses are restricted to single lanes. The software requires calibration with the
recommended Wisconsin headway values listed in Table 20.3.
Use Table 20.2 as guidance in choosing the most appropriate approved analysis tool for the roundabout lane
Page 24
configuration being analyzed. Refer to the sections listed in the table for additional details on completing the
operational analysis.
Table 20.2 Choosing Appropriate Analysis Tool
Analysis Tool
Appropriate Situations
Section
HCS 2010 (version
6.41 or newer)
One or two lane entries, single lane partial bypasses, no
more than four approach legs
FDM 11-26-20.4.5
Sidra Intersection
(version 5.1 or newer)
One, two or three lane entries, one or two lane partial
bypasses, up to 8 approach legs
FDM 11-26-20.4.6
Sidra Intersection is capable of analyzing roundabouts with multiple models. The HCM capacity and delay
models shall be used when analyzing Wisconsin roundabouts. The limitations of the HCM 2010 methodology on
lane configuration has been expanded by Sidra (U.S. mode) and the analysis can be used for all roundabouts
but is specifically required for evaluating roundabouts with three entry lanes, dual partial right turn bypass lanes,
and/or five or more approaches. Sidra applies the basic HCM 2010 procedures and will provide essentially the
same results as the HCS 2010 software procedure. Sidra (U.S. mode) also requires calibration with the
headway values listed in Table 20.3.
20.4.5 HCS 2010 Analysis
Critical headway (also referred to as ‘critical gap’) and follow-up headway are the driver behavior parameters
that influence the capacity of a roundabout approach and the roundabout as a whole. Critical headway is the
smallest gap in circulating traffic that an entering driver would accept to enter the roundabout. Follow-up
headway is the time between two successive entering vehicles accepting the same gap in circulating traffic.
Figure 20.4 diagrams the concept of critical headway and Figure 20.5 diagrams the concept of follow-up
headway.
Figure 20.4 Critical Headway
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Figure 20.5 Follow-up Headway
The authors of the NCHRP Report 572 collected and analyzed critical and follow-up headways at several
roundabouts across the country to determine that an exponential gap-acceptance theory combined with field
determined headway values can provide an acceptable empirical capacity equation for estimating the operations
of a U.S. roundabout. This method of analyzing roundabouts is described in the Highway Capacity Manual
(HCM) 2010 and is the basis for Wisconsin’s driver behavior-based approach to analyzing roundabout
operations.
The general form of the capacity equation for a roundabout is provided in Equation 20.1 – Equation 20.3:
[Equation 20.1]
[Equation 20.2]
[Equation 20.3]
where
The capacity equation in Equation 20.1 can be calibrated for local sites by adjusting the critical and follow-up
headways. The HCM 2010 lists default capacity equations based on national averages for critical and follow-up
headways. Research funded by WisDOT and conducted by the Traffic Operations and Safety (TOPS) Lab at the
University of Wisconsin - Madison observed headways at Wisconsin roundabouts that are mainly lower than the
HCM 2010 defaults, resulting in higher capacities for most entry-circulating configurations. As a result of the
study, Table 20.3 lists the recommended headway values and the corresponding parameters A & B that shall be
used in roundabout capacity analyses state-wide. HCS will require the analyst to enter the headway values
when calibrating the analysis while Sidra’s most efficient way to calibrate the analysis is by entering Parameters
A & B. These values represent the latest headway numbers based on Wisconsin research. Improvements in
driver behavior and potential increases in congestion levels, may result in an increase in roundabout capacity.
Driver behavior will continue to be monitored by WisDOT and headway values will be adjusted as necessary in
the future.
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Table 20.3 Recommended Headway Values
Number of Circulating
(Conflicting) Lanes
Critical
Headway, tc
Follow-up
Headway, tf
Parameter A
Parameter B
One
4.2* sec
2.8 sec
1286
0.000778
Two or Three
4.0 sec
2.8 sec
1286
0.000722
* Based on NCHRP 572, not Wisconsin Research
The resulting capacity equations for Wisconsin roundabouts using the headways listed in Table 20.3 are
presented in Equation 20.4 for roundabout entries with one lane circulating past the entry and Equation 20.5 for
roundabout entries with two lanes circulating past the entry. In theory, entries with two lanes circulating past the
entry have higher capacities than entries with one lane circulating.
[Equation 20.4]
[Equation 20.5]
where;
The HCM 2010 includes separate capacity equations for the left lane and the right lane. The capacity equations
listed above are appropriate for the left and right lanes of a two lane entry and for single partial right turn bypass
lanes. The general approach to analyzing, building, and adjusting existing roundabouts begins by establishing
the general footprint of a new roundabout, once traffic forecasts for the studied intersection are obtained.
Following FDM guidance and HCM methodologies, a lane configuration for acceptable operations is determined
and the detailed design completed. Existing roundabouts may need to be field adjusted to improve capacity;
supplemental tools may be used to determine potential improvements for an existing roundabout. Figure 20.6
provides an overview of the general procedures.
Figure 20.6 Operational Analysis Process, Inputs, and Outputs
20.4.5.1 HCS 2010 Roundabout Analysis Module
In the HCS 2010 roundabout analysis module enter the lane configuration determined in FDM 11-26-20.4.3.
Refer to the User Guide, or press F1 when the cursor is in the entry lanes field, for help with coding the lane
Page 27
configuration. Some common entry lane configurations and their corresponding HCS 2010 coding are provided
in Table 20.4.
Table 20.4 Common Lane Configurations and their HCS 2010 Coding (Partial Listing)
Both “Shared” buttons are
depressed
depressed
Only the Thru/Right “Shared”
button is depressed
depressed
Next enter the “Percent of Entry Vehicles using Left Lane.” The default percentage is usually acceptable, unless
different lane utilizations were observed at the existing intersection. Then choose the number of lanes conflicting
with the entry and identify the presence and type of bypass lanes. If a bypass lane exists, the lane configuration
is coded as if the bypass did not exist, and the type of bypass is selected. For example, if any of the lane
configurations shown in Table 20.4 had partial right turn bypasses, the lane configuration would be coded as
shown in Table 20.4 and the “Yielding” option would be selected under “Right-Turn Bypass.” Full bypasses are
coded as “Non-Yielding.” Make sure to identify the number of conflicting lanes on the bypass entry; this is the
number of exit lanes passing the bypass yield line. Also enter the number of pedestrians crossing the entry, if
applicable.
The volumes from the Traffic Flow Worksheet are then entered along with the peak hour factor and percent
trucks. The peak hour factor should be one number for each approach or the intersection as a whole, do not
enter different peak hour factors for each movement of an approach. If U-turns exist, the volume, peak hour
factor, and truck percentage is entered after the main truck percentages.
Critical and follow-up headway values shall match the accepted Wisconsin headways listed in Table 20.3. The
headway values entered depend on the number of lanes circulating past a given entry. The left lane and right
lane of a two lane entry will have the same headway values.
Review the results and adjust the lane configuration if needed. Remember to change the headway values if the
number of circulating lanes changes. Once an acceptable lane configuration has been achieved, print the
formatted report. The format for results should follow intersection control evaluation (ICE) FDM policy (FDM 11
25-3) and the Traffic Impact Analysis (TIA) guidelines for reporting on operational analysis. Include the analysis
files as attachments and report all queues in feet.
20.4.6 Sidra Intersection Analysis (US Mode)
In Sidra, change the Model Settings to “US HCM (Customary)” in order to analyze roundabout operations in
HCM mode. Under the “Input” folder, enter the geometry for the roundabout. Add approach and exit lanes as
necessary and enter the lane configuration. Make sure to add U-turns if applicable. Flared lanes can be entered
by changing the “Short Lane” lane type to “Turn Bay” and adding a lane length. Bypasses are entered by
changing the “Lane Type” to “Slip (Give-way/Yield)” for partial bypasses or “Continuous” for full bypasses. The
measurement parameters listed in the “Roundabout Data” section do not affect roundabout capacity and do not
need to be changed; however, the number of circulating lanes for each approach does need to be entered.
Viewing the “Layout” is an easy way to determine that the lane configuration was entered correctly.
If special lane utilization patterns were observed at the existing intersection, the “Utilization Ratio” can be
changed in the “Lane Data>Approach Data” section of the Geometry window. A percentage is entered in the
lane with the lowest traffic volume as a percentage of the lane with the highest volume. The lane with the
highest volume is considered to be 100% utilized.
After entering the geometry, parameters A and B must be changed in the “HCM 2010” dialog box under the
“Input” folder to calibrate the equations so that they reflect the Wisconsin headway values. Table 20.3 and
Equations 20.4 and 20.5 shown in FDM 11-26-20.4.5 show the calculated parameters A & B using the
Wisconsin headway values. Next enter the volumes from the Traffic Flow Worksheet in the “Volumes” dialog
box under the “Input” folder. Also in this dialog box, enter the truck percentages and peak hour factors (called
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“peak flow factor” in Sidra). Reference the Input Guide for more help on any of these topics.
Process the model, review the results, and adjust the lane configuration if needed. It is critical that parameters A
& B are not changed once the file is processed for analysis as adjusting the values higher or lower than the
recommended values in the same file once it’s processed will under or over estimate a roundabouts capacity.
“Cloning” the file before processing it for analysis will avoid this issue if the analyst desires to investigate the
capacity prediction of a roundabout with other headway values for his/her own use. Once an acceptable lane
configuration has been achieved, print the Lane Summary report. The format for the results of the operational
analysis should follow the intersection control evaluation (ICE) FDM policy (FDM 11- 25-3) and the Traffic
Impact Analysis (TIA) guidelines. Include the analysis files as attachments and report all queues in feet.
WisDOT reviewers are encouraged to recreate the analysis in Sidra to confirm the results as multiple
parameters can be used or adjusted that may influence the capacity prediction.
20.5 Supplemental Tools for Operational Analysis & Design
Once the general lane configuration of a roundabout has been determined with HCM procedures, Paramics and
Sidra Standard can be used to account for operational considerations that the HCM methodology does not
account for as discussed in FDM 11-26-20.5.1 Special Considerations. Sidra Standard, Rodel and any other tool
that designers have available to assist them in the design process can prove beneficial for the final design of the
roundabout. Geometric sensitivity can be tested in these programs, allowing the user to test the effects of size
and key geometric parameters i.e. (inscribed circle diameter, entry radius, phi angle, lane width, and flared
entry) on an existing or proposed roundabout design.
Rodel applies UK research producing a model that relates geometry to capacity, for roundabout capacity
calculations. A Paramics simulation model may also be used to refine the roundabout design. Micro simulation
that provides for animation and visualization of operating predictions is useful for public outreach. Sidra, when
used in Standard mode, implements a capacity estimation method that assumes a dependence of gap
acceptance parameters on multiple factors. Roundabout geometry, circulating flows, entry lane flows, and model
designation of dominant or subdominant lanes all influence gap acceptance parameters to account for lane-by
lane capacity variation. Sidra Standard utilizes what they call the Environment Factor as one of the main
parameters to calibrate the capacity model. The recommended Environment Factor for U.S. roundabouts is 1.2.
See the Sidra Intersection Users Guide, Part 3 Input Guide; Section 4.6.3 Roundabout Calibration Data for a
detailed description of this factor and how it is used in the Sidra capacity model.
20.5.1 Special Considerations
Lane designation or lane assignments are critical to the success of the roundabout lane configuration and
design. Conditions can be very complex with subtle problems that can reduce capacity and cause severe lane
imbalance. Examples of the kind of impacts that can be experienced include the presence of signals upstream
or downstream with a heavy turn proportion at the roundabout. Great care and sensitivity are required to
achieve lane utilization balance. Supplementary software is especially suited to these situations.
Unbalanced Conflicting Flows:
At a roundabout with unbalanced conflicting flow patterns, a traffic stream with a low flow rate
enters the roundabout having to yield to a circulating stream with a high flow. The opposite is
also considered an unbalanced flow. The HCM method does not account for unbalanced
circulating flows; accordingly, the analyst should be aware that the delay/LOS results may not
reflect an accurate prediction of operations/performance when this situation is apparent. In the
reporting of results the analyst must indicate if there are significant unbalanced conflicting flows
for any approach and discuss how this may influence the performance of the
roundabout/approach.
Unbalanced circulating flows highlight an operational condition that traffic engineers and
designers should understand and interpret by taking into consideration all aspects including but
not limited to the results of the analysis, the existing and future field conditions and traffic
patterns in order to better inform the findings on the analysis. The Sidra Standard capacity
model is sensitive to the ratio of entering to circulating flow, and therefore may be able to reflect
expectations of capacity when unbalanced flow conditions are expected. A Paramics
microsimulation model can also supplement the analysis but the level of data and effort to
calibrate this model can be significant for an isolated roundabout analysis.
Roundabouts in a Corridor - As a System:
Closely spaced roundabouts or roundabouts in a corridor are defined as locations where there
is interaction between the traffic arriving and traveling between the roundabouts. When a project
contains multiple roundabouts in one corridor, analyze all of the roundabouts with the same
software analysis tool for consistent reporting. The HCM method does not consider the impacts
of closely spaced roundabouts or roundabouts in a corridor as a system. Further analysis with
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supplemental tools may be necessary.
Paramics is a micro simulation tool that is capable of system level analysis and can be used to
adjust roundabout designs indirectly. Lane imbalances or lane use problems within a series of
intersections can be discovered with micro simulation to make the design of any single
roundabout more robust. The current version of Sidra is also capable of analyzing closelyspaced roundabouts or signalized intersections near roundabouts with the adjustment of the
Extra Bunching parameter. See the Sidra Intersection Users Guide, Part 3 Input Guide; Section
4.2 Approach Data and Approach Control for a detailed description of this factor and how it is
used in the Sidra capacity model.
With a system of roundabouts such as diamond interchange with adjacent frontage roads the
capacity of the roundabout can be impacted by lane utilization patterns. The analyst should test
alternative lane use and lane balance scenarios by adjusting traffic origin and destination
combinations for the same turning movement proportions at each intersection. This helps to
determine the lane assignment options for drivers originating one or two roundabouts upstream
of a particular turning movement. This scenario should also be considered for upstream
intersections with any other traffic control and geometry.
Capacity Considerations of Flared Entries:
In some situations the use of appropriate lane arrows can encourage balanced lane use, thus
improving capacity. Traffic often has a bias towards the right-most lane. Lane arrows can either
encourage this bias, or can encourage lane balance. Figure 20.7 shows the pavement marking
scheme preferred to encourage balanced lane demand. It is important to note that flared entries
at roundabouts cannot be assumed to always provide a balanced lane use and therefore add
capacity to that entry as HCS and Sidra will predict. For example, this happens on the approach
to a roundabout that has little to no conflicting circulating traffic, such as a roundabout at an
interchange ramp or any roundabout with a one-way street. The suitable marking for an
approach will depend on the turning volume proportions. A methodology similar to that
described in FDM 11-26-20.4.3 is used to asses lane designation alternatives.
Figure 20.7 Capacity Considerations of Flared Entries
In addition, assessment of the potential for one lane to fill and block back across the flared lane
is necessary to achieve the predicted levels of service, i.e. the geometry must be effective to
match the capacity prediction. Lane starvation is a primary failure mechanism for flared entries.
Paramics has various forms of lane-by-lane simulation features, which allow the analyst to test
alternative lane configurations with visualization of the simulated flows accumulating and filling
the flared lanes. Similarly, for departures, the function is based on the lane’s capacity at the
yield line.
20.6 Capacity Analysis of an Existing Roundabout
The HCM procedure shall also be used to analyze the capacity of existing roundabouts. The headway data at
the existing roundabout should be collected and adjusted in the HCM procedure for roundabouts experiencing
delays. Consult with the Bureau of Traffic Operations for the specifications to collect capacity data at existing
roundabouts. Using the HCM procedure may indicate that the existing roundabout requires additional lanes to
achieve increased capacity; however, depending on the site specific conditions capacity can be added through
changes in pavement markings, signage and geometry. Changes to pavement markings, signage and geometric
parameters are often less expensive and easier to implement than the construction of additional lanes. Sidra in
standard mode can be used, as one of the geometric sensitive tools to determine if geometric changes will
Page 30
increase the capacity of the existing roundabout without adding more lanes. While geometric sensitivity has
been demonstrated to be secondary to driver behavior in influencing the capacity of a roundabout, capacity data
for an in-service roundabout can be collected to calibrate the model’s equation to take advantage of geometric
sensitivity of parameters such as radius at the entry, inscribed circle diameter, conflict angle and flare length.
Rodel can also be used to supplement the analysis of an existing roundabout when calibrated.
20.7 References
[1] NCHRP 572, Roundabouts in the United States, 2007
[2] Akcelik, R., E. Chung, and M. Besley. Roundabouts: Capacity and Performance Analysis. Research Report
ARR No. 321, 2nd ed. ARRB Transport Research Ltd, Australia, 1999.
[3] The Traffic Capacity of Roundabouts TRRL Report LR 942, 1980. Kimber, R.M.
LIST OF ATTACHMENTS
Attachment 20.1
Roundabout Traffic Flow Worksheet
FDM 11-26-25 Access Control
March 4, 2013
25.1 Access Management
Management of access to arterial roads is vital to creating a safe and efficient transportation system for
motorists, bicyclists, and pedestrians. Access guidance is provided through the Region access coordinator,
Chapter 7 of the FDM, and the WisDOT Traffic Impact Analysis (TIA) Guidelines.
The operational characteristics of roundabouts may offer advantages when compared to existing conventional
approaches to access management. Some roundabout benefits include:
- Increased capacity along arterial roads
- Reduction of traffic congestion and delay
- Improved safety
- More efficient use of land
- Savings on infrastructure investments
For example, connecting two roundabout intersections with a raised median will preclude lefts in/out from the
side street or business access to protect main-line capacity and improve safety. U-Turns are not problematic at
roundabouts and can increase safety. This provides the desired capacity protection and safety along the
mainline with less impact to business accessibility.
The preliminary planning phase for any intersection including roundabouts should include a comprehensive
access management plan for the site. Consider the possible need to realign/relocate existing driveways, and
include their associated costs in the project’s preliminary estimate. Account for pedestrian accessibility and
safety during all stages in the development of a comprehensive access management plan.
25.2 Functional Intersection Area
As addressed in FDM 11-25-2, the functional area of an intersection includes the physical area, but also extends
upstream and downstream, along all of the intersection roadways, from the physical area. The functional area
for a roundabout is generally less restrictive due to low speeds and less queuing, when compared to a
traditional signalized intersection. Roundabouts will reduce queuing and minimize the need for exclusive turning
lanes that may be required at a signalized intersection. Also different sight requirements at a roundabout require
drivers to judge gaps at higher perception-reaction time (PRT) than stated in FDM 11-25, Table 2.4. A
roundabout’s functional intersection area should be determined by the length of the splitter island and the
estimated queue length back from the yield line. Use the approved analysis software to analyze the length of
queue as discussed in FDM 11-26-20. Also, consider the sight distance and high speed approach requirements
discussed in FDM 11-26-30.5.15.
25.3 Corner Clearance and Driveway Location Considerations
Corner clearance represents the distance that is provided between an intersection and the nearest driveway.
FDM 11-25-2.5 discusses the four types of corner clearance and corner clearance distances for STHs. Corner
clearance for roundabouts is generally less restrictive than a signalized intersection because a roundabout
reduces speed and queuing. On a case by case basis it may be feasible to consider a full access driveway
closer to a roundabout than would be considered for other types of control, e.g. a traffic signal. There are three
main considerations for driveway location relative to a roundabout entry or exit:
1. Volume of the driveway: If it is only occasional traffic during the peak hour, entering the driveway from
the highway, i.e. a low volume case, there may be no storage required for left turns in advance of the
roundabout. The driveway may be located closer to the roundabout subject to criteria 2 and 3. If the
Page 31
volume entering the driveway from the highway is moderate and the arterial flow impeding the
driveway results in a predicted queue spillback then the queue length must be accounted for in the
driveway location. In cases where a driveway location is downstream of a roundabout exit, there is a
potential for the left turning traffic to back up into the roundabout.
2. Operational impacts of the roundabout (queue spillback from the entry across the driveway opening):
From the queue prediction results generated from the approved capacity analysis software, the
designer can assess how often the entry queue will spill back across the driveway.
3. Sight distance between users: The driveway exit must have proper sight distance of the roundabout
exit, the speed of exiting traffic from the roundabout and to the left of the approaching upstream traffic.
The approach sight to the driveway from the roundabout or approaches to the roundabout must also
meet intersection sight criteria for the approach speeds.
Major commercial driveways may be allowed as one leg of the roundabout. However, installation of a signal or
roundabout strictly for access to private development is discouraged. They may be designed at a public road
access point as an intersecting leg of a roundabout. Moreover, the roundabouts may reduce the need for
additional through-lanes thus narrowing the overall footprint of the roadway system.
Minor commercial and residential driveways are not recommended along the circulating roadway unless
designed as a leg of the roundabout. Some situations may dictate the need for a driveway and must be
analyzed on a case-by-case basis. For a driveway to be located with direct access into the circulatory roadway
of a roundabout, the following items should exist:
- No alternative access points are feasible.
- Traffic volumes are low enough that the likelihood of erratic vehicle behavior is minimal; driveways
with higher traffic volumes, or higher proportion of unfamiliar drivers should be designed as a regular
roundabout approach with a splitter island.
- Drivers must be able to exit facing forward; no backing into the roundabout.
- Driveways may be located along entrances and exits, but need to be set back to not interfere with
pedestrian movements in the crosswalks, and to minimize the number of conflict points with vehicles
approaching or exiting the roundabout. Driveways located along entrances and exits may be blocked
by the splitter island and will have restricted access, (right-in/right-out). Generally, these should be
avoided unless minimal impacts are expected or no other feasible alternatives exist.
25.4 Parking near Roundabouts
Prohibit on-street parking; within 75 feet of the roundabout entry/exit or further depending on site-specific
conditions. Factors that influence the decision to prohibit on-street parking near a roundabout may include:
adjacent access, location of pedestrian crossing, and approach or departing curvature. Generally, it is not
desirable to allow parking on either side of the roadway within the splitter island area or in the transition to the
splitter island.
25.5 Interchange Ramps
According to FDM 11-5-5 a desirable distance of 1320 feet between a ramp terminal and any adjacent
intersection is required. This distance (1320 feet) is typically needed to provide progression for a series of
signalized intersections. Roundabouts need less space between adjacent intersections to operate at a high level
of service. Operational concerns at an interchanges resulting from reduced access spacing, such as traffic
blocking adjacent intersection, can be better understood through the analysis of forecasted queue lengths.
Queue lengths for a roundabout should be predicted with the use of traffic modeling and the impacts to the
adjacent intersections reviewed using other appropriate traffic modeling software. A traffic analysis is required to
justify a less than desirable distance (1320 feet) of access control.
25.6 References
[1] A Policy on Geometric Design of Highways and Streets, AASHTO, 2004.
FDM 11-26-30 Principal Based Design Guidance March 4, 2013
30.1 Introduction
In a general sense, roadway engineering is often an iterative process of design exploration against a set of
project constraints. The geometric design of a roundabout requires the balancing of competing interests. Design
considerations of safety, capacity and cost. Roundabouts operate most safely when their geometry positively
guides traffic to enter and circulate at slow speeds. Poor roundabout geometry has been found to negatively
impact roundabout operations by affecting driver lane choice and behavior through the roundabout. Roundabout
layouts are also governed by the space and swept path requirements of the design vehicle.
Thus, designing a roundabout is a process of determining the optimal balance between safety provisions,
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operational performance, and accommodation of the design vehicle. Even though a step-by-step design process is presented in this section, the designer must understand that adherence to design principles, awareness and understanding of the inherent design tradeoffs are the central points of design regardless of whether any design procedure is followed. The geometric design, signage and pavement markings of roundabout intersections can influence their capacity and operational performance. Therefore it is essential that a roundabout be properly designed to ensure that its expected capacity is not limited by the design.
30.2 Design Principles
This section describes the principles and objectives common to the design of all categories of roundabouts.
Note that some features of multilane roundabout design are significantly different from single-lane roundabout
design, and some techniques used in single-lane roundabout design may not apply to multilane design.
However, several overarching principles should guide the development of all roundabout designs. With the
primary goal of an operationally adequate facility that also provides good safety performance.
The principles that should be applied to achieve a safe and efficient roundabout design are:
- The roundabout should be clearly visible from the approach sight distance at the road operating speed
in advance of the roundabout approach (See FDM 11-26-30.5).
- The number of legs should desirably be limited to four (although up to six may be used at an
appropriately designed roundabout).
- Legs should desirably intersect at approximately 90-degrees, especially for multilane roundabouts
(See also NCHRP 672, §6.3).
- It is essential that appropriate entry curvature is used to limit the entry speed (See also NCHRP 672,
§6.2.1 and FDM 11-26-30.5.2).
- Exits should be designed to enable large vehicles to enter, circulate and depart efficiently either using
a large exit radius or more tangent exits (See also NCHRP 672, §6.2.4). The circulating roadway with
truck apron should be wide enough to accommodate the swept paths of the design vehicle (generally
1.0 to 1.2 times the widest entry).
- Entering drivers must be able to see from the left early enough to safely enter the roundabout.
However, excessive intersection sight distance can lead to higher vehicle speeds that reduce the
safety of the intersection for all road users (motorists, bicyclists, pedestrians). Landscaping within the
central island can be effective in restricting sight distance to the minimum requirements while creating
a terminal vista on the approach to improve visibility of the central island (See also NCHRP 672 and
FDM 11-26-30.5).
- Provide the appropriate number of lanes and lane assignment to achieve adequate capacity, lane
volume balance, and lane continuity to ensure that the roundabout operates at an appropriate level of
service. (See also NCHRP 672, §6.2.2).
- Design such that the driving task is as simple as possible, avoiding the use of spiraled designs unless
it’s clearly warranted by traffic (i.e. high left turning traffic volume).
- Provide smooth channelization that is intuitive to drivers and results in vehicles naturally using the
intended lanes. (See also NCHRP 672, §6.2.3 and FDM 11-26-30.5).
- Design to meet the needs of pedestrians and cyclists. (See also NCHRP 672, §6.2.5)
The design criteria for potential non-motorized roundabout users (e.g., bicyclists, pedestrians, skaters,
wheelchair users, strollers) should be considered when developing many of the geometric components of a
roundabout design. These users span a wide range of ages and abilities and can have a significant effect on the
design of a facility. There are two general design principles that are most important for non-motorized users.
First, slow motor vehicle speeds make roundabouts both easier to use and safer for non-motorized users.
Second, one-lane roundabouts are generally easier and safer for non-motorized users than multilane
roundabouts; therefore, if a single lane roundabout is feasible for most of the design life of the intersection that
has pedestrian traffic then due consideration is given for the sake of pedestrian comfort and safety.
While the basic form and features of roundabouts are usually independent of their location, many of the design
outcomes depend on the surrounding speed environment, desired capacity, available space, required number
and arrangements of lanes, design vehicle, and other geometric attributes unique to each individual site. In rural
environments where approach speeds are high and bicycle and pedestrian use may be minimal, the design
objectives are significantly different from roundabouts in urban environments where bicycle and pedestrian
safety are a primary concern. Additionally, many of the design techniques are substantially different for singlelane roundabouts than for roundabouts with two or more lanes. Maximizing the operational performance and
safety for a roundabout requires the engineer to think through the design rather than rely upon a design
template.
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30.2.1 Designing with Trade-offs in Mind
The selection and arrangement of geometric design elements and their relationships to one another is referred
to as design composition. Minor adjustments in geometry can result in significant changes in safety and/or
operational performance. The relationship between safety and capacity, that exists for a roundabout is in most
cases inverse that of a standard intersection. Table 30.1 below identifies the trade-offs of adding to one element
at the expense of another. When composing an initial layout the tradeoffs of safety, capacity and cost must be
recognized and assessed throughout the design process. The effect of improving one aspect of design impacts
another.
Table 30.1 Effects of Design Elements on Safety and Operations
Safety
Capacity
Speed
Wider entry (gore area)
Less safe
Increase
Increase
Wider Circulatory lanes
Less safe
Better
Increase
Larger entry radius
Less safe
Better
Increase
Larger inscriber circle diameter
Less safe
Better
Increase
Safer
Decrease
Neutral
Poorer sight
Better
Increase
Neutral
Better
Neutral
Element
Larger angle between approach legs
Smaller entry angle (phi)
Longer flare length
30.2.2 Staging and Expandability
Providing excess capacity at standard intersections usually has very little (if any) negative effect on safety or
crash rate and usually improves safety. Inadequate capacity, at a standard intersection could result in reduced safety and increased crash rates. These traits of standard intersections have encouraged traffic engineers to estimate future traffic volumes conservatively high. The process of providing safe roundabouts, for the public, would benefit from conservatively low traffic volume projections, design criteria which requires satisfactory levels of service for ten year or fifteen year projected traffic volumes (with phased designs which allow cost effective expansion, if needed).
The design and analysis process should consider the potential to stage improvements to reduce excessive capacity in the early years and improve safety, and driver/public acceptance. The capacity analysis evaluates the duration of time that for example a single-lane or dual-lane roundabout would operate acceptably before requiring additional lanes. When sufficient capacity is provided for much of the design life of a roundabout, designers should evaluate whether it is best to first construct a roundabout that is easy to convert when traffic volumes dictate the need for expansion and additional capacity. Reducing the number of entry and exit lanes reduces the number of potential conflicts and reduces navigation complexities associated with multilane roundabouts. Minimizing the necessary entry, exit and circulating lanes improves safety for all modes. Pedestrian safety is improved by minimizing the crossing distance and limiting their exposure time to vehicles
while crossing an approach. When considering an interim roundabout that may be converted, the designer should evaluate the right-of-way and geometric needs for both the interim and multilane configurations as part of the initial design exercise. Consideration should also be given to the future construction staging for the additional lanes.
Specific expansion design is a function of many variables. Some situations will dictate that expanding from the inside is more advantageous while other locations may benefit from widening to the outside. For additional information, see NCHRP 672, §6.12.
30.2.3 Impact of Cost Reduction on Roundabouts
In many cases, the process of developing and designing a roundabout involves many design modifications,
which are intended to effect cost savings. While this is common to conventional design practices it can have a
hidden detrimental effect on design and operations of roundabouts.
Landscaping is often considered an aesthetic feature, which can be removed from the plan to reduce cost
savings. Reduction of right-of-way take is often seen as an obvious cost reduction measure but the trade-offs of
safety and operations may not be apparent to the deciding authority. Other elements such as overhead signing
(on approaches) is similarly looked at as excessive and is often replaced with terrace signing, despite the
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rationale that these features improve the function and safety of the intersection. Designers should be sensitive
to the need for cost savings and should strive to effectively document and communicate the impact that the
proposed design modifications will have on the function and safety of the roundabout. The designer should be
given the opportunity to recommend an alternate modification, which will provide required cost savings while
having the minimum amount of impact on function and safety.
30.3 Roundabout Design Process
The process of designing roundabouts may require a considerable amount of iteration among geometric design,
operational analysis, and safety evaluation (refer to Figure 30.1). Minor adjustments in geometry can result in
significant changes in safety and/or operational performance. Thus, the designer often needs to revise and
refine the initial design to enhance the roundabouts capacity and safety. It is not typically possible to produce an
optimal geometric design on the first attempt.
It is advisable to prepare the initial concept drawings at a sketch level detail. It is important that the individual
components are compatible with each other so that the roundabout will meet its overall performance objectives.
Before the details of the geometry are finalized, three fundamental elements must be determined in the Scoping
and Feasibility stage.
1. The optimal size
2. The optimal position
3. The optimal alignment and arrangement of the approach legs
An initial estimate of the space (footprint) required for a roundabout is a common question at the planning stage
and may affect the feasibility of a roundabout at any given location. At this planning level, important questions
may begin to be explored including:
- Is sufficient space available to accommodate an appropriately sized roundabout?
- What property impacts might be expected?
- Is additional right-of-way likely to be required?
- Are there physical constraints that may affect the location and design of the roundabout?
Due to the need to accommodate large trucks through the intersection, roundabouts typically require more
space than conventional intersections. However, this may be offset by the space saved compared with turning
lane requirements at alternative intersection forms. The key indicator of the required space is the inscribed circle
diameter.
There are no easy ten-steps to roundabout design. Much of the knowledge in roundabout design is counterintuitive to the technically minded engineer. Designing roundabouts can range from easy to very complex.
Although it may appear inherently otherwise and extensively attempted, roundabouts are not homogeneous and
cannot be standardized. There are many different types of roundabouts, such as single lanes, two-lanes, threelanes, circles, ellipses, bypass lanes, “snagged” partial bypass lanes, double roundabouts, spirals, etc., in which
a number of combinations or multiple combinations of the above can be in one roundabout (See Figure 1.2).
Each roundabout is unique where each potential “type” of roundabout is applied in different situations in which
site-specific problems require special and distinctive solutions. The major differences in design techniques and
skill levels fall between single-lane roundabouts and multilane roundabouts where different principles apply.
Figure 30.1 depicts the steps and process that guide a designer through the entire Roundabout Design Process
(see also NCHRP 672, Exhibit 6-1).
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Figure 30.1 Roundabout Evaluation & Design Process
30.4 General Design Steps & Explanation
The following general design steps will typically apply to most roundabout design practices. However, each
roundabout requires a different design and thinking process depending on the unique design constraints, traffic
volumes, roadway speeds, existing topography, and geometric alignments of the roadways. Not all aspects of
design or the design process are included herein, however, the provided general design steps should be
sufficient to get most designers started in an initial conceptual roundabout design.
Step 1 - Document Existing Conditions
Review the most recent site plans and roadway alignment information in an electronic format
(e.g. CAD-based software). Review existing roadways with respect to surrounding topography,
centerlines, curb faces, edge of pavement, roadway lane markings, existing or proposed bike
lanes, nearby crosswalks, environmental constraints, buildings, drainage structures, adjacent
access points, shared-use paths, rail crossings, school zones, and right of way constraints. This
should include any special design constraints such as specific properties that cannot be
encroached or specific desired lane widths. Review any traffic study, which should include final
future design year traffic volumes and assumptions of the proposed intersection or corridor
project.
These items should provide adequate background traffic conditions, existing traffic conditions
within and outside the project area, as well as the level of detail, design parameters, right-of
way constraints, restricted historical or wetland areas, and location for the proposed
roundabout.
Step 2 - Document Future Conditions
The future traffic flows of the existing roadways should be reviewed and possibly discussed with
the lead jurisdiction for project understanding and existing operational issues. These operational
issues, including potential excessive delay, should be recognized in the design process and
geometric criteria. In addition, any potential changes to adjacent sites, access points, or
roadway cross-sections that may affect the roundabout design should be provided, reviewed,
and incorporated. Review the future AM & PM peak-hour, and off-peak turning movement volumes (also include mid-day in tourist areas) at the intersection developed from the design year projected traffic
volume data. Use the Traffic Flow Worksheet in FDM 11-26 Attachment 20.1 and a simple
schematic diagram consisting of the final future peak hour turning movement volumes at the
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intersection(s). In order to accurately identify the roundabout geometric and capacity needs, the
following are required:
- Traffic Conditions
- Future turning movement volumes: AM & PM peak, off-peak, and mid-day (in tourist
areas)
- Future percent heavy vehicles (by type and approach) for each peak hour
- OSOW freight network considerations
- Design vehicle type by turning movement (i.e. legal large vehicles: WB-65, WB-92
(formerly WB-67 Long), farm combine and an 80 foot mobile home transport; and,
where applicable based on STH routing, OSOW vehicles – all 8 vehicle
combinations)
- Constraints
- Vertical constraints
- Right-of-way constraints
- Existing and proposed roadway alignment base map (with travel lanes, proposed
curb tie-in, pavement marking, bike lanes, right-of-way, etc.)
- Other Modes
- Pedestrian volumes (if significantly high)
- Identify if bike lanes and sidewalks will be needed
Step 3 - Understand the Specific Design Problem(s)
Prior to commencing a design, the designer must first understand the basic intersection
problem; is it safety, congestion, or a combination of both and what is the design problem(s) to
be solved (right-of-way issues, acute angles, grades, approach legs, roadway alignment, etc.)?
After evaluating the traffic volumes, the designer should have an understanding of how many
lanes may be initially required.
A general roundabout diameter can then be chosen based on the traffic needs, proximity to
constraints, design vehicle, and the relative speeds of the roadways (i.e. if high speed
approaches present). The designer must be conscious of the design vehicle when choosing a
diameter. Refer to FDM 11-25 Table 3.1 as a first step in the evaluation process if no other
values have been stated.
Step 4 - Perform Capacity Analysis for Lane Configuration Development (also refer to FDM 11-26-20)
After obtaining all of the pertinent information regarding the roadways, site, and traffic volumes,
and a general roundabout diameter has been initially identified, the designer should perform a
geometric analysis of the proposed roundabout using roundabout design software. Refer to
FDM 11-26 Attachment 20.1 Traffic Flow Worksheet to assist with traffic volume data entry. The
capacity analysis results will assist in developing the initial lane geometry and capacity requirements for the roundabout based on the future design volumes.
This will set the design requirements for the conceptual roundabout design. The AM and PM, and sometimes a weekend peak, traffic volumes will need to be analyzed at the intersection.
This analysis should ensure that the roundabout will operate appropriately under all peak hour
traffic conditions. The final results of this analysis will produce key information to include in the
roundabout design, some of which are:
Geometry
Operations
Initial roundabout diameter
- Future traffic volume capacity by approach
(estimated size)
- Delay of each approach and the overall
- Entry lane configurations at each
delay of the intersection
approach
- Predicted 95th percentile queue lengths for
each approach
- Minimum approach widths and
- Future level of service
entry radii of the roundabout
The allowed movements assigned to each entering lane are key to the overall design. Basic
pavement marking layouts should be considered integral to the preliminary design process to
ensure that lane continuity is being provided. In some cases, the geometry within the
roundabout may be dictated by the number of lanes required or the need to provide spiral
transitions (see FDM 11-26-30.5.22 for more information). Lane assignments should be clearly
identified on all preliminary designs in an effort to retain the lane configuration information
-
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through the various design iterations. In some cases, a roundabout designed to accommodate
design year traffic volumes, typically projected 20 years from the construction year, can result in
substantially more entering, exiting, and circulating lanes than needed in the earlier years of
operation. To maximize the potential safety during those early years of operation, the engineer
may wish to consider a phased design solution that initially uses fewer entering and circulating
lanes. As an example, the interim design would provide a single-lane entry to serve the nearterm traffic volumes with the ability to cost-effectively expand the entries and circulatory
roadway to accommodate future traffic volumes. To allow for expansion at a later phase, the
ultimate configuration of the roundabout needs to be considered in the initial design. This
requires that the ultimate horizontal and vertical design be identified to establish the outer
envelope of the roundabout. This method helps to ensure that sufficient right-of-way is
preserved and to minimize the degree to which the original roundabout must be rebuilt.
Step 5 - Sketch
Once the minimum design requirements have been established, a modern roundabout design
can be sketched by initially identifying the flow of traffic, lane configuration, and approach lane
assignment requirements, the circulatory roadway width and the exits of the roundabout. This
task includes the placement of the roundabout’s circle to roughly determine its location. Special
consideration should be taken for any skewed intersection or right-of-way constraints. A general
roundabout diameter can then be chosen based on the traffic needs, proximity to constraints,
design vehicle, and the relative speeds of the roadways.
Step 6 – Refine the Initial Layout
The hand sketch or initial conceptual layout should be refined. The designer should refine the
concept iteratively to suit the site constraints while attending to the design performance criteria
of speeds, truck space and site distance. The purpose of this process is to achieve an optimal
layout that serves the design objectives without excessive CAD effort. Often designers are
wrongly focused on details and do not have the patience to produce multiple iterations of a CAD
design.
Step 7 - Formalize the Preliminary Design
Once the general location and roundabout configuration has been developed and all of the
design issues have been resolved, a full conceptual design can be initiated.
In multilane designs, the lane pavement marking is applied to establish natural entry and exit
paths, i.e. to minimize entry and exit path overlap. Applying the lane pavement marking ensures
proper lane widths and widening, and confirms the lane designations and possible spiraled lane
movements.
Step 8 - Safety and Fastest Path Review
Fastest path design speeds as well as a number of other safety factors and design features,
such as the phi angle, must be checked. The fastest paths should be developed and reviewed
to see if they are adequate and reasonable. If deficiencies or deviations in any of the design
features or safety factors are found, the design must be modified, either with many small
changes or by shifting alignments, geometry, or placement of the circle. This is an iterative
process which may require an entire redesign.
Step 9 - Design Vehicle Check & Modifications
A CAD-based software program such as AutoTURN or AutoTrack should be used to verify
proper accommodations are provided through the roundabout for each approach and every
truck turning movement. In addition, the truck apron minimum width is 12-feet and may be wider
in some situations to better accommodate OSOW vehicles. All truck movements should have a
buffer space between the swept path of trucks and the face of curb equal to 2 feet. Contact the
Regional Freight Operations Unit for the OSOW vehicle.
Step 10 - Accessorize the Design
When a preliminary design (and pavement marking for multilane roundabouts) has been
completed, additional amenities should such as crosswalks, detached sidewalks, bike paths and
ramps, truck aprons, disabled access (ADA) ramps, etc. should be added. All efforts should be
made to avoid any right-of-way issues.
At the 30% stage of the design process, some form of approval or review consultation should be
performed by a qualified designer. Once a roundabout design has been properly designed with
respect to horizontal geometry, there are many other geometric and non-geometric design
components that must now be completed in order for a roundabout to function as it was
designed. These design components are key to the public driving the roundabout as it was
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intended without further safety or operational issues. These items are identified in the three
stages of the design process (Figure 30.1).
The 60% and 90% design aspects of roundabout design including horizontal geometry, vertical
profiles, signing, pavement marking layout, landscaping, lighting, and construction materials
should either be designed by or reviewed by a qualified roundabout designer. Nothing can
replace real-world design and field experience.
Continual practice, mentoring from experts, training & education, and quality roundabout review greatly assists
the designer in understanding all aspects of the design of modern roundabouts. However, all designers must
spend time in the field reviewing roundabout construction and completed roundabouts in order to understand
roundabouts and their design completely. After years of daily practice, one can still learn. Small changes in
roundabout design elements can influence the operation and safety of a modern roundabout.
30.5 Design Considerations
This section provides guidelines for each geometric element. Further guidelines specific to two-lane entries are
provided in the latter part of Chapter 6 of NCHRP 672. Note that two-lane entry roundabout design is
significantly more challenging than one-lane entry design. Many of the techniques used in one-lane entry
roundabout design do not directly transfer to multilane design. This procedure provides recommended changes
to NCHRP 672, Chapter 6. Therefore, designers must become very familiar with Chapter 6 in the NCHRP 672.
30.5.1 Alignment of Approaches and Entries
Adherence to the principles of deflection is crucial to the operation and safety of roundabouts. WisDOT
considers this design element to be of the utmost importance. Figure 30.2 shows the typical composition of
approach alignment and curves to generate desired speed reduction at entries. It is not good practice to
generate entry deflection by sharply curving the approach road to the left close to the roundabout and then to
the right at entry.
It is recommended design practice (especially in multilane roundabouts) to provide an offset to the left of the
center of the central island. In some situations it may be appropriate to provide an offset of approximately 20 to
30 feet (or more), left of the center of the roundabout to achieve proper deflection and appropriate entry speeds.
For additional information, see NCHRP 672, §6.2.1 & 6.7.1.
Figure 30.2 Entry Deflection
30.5.2 Assessing Vehicle Paths
Determine the smoothest, fastest path (using a spline curve) possible for a single vehicle, in the absence of
other traffic and ignoring all lane line markings, traversing through the entry, around the central island, and out
the exit. A step by step process for creating a AutoCAD Civil 3D and MicroStation spline curve are provided in
FDM 11-26, Attachments 50.1 and 50.2. Usually the critical fastest path is the through movement; but,
depending on the angle between arms, in some situations it may be a right turn movement.
Fastest speed path is a critical performance measure in the design of roundabouts. Use NCHRP 672, Exhibit 6
46 for the definition of vehicle path radii. NCHRP 672 Exhibit 6-48 and Exhibit 6-49 illustrate the definition of
fastest vehicle path for single-lane and multilane designs. Use Figure 30.3 to determine the radii values for R1
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based on the arc and spline definitions. Vehicle speed estimation is in accordance with NCHRP 672, Section
6.7.1.2 Equations 6-1 and 6-2. Equation 6-3 may be used to estimate actual entry speed, but it will not govern
the design.
R2 and R4 are determined using the same vehicle path offsets for R1. The R3 exit radius fastest speed path is
determined based on the R2 speed plus acceleration over the distance to the point where R3 is measured. Use
NCHRP Exhibit 6-50 to determine the radius value for R5 fastest speed path. The vehicle path offsets of 5 feet,
as shown in Figure 30.3, are measured from the curb face (not the flange line). In the situation where the
approach to the roundabout has centerline pavement marking on the left side with no curb face, then the offset
is 3 feet from the centerline pavement marking.
a. The radius should be measured over a distance of 65 to 80 feet. It is the minimum that occurs along
the approach entry path near the yield point but not more than 165 feet in advance of it.
b. The beginning point is 3 feet from a pavement marking (if no raised median), or 5 feet from the left
curb face (if raised curb median) at a point approximately 165 feet from the yield line. This point is a
continuation of a vehicle path spiraling from tangent to a curve, not a point with deflection.
c. Vehicle entry path curvature.
Figure 30.3 Determination of Entry Path Curvature (See NCHRP 672 Exhibit 6-49 for multilane entries and Exhibit 6-50 for right turns)
The radii described in Table 30.2 are used to define the fastest path through a roundabout. They are illustrated
in Exhibit 6-12 of NCHRP 672.
Table 30.2 Roundabout Radii
Radius
Description
Range of Speeds
Entry Path Radius, R1
The minimum radius on the fastest
through path prior to the yield line. This is
not the same as Entry Radius.
Single Lane 20 to 25 mph*
Circulating Path Radius, R2
through path around the central island.
Exit Path Radius, R3
through path into the exit.
Left Turn Path Radius, R4
The minimum radius on the path of the
conflicting left-turn movement.
10 to 20 mph
Right Turn Path Radius, R5
The minimum radius on the fastest path
of a right-turning vehicle.
15 to 20 mph*
Multilane 25 to 30 mph*
15 to 25 mph
R2 + Acceleration over the path to the
exit crosswalk*
* Notes: Under conditions where sufficient numbers of pedestrians are present, desirable values of fast path speeds
should be lower than maximum values shown in the table. Check the design speed control of sensitive designs that
may have high entering or circulating speeds or where the pedestrian activity is anticipated to be medium to high,
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check for a conservative design by determining the fastest speed paths using a 3.28 ft (1 m) offset to each of the
critical controlling feature locations (i.e. raised curb face on the approach and exit median, curb face at the central
island, or centerline pavement marking between opposing traffic).
30.5.3 Speed Consistency
In addition to achieving the appropriate design speed for the fastest path movements, the relative speeds
between consecutive geometric elements should be minimized as well as between conflicting traffic streams.
Ideally, the relative differences between all speeds within the roundabout will be no more than 10 to 15 mph.
Typically, the R2 values are lower than the R1 values. With either single or multilane entries, R2 values should
be lower than the R3 values.
The desirable maximum R1 radius is 250 ft. Generally for urban roundabouts with pedestrian accommodations a
lower speed entry is desirable. A typical R1 may range between 150 and 230 feet. Rural roundabouts typically
allow slightly higher entry speed than urban roundabouts. The R1 and R2 should be used to control exit speed.
Typically, the speed relationships between R1, R2, and R3 as well as between R1 and R4 are of primary
interest. Along the through path, the desired relationship is R1> R2< R3, where R1 is also less than R3.
Similarly, the relationship along the left-turning path is R1> R4.
For most designs, the R1 - R4 relationship will be the most restrictive for speed differential at each entry.
However, the R1 - R2 - R3 relationship should also be reviewed, particularly to ensure the exit speed is not
overly restrictive. Design criteria in past years advocated relatively tight exit radii to minimize exit speed; recent
best practice suggests a more relaxed exit radius for improved drivability.
30.5.4 Design Guidance for all Trucks
WisDOT is a transport friendly state, and accommodates not only for the standard large legal size trucks, but
also the OSOW vehicles that use our highways. The standard design vehicle for the STH system in Wisconsin is
the WB-65. Additionally, intersections of two state trunk highways and state highways that make an abrupt turn
at an intersection should accommodate these check vehicles, the WB-92 (formerly WB-67 Long), farm combine,
and 80 foot mobile home transport vehicles providing these vehicles could make the various turns while staying
inside the curb face prior to the proposed improvement. If the existing intersection geometrics did not
accommodate a Multiple-Trip (OSOW MT) permitted vehicle by staying within the curb face prior to the
proposed improvement then it does not have to accommodate that vehicle after the improvement is completed.
The designer should try to design the proposed intersection such that all the OSOW MT permitted vehicles will
be accommodated within the curb face. See FDM 11-25-2 for additional information on OSOW MT permitted
vehicles and Single Trip (OSOW ST) permitted vehicles. The location of the regional OSOW Freight Network
maps are located at: http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf
1. Slope truck apron at 1% toward the roadway on all roundabouts. In order to ensure that light vehicles
encounter sufficient entry deflection at normal roundabouts, a truck apron (i.e. a raised low profile area
around the central island) is necessary. It should be capable of being mounted by the trailers of large
goods vehicle, but unattractive to cars and SUVs.
2. The truck apron width is a minimum of 12 feet wide on single lane, as well as, multilane roundabouts.
Sometimes additional space is needed for trucks to off-track onto the truck apron that may exceed the
12 foot width.
3. Widen the truck apron as needed to accommodate the anticipated OSOW turning maneuver. Discuss
with the Regional Freight Network coordinator.
4. Roundabouts must have the recommended circulatory roadway crown installed, 2/3 inward and 1/3
outward on all roundabouts, not just those on the OSOW Freight Network. Refer to Figure 30.8 for
cross-section clarification.
5. Install a 4-inch sloped Type A/D curb and gutter on the outside of the approach where large vehicles
may off-track onto the curb, and when necessary install an outside concrete pad .
6. Install an 8-inch, reddish-colored concrete pad behind the back of curb along the outside entrance
area where off-tracking is anticipated. The slope of the pad should be a maximum of 1%. Evaluate the
entrance for pedestrian crossings and placement of the concrete pad to prevent these areas from
overlapping. The width of this pad will depend on the amount of off-tracking anticipated. The same 8
inch reddish colored concrete pad, without stamping, should be installed in the splitter islands where
OSOW vehicles may drive to negotiate the roundabout.
The following items are a reminder for additional roundabout design guidance:
- Keep drainage structures away from the travel path of the possible OSOW vehicle wheel tracking.
- The compaction levels under the 8-inch concrete pad along the back of curb near the entrance and in
the splitter island areas must be equal to the compaction levels under the roadway and truck apron.
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- With the wider 12 foot minimum truck apron required by WisDOT for single-lane and multilane entries,
it is rare that additional intersection sight distance is needed directly in back of the curb on the inside
of the truck apron. If a central island landscape buffer area located adjacent to the back of the most
inside curb and gutter is desired, avoid the use of hard surfaces that look like concrete sidewalk.
- 2% cross-slope is the maximum in the roadway area.
- Avoid approach vertical break-over grades over 3% within 200 feet of the entry yield line location.
- Provide a note to the construction engineer that the plans, including the vertical and horizontal design,
shall not be adjusted in the field without the design engineer’s approval.
- Refer to FDM 11-26-35.1.12 for guidance on removable signs at roundabouts.
- For the roundabouts located on the OSOW Freight Network, their grading plans should be verified with
3D design software for any conflict points. The tractor should be placed 100 feet back from the yield
line.
- Produce a swept path diagram showing the vehicle movements and directions for the purpose of
supplying the permitting office with diagrams to aid route choice.
30.5.5 Geometric Design Guidance for Legal Trucks
The inscribed circle diameter, the width of the circulatory roadway and the central island diameter are
interdependent. Once any two of these are established, the remaining measurement can be determined.
However, the circulatory roadway width, entry and exit widths, entry and exit radii, and entry and exit angles also
play a significant role in accommodating the design vehicle and providing deflection.
In all cases, the designer will test swept paths and iterate through combinations of circle size and lane widths. A
recent roundabout design study identified three cases or categories for accommodating trucks. Case 1, Case 2,
and Case 3 categories are determined by a number of factors, primarily whether a truck can stay in lane or not,
as explained below.
Roundabouts are designed with a truck apron. Truck drivers that use the inside lane are expected to off-track
onto the truck apron. Regardless of the case category the outside lane of a dual lane roundabout is typically
wider than the inside lane to better accommodate trucks. Multilane roundabouts can be designed in three
different ways to accommodate legal size large trucks. Three categories of design for legal trucks have been
identified as Case 1, Case 2, and Case 3:
- Case 1:
Roundabouts which are designed to allow trucks to encroach into adjacent lanes as they approach,
enter, circulate, and exit the intersection. Refer to Figure 30.4 for an example of a Case 1 design.
- Case 2:
Roundabouts which are designed to accommodate trucks in-lane as they approach and enter the
roundabout, but may require trucks to encroach into adjacent lanes as they circulate and exit the
intersection. Case 2 roundabouts have a painted “gore” area between lanes on the approaches. Refer
to Figure 30.5 for an example of a Case 2 design.
- Case 3:
Roundabouts which are designed to accommodate trucks in-lane as they approach and traverse the
entire intersection. Case 3 roundabouts have a painted “gore” area between lanes on the approaches.
Case 3 roundabouts typically are designed to allow trucks to stay in lane for through and left turning
movements, while right turning trucks may occupy multiple lanes as they exit. With few Case 3
roundabouts implemented to date, these designs typically require significantly more designer skill than
other case types to ensure proper operations, geometrics, speeds, and safety. Refer to Figure 30.6 for
an example of a Case 3 design.
Well-designed Case 2 and Case 3 roundabouts do not compromise accepted design principles, as outlined in
this chapter. Tables 30.3, Table 30.4, and Table 30.5 show the advantages and disadvantages of Case 1, Case
2, and Case 3 roundabout designs.
Table 30.3 Advantages and Disadvantages for Case 1 Roundabout Designs
Advantages
Disadvantages
Wide variety of approach alignment design methods
can be used
May result in increased delays due to trucks occupying
both lanes on entries and while circulating
More likely to fit in tight right-of-way locations, including
built-up urban environments
Trucks may off-track over outside curbs, resulting in
more damage and maintenance
Potentially lower costs in some situations
May result in additional truck-car crashes
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Less pavement marking maintenance
Advantages
Surveys indicate this entry design is preferred over
Case 1 by truck drivers
Safety benefits at entries due to no truck encroachment
Potentially less damage to curbs
Trucks can maneuver more freely at entries
Disadvantages
Fewer approach alignment design methods can be used
May require geometry with more right-of-way
Potentially higher cost in some situations
May require more pavement marking maintenance
May have greater entry capacity/less delay
Slightly higher circulating speeds and worse lane
discipline possible
Can be used in urban or rural environments
Requires greater designer and contractor skill
May have greater public acceptance
Poor design could result in more crashes
Possibly lower safety in circulatory roadway due to truck
encroachment
Advantages
Disadvantages
Surveys indicate this design is preferred by truck drivers
and the trucking industry
Fewer approach alignment design methods can be used
May require larger geometry with more right-of-way
Safety benefits at entries and in circulatory roadway
due to no truck encroachment
Potentially higher cost in some situations
Less damage to curbs
May require more pavement marking maintenance
Trucks can maneuver more freely at entries and in the
circulatory roadway
Slightly higher circulating speeds and worse lane
discipline possible
May have greater entry capacity/less delay
Requires greater designer and contractor skill
Can be used in urban or rural environments
Poor design could result in more crashes
Better operations in the circulatory roadway
No truck/trailer encroachment required for turning
movements - more lateral clearance
May have greater public acceptance
Case 3 design is a priority where practical and feasible and there are approximately 100 large trucks
(forecasting classification 3S2) per day using the intersection. In general, it is believed that a well-designed
Case 3 roundabout which meets applicable geometric design requirements will provide safe and efficient
operations while providing optimal truck accommodations. Where costs or right-of-way impacts are prohibitively
expensive or at locations where design truck numbers are very low, other design case types may be more
advantageous.
Certain specific locations should warrant additional consideration of a Case 3 design. These would include
locations where designated OSOW routes exist, multilane approaches on arterial routes, at interchange ramps,
near truck stops, and in industrial/warehouse districts. If a Case 3 is an alternative based on large truck
numbers but there are serious adverse impacts, such environmental, historic, real estate or other impacts then
evaluate a Case 2. Consider a step-down approach to the evaluation process and perhaps a Case 1 is all that
will fit into the intersection without serious adverse impacts. In the end evaluate the selected roundabout case
number option appropriate for the intersection and compare it to the other intersection alternatives such as a
signal, or other type.
In the case of three lane entries, off-tracking is assumed to overlap lane lines. If high volumes of large trucks are
present and capacity is a concern, a painted gore width of 4 to 6 feet may be placed between the right two
lanes.
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Table 30.6 depicts typical design parameters for each of the three design cases. Refer to FDM 11-25-1.4, FDM
11-25-2 and FDM 11-26-10.2 for additional information on OSOW routes and vehicles.
Table 30.6 Typical Design Parameters for Two-Lane Roundabouts*
ICDA
Case 1 - No lane discipline
entering or circulating
Case 2 – Lane discipline
entering only
Case 3 – Lane discipline
entering and circulating
150-190 ft
160-210 ft
180-220 ft
Inner Circulatory Lane
B
Width
11-13 ft
12-14 ft
Outer Circulatory Lane
WidthB
13-15 ft
15-18 ft
Approach Gore Widths
A
Entry Width
Not used
2-6 ft
4-8 ft
28-32 ft
32-34 ft
32-34 ft
Entry Radius
Controlling Radius
65 ft or greater
65 ft or greater
65 ft. or greater,
100-130 ft. typical
Controlling Radius Length
No MAX.—typically 70 ft or
less
No MAX.—typically 80 ft or greater
Entry Angle (measured per
FDM 11-26-30.5.2)
16-30 degrees
Flared Entry Lane Addition
> 100 ft
(based on 95%ile Queue)
Generally 100 ft to 300 ft
Exit WidthsA
28-32 ft
28-32 ft
(where large radius or
tangential exit is used)
* Based on site conditions, right-of-way constraints, specific design vehicle, and other factors, designers
may choose to implement geometries outside these recommended ranges; however the overall design
should comply with WisDOT general roundabout design practices
A
Measurements are from the face of curb to face of curb, (includes 2-ft gutter pans on each side)
B
Measurements are from flange line to lane line
30.5.5.1 Geometric Design Guidance for Case 1 Roundabouts
Case 1 roundabouts are designed with a single solid white paint line dividing the entry lanes. Trucks encroach
on adjacent lanes at the approaches and when circulating and exiting the roundabout. Designers should
consider implementing features that would result in a clear encroachment by trucks into adjacent lanes rather
than a subtle encroachment (such an approach would typically include avoiding wide lanes, long sweeping
curves, large ICDs, and large radii).
Additionally, Case 1 designs can allow for the approaching roadways to have more tangential alignments with
short, tighter entry radii. In some rare Case 1 design locations, implementing outside curb truck aprons (i.e., a
sloped/mountable curb with a concrete/pavement area behind the curb) may be beneficial to repair and prevent
rutting behind the entry radius curb, curb damage or damage to signs and landscaping from truck off-tracking.
The implementation of outside truck aprons in new designs is discouraged due to potential concerns about
pedestrian safety and optimal operations. As such, designers should not typically consider outside truck aprons
as a preferable option when sidewalks or shared-use paths are present. The width of this apron should be
determined through the use of software that generates swept paths for trucks. Figure 30.4 shows the basic
design features of a Case 1 roundabout.
A sub-option for Case 1 designs is to use a short flare from a single lane approach to a two-lane entry. With
approximately a 100 foot flare, the design may be acceptable without the gore pavement marking. If the flare is
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long, e.g. approaching 250 feet to 300 feet, then a Case 2 design with the gore area between lanes would be
desirable.
Figure 30.4 Case 1 Roundabout Design (Single lane line dividing the entry lanes)
30.5.5.2 Geometric Design Guidance for Case 2 Roundabouts
Once the primary design principles from this guidance have been met (speed control, sight distance, adequate
space for a design vehicle), the designer will typically revise the design iteratively to allow trucks to stay in lane
at the entry while still maintaining the primary design. Although there are some specific design characteristics
which are unique to Case 2 roundabouts, the overall approach, methods, and iterative design process remain
the same as multilane roundabouts in general.
Case 2 roundabout ICDs are typically 10-20 feet smaller than for Case 3 roundabouts. Designers must maintain
appropriate fastest path entry speeds and speed differentials between entering and circulating traffic. Figure
30.5 shows the basic design features of a Case 2 roundabout.
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Figure 30.5 Case 2 Roundabout Design (6-ft gore pavement marking between lanes)
30.5.5.3 Geometric Design Guidance Common to Case 2 and Case 3 Roundabouts
1. Often have slightly wider entries (typically 2 to 6 feet wider) than a comparable Case 1 roundabout at
the same location. For example, a Case 1 roundabout may have an entry width of 28 to 32 feet
(including gutter pan width) wherein a typical Case 2 or 3 roundabout could increase the entry width to
about 32 to 34 feet (including gutter pan width and gore pavement marking area) to allow trucks to
stay in lane in entry.
2. Usually have longer curve lengths than Case 1 roundabouts on the approach geometry and within the
entries. Offset left alignments (i.e., alignment directed to the left of the center of the ICD) are generally
preferred where possible.
3. Should avoid tight entry radii curves and closely spaced curves in opposite directions. Instead, larger,
longer radii with straight tangent sections between curves are common at Case 2 and 3 roundabouts,
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4.
5.
6.
7.
resulting in gradual sweeping curvature which makes it easier for trucks to stay in lane. Optimal entry
radii values will vary based on the ICD, approach alignment, and entry design method. Typically, an
urban Case 2 or 3 design may have a controlling curb radius value of 100 feet or greater, while a
larger rural Case 3 design may range as high as 120 feet or more (note: per definition above,
controlling radius is not the same as the R1 radius). Regardless of the actual values (which are site
specific), the designer still must maintain other design requirements such as appropriate fast path
speeds, while still accommodating for trucks in-lane. Considerable designer skill is typically needed to
accomplish these competing objectives.
Use of width transitions. With Case 2 and 3 roundabouts relatively long width transitions may be
needed to allow trucks to use more roadway width to stay in lane. Designers should ensure that the
total length of the combination of the taper and the second full lane width utilized accommodates the
design truck as well as queuing and capacity needs. Not including the gore area between entry lanes,
the lanes should typically have continual tapers between the normal width upstream location and the
entry, (Figure 30.5 and FDM 11-26-35.2.1), and at no point should lane widths become narrower over
this distance. The design of the gore area may require variable widths, including narrowing toward the
entry as needed.
A slightly wider entry width than usually provided at Case 1 roundabouts. The designer should keep
the entry width as narrow as possible while still allowing trucks to stay in lane. Total two-lane entry
width should typically not exceed 34 feet (from curb face to curb face, including painted gore area)
unless special circumstances are present. Lane widths at the entry typically vary from 12 to 14 feet,
not including the two-foot gutter or gore area.
The relationship between width transitions, entry widths, lane widths, and gore widths should be
carefully considered by the designer when determining how to optimally serve trucks and passenger
vehicles. As a general principle, widths should be minimized while still accommodating the design
truck.
Typically, a Case 1 design would have a controlling radius value of 65 feet or greater, while a more
common range is 100 to 130 feet for Case 2 and 3 designs.
30.5.5.4 Additional Geometric Design Guidance for Case 3 Roundabouts
The Case 3 design is preferred as the initial consideration at intersections that experience 100 or more large
trucks (traffic forecast classification 3S2) When preparing a Case 3 design, once the primary design principals
from this guide has been met (speed control, sight distance, adequate space for a design vehicle), the designer
will typically revise the design iteratively to allow trucks to stay in lane at the entry and circulating road while still
maintaining the primary design principles. Although there are some specific design characteristics which are
unique to Case 3 roundabouts, the overall approach, methods, and iterative design process remain the same as
multilane roundabouts in general.
Overall, Case 3 roundabouts embody similar geometric characteristics as Case 1 and 2 roundabouts. However,
there are specific geometric elements where Case 3 roundabouts differ from Case 1 and 2 designs.
1. The outside circulating lane is often in the range of 15 to 18 feet (from edge of gutter flange line to
lane line). Inside lanes range from 13 to 15 feet (from edge of central island gutter flange line to
nearest lane line).
2. Usually include relatively large or flat exit radii which allow trucks to depart from the circulating road
with minimal curvature to the right, thus allowing them to stay in lane more easily. Case 3 roundabouts
may have larger ICDs in some situations where a double left turn is required. This type of design may
be quite complex. Figure 30.6 shows the basic design features of a Case 3 roundabout.
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Figure 30.6 Case 3 Roundabout Design (6-ft to 8-ft gore pavement marking between lanes)
30.5.6 Vertical Considerations for OSOW Vehicles
Prior to the preliminary design, check with local officials and the public to determine if there are any special
OSOW vehicles that regularly use the intersection and refer to the WisDOT OSOW vehicle inventory in FDM 11
25, Attachment 2.1.
If a roundabout is located on the OSOW Freight Network or know the OSOW vehicles may use the intersection,
conduct a vehicle horizontal turning and a low vertical clearance check with the OSOW vehicle inventory (the
regional OSOW Freight Network maps are located:
http://dotnet/dtid_bho/extranet/maps/docs/freightnetwork.pdf).
AutoTurn or AutoTrack software may be used for the horizontal checks. AutoTURN Pro may be used for
horizontal analysis and is required to determine if low vertical clearance conflict points are present. Use a low
clearance of 5 inches for the DST lowboy evaluation. If clearance issues are found, reconfigure the slopes within
the conflict areas and check the surrounding area (i.e. approaches) for additional conflict points. Refer to Figure
30.7 for typical ground clearance problem areas.
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Figure 30.7 Typical Ground Clearance Problem Areas
1. Off‐tracking at the entry curve/lowboy hitting the outside curb head
a. Consider 4‐inch sloped Type A/D curb and gutter with additional 8-inch concrete pavement
behind the back of curb along the outside entrance area. The slope of the pad should be a
maximum of 1%. Evaluate the entrance for pedestrian crossings and placement of the
concrete pad to prevent these areas from overlapping.
2. Entry and exit rollover
a. Consider flattening the circulatory roadway crown in these areas if needed, while providing
approximately 2/3 sloped inward and 1/3 sloped outward.
b. Avoid break-over grades over 3% within 200 feet of the entry yield line location and exiting the
roundabout
3. Truck Apron
a. Slope truck apron 1% toward the roadway on all roundabouts (not 2% as in the past).
Consider a pill shaped central island or other shape where appropriate to accommodate the
anticipated OSOW turning maneuver.
i. See if the vehicle can track more on the circulatory roadway. In rare situations the
designer may consider a 3 inch height R/T type curb and gutter. This will require
an evaluation of the inlet casting height/location (out of the vehicle path) and will
require a C & G special detail.
b. Look at the circulatory roadway profile
i. Keep it as flat/gentle as possible and still maintain drainage (0.75% - 1.0%)
ii. Locate the crest away from the area(s) of concern
Figure 30.8 Cross-Section Example
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In some cases, abnormally long vehicles may not be able to negotiate roundabout regardless of geometric
adjustments to the truck apron and approaches when making left turns. In some cases, special median
crossings may be required, which allow the vehicle to bypass the circle portion of the roundabout by traveling
the opposite direction down a right turn bypass. Such maneuvers should be avoided, if possible, due to the extra
planning required for escorting a vehicle in such a maneuver. Discuss such alternatives with the Regional Traffic
Section and the OSOW Freight Network coordinator and document route testing produced by turn analysis
software for future use by the OSOW Permitting Unit.
30.5.7 Overturning Considerations for Large Vehicles
A further consideration associated with large trucks in roundabouts is the potential for overturning or shifting of
loads. There is no simple solution in relation to layout geometry to completely prevent load shifting and rollovers. Experience suggests that at roundabouts where these problems persist, there are frequently
combinations of the following geometric features:
- Long straight high speed approaches
- Inadequate entry deflection or too much entry deflection
- Low circulating flow combined with excessive visibility to the left
- Significant tightening of the turn radius partway around the roundabout (spirals with arcs that are too
short).
- Cross-slope changes on the circulatory roadway or the exit
- Outward sloping cross-slope on the entire width of the circulatory roadway
A problem for some vehicles may be present even if speeds are low because of a combination of grade,
geometry, sight distance and driver responsiveness. Research has shown that an articulated large goods
vehicle with a center of gravity height of 8 feet above the ground can overturn on a 65 foot radius curve at speeds as low as 15 mph. See Transport Research Laboratory Report LR788.
Layouts designed to mitigate the above noted characteristics will be less prone to load shifting or load shedding. In addition, pay attention during design and construction to ensure that pavement surface tolerances are
complied with and that abrupt change in cross-slopes are avoided.
30.5.8 Roadway Width
The width of the roadway at locations with curb and gutter on both sides should accommodate the design
vehicle and allow for passing a stalled vehicle. The design width for entries, exits and bypass lanes is shown in
Exhibit 3-51, page 220, GDHS 2004 as a 19-foot face-face minimum and 20-foot face-face desirable to allow a
stalled vehicle to pass.
30.5.8.1 Entry Width
Entry width is measured perpendicularly from the outside curb face to the inside curb face nose P.C. at the
splitter island point nearest to the inscribed circle.
Narrow entries tend to promote lower speeds and improved safety. However, a WB-65 may require a 19 to 22
foot wide entry path for single lane approaches to be able to make a right turn. Design single lane roundabouts
to accommodate a WB-65 without encroachment onto the truck apron or the curb and gutters. Wide entries may
cause concerns about whether to pavement mark the entry as a multilane or keep as a single lane. Increasing
the flare length without changing entry width will increase entry capacity and is crash-neutral (see NCHRP 672
Exhibit 6-25). Increasing both flare width and entry width may produce a substantial increase in capacity but will
degrade safety by promoting higher entry speeds. Effective flare length may be as short as 15 feet or as long as
330 feet. Once the effective flare length exceeds 330 feet it will have a minimal benefit to capacity; therefore,
adding a full approach lane would be advised.
30.5.8.2 Entry Flare
Flaring an entry from one lane to two or from two to three creates additional entry capacity without extensive
mid-block widening. When lane choice options are even or no preference is given to either lane, it is ideal to split
the approach width at a point where the lane width reaches 9.5 feet or 19 feet overall (flange of curb
dimensions).
The development of horizontal geometry and pavement marking of a flared entry is balanced and smooth
making lane choice options obvious and entry paths clear.
30.5.9 Exit Tapers
Tapering the number of lanes on an exit from two lanes to one lane or from three lanes to two lanes allows for
additional roundabout capacity without extensive mid-block widening. The continuous flow nature of
roundabouts typically results in less saturated traffic streams exiting the intersection. This is in sharp contrast to
a signalized intersection where platoons of traffic are much more concentrated, and consequently typically
Page 50
require more downstream distance to merge. Speeds are also much slower for traffic exiting roundabouts which
eliminates the need for long parallel section downstream of the roundabout exit.
Design exit tapers from roundabouts based on the anticipated in lane exiting speed, not the fastest path,
typically in the range of 15 to 25 mph. Merging taper rates should be based on the lengths shown in FDM 11-25
Attachment 2.2., typically 20:1 to 30:1. The length of full width lanes beyond the circulating roadway to
beginning the merging taper may vary between 100 and 300 feet depending on volume, potential for upstream
lane choice, and other factors that may be unique to the site, Consider the farther the full lane widths are
extended upstream, the potential for increase in speed and the potential for a longer merge taper. See Figure
30.9.
Figure 30.9 Exit Lane Taper
30.5.10 Circulatory Roadway Width
Circulatory roadway width is the width between the outer edge of the inscribed diameter at the curb face and the
central island curb face. It is typically 1.0 to 1.2 times the width of the widest entry with potential exceptions for
Case 2 and Case 3 designs. It does not include the width of any traversable apron, which is defined to be part of
the central island. The circulatory roadway width defines the roadway width, curb face to curb face, for vehicle
circulation around the central island. The circulatory roadway width does not need to remain constant. A twolane entry may be appropriate for the major through highway, however, the minor side road may be single lane
approaches. The circulating roadway may often have a different width to accommodate the through traffic than
for the side road traffic. Alternative lane configurations also produce varying circulatory widths as shown on
NCHRP 672 Exhibit 6-27.
30.5.11 Central Island
The central island of a roundabout is always a raised, non-traversable area encircled by the roundabout circulatory roadway. The central island is stepped up from the traversable truck apron to the non-traversable island area. The central island is raised and landscaped to enhance driver recognition of the roundabout upon approach and to limit the ability of the approaching driver to see through to the other side. The inability to see through the roundabout reduces or eliminates headlight glare at night and driver distraction by other vehicles on the circulating roadway.
The center or highest portion of the central island ground surface elevation should be raised a minimum of 3.5 feet and maximum of 6 feet from the circulatory roadway surface. The ground slope in the central island shall not exceed 6:1. Concrete, stone, wood or other non-forgiving material used to make a wall within the central island is prohibited. Landscaping the central island and the roundabout area is further addressed in FDM 11-26-40. The outside 6 feet of the central island should be a low mowed grass surface or low maintenance surface to maintain good visibility to the left upon entry as well as good forward and circulatory visibility on the circulatory roadway.
30.5.12 Entry Curves
The minimum entry radii should be approximately 65 feet. Capacity will increase with increased entry radii, but
so may the entry speed. Entry radius is not R1.
NCHRP 672 Exhibit 6-14 illustrates the composition of entry curves to produce natural entry paths. This method
is useful but has limitations where large trucks making right turns will require even larger outside radii,
particularly on single lane roundabouts with narrow entry widths. In such cases, the larger outside radius may
increase entry speeds undesirably. A preferred design technique for single-lane roundabouts is not to make the
Page 51
inside radius/arc tangential to the central island, but to create a flare in the entry such that the large truck path
can preserve the outside radius which controls entry speed. The effect gives the entry a flare, typically ranging
from 18ft to 24ft. To avoid misleading drivers to expect multilane operation at wider single-lane entries, the left
hand side of the entry may be pavement marked as shown in Figure 30.10 to reinforce single lane operation.
Also refer to FDM 11-26-35.2.1, and Attachment 35.1 for further pavement marking procedures.
Figure 30.10 Example of alternative pavement marking design for single entrance lane
not in the NCHRP Report 672
30.5.13 Non-motorized Users
Roundabouts like other intersections need to accommodate bicyclists and pedestrians. The types of facilities
provided vary based on the existing urban, suburban and rural conditions as well as future land uses. Evaluate
regional and local land use plans including stand-alone bike and pedestrian plans for communities when
determining the appropriate bike and pedestrian facilities at a roundabout. See FDM 11-46-1 for guidance on
including bike and pedestrian facilities on projects.
Pedestrian accommodations include sidewalks, shared-use paths and roundabout sidepaths.
Bicycle accommodations include bike lanes, wide curb lanes, urban paved shoulders, rural paved shoulders,
shared-use paths and roundabout sidepaths. Although a shared roadway is not a bicycle accommodation,
shoulders or bike lanes taper down and end just prior to the entrance to a roundabout. Tapers are necessary to
help achieve proper speed control for vehicles at entry. Design requirements do not allow bike lanes or
shoulders at the yield line or within the circulatory roadway of a roundabout. Bicyclists in Wisconsin have the
right to use the roadway in the same manner as motor vehicles. Bicyclists may have concerns when traveling
into, through, or around roundabouts depending on traffic volume, vehicle type composition, experience of the
bicyclist, lighting or other factors. Therefore, a bicyclist approaching a roundabout may proceed in a travel lane
(“take the lane”), or exit the roadway by way of a ramp and ride on a roundabout sidepath (or a shared use path,
if applicable). See FDM 11-26-30.5.13.1 and Figure 30.11 for guidance on bike exit and entrance ramps). These
ramps are where the shoulder or bike lane tapers and a typical 5-foot sidewalk transitions to/from a roundabout
sidepath.
A sidewalk transitions to/from a roundabout sidepath as it approaches/departs an isolated roundabout. At
locations with consecutive closely spaced roundabouts, a sidewalk transitions to a roundabout sidepath at the
first upstream roundabout, and transitions from a roundabout sidepath at the last downstream roundabout. See
FDM 11-20-1, FDM 11-46-5 and FDM 11-46-10 for design guidance on sidewalks.
Shared-use paths are typically community or regional facilities in their own corridors that may extend for miles.
Shared-use paths support a wide variety of non-motorized travelers like bicyclists, in-line skaters, roller skaters,
wheelchair users, walkers, runners, people with baby strollers or people walking dogs (typically not equestrian
users or motorized users - although some state trails in Wisconsin allow snowmobiles). Shared-use paths are
designed for bi-directional bicycle travel. Continue a shared-use path around roundabouts (and between
consecutive roundabouts if applicable) following shared-use path design standards. See FDM 11-46-15.6 and
the Wisconsin Bicycle Facility Design Handbook for more guidance on shared-use paths. Also, see FDM 11-35
1.6 and FDM 11-35 Attachment 1.1. Roundabout sidepaths are a variant of shared-use paths that apply specifically to roundabout intersections and between consecutive closely spaced roundabouts. A roundabout sidepath is a sidepath around the perimeter of an isolated roundabout, or a sidepath between two consecutive closely spaced roundabouts and around their Page 52
perimeters. Consecutive roundabouts are closely spaced if they are 1,000-feet or less from center to center.
Roundabout sidepaths are designed with the expectation that bicyclists will travel in a unidirectional manner
(i.e., one-way bicycle travel in the same direction as traffic flow on that side of the roadway) and do not connect
to shared-use paths. If bicyclists choose to leave the roadway and enter the path, they must yield the right-of
way to pedestrians. If bicyclists stay on the roadway they are expected to position themselves near the middle of
the travel lane to circulate around the roundabout.
The roundabout splitter islands provide pedestrian refuge and pedestrian crossings. At roundabouts with high
traffic volumes, or where pedestrian or bicyclist volumes are high, consider accommodating both users by
enhancing the pedestrian crossings with features such as:
- 6-inch white crosswalk marking next to colored concrete (2011 Wisconsin MUTCD Supplement,
§§3B.18, 3G.01, 7C.02)
- Colored concrete with 6-inch wide patterned borders with white crosswalk markings, note main
walking surface is smooth
- Activated (push button or automatic detection) warning beacons (e.g. Rectangular Rapid Flashing
Beacon or pedestrian hybrid beacon)
30.5.13.1 Bike Ramp Entrance and Bike Ramp Exit Design Guidance
End the on-road bicycle accommodations approximately 75 to 150 feet upstream of the yield line and allow the
bicyclist an opportunity to leave the roadway by way of a bicycle exit ramp. More distance is needed when a
right turn bypass lane is provided. The bike ramp exit should have relatively flat angles as shown so that
bicyclists are not directed into the path of pedestrians. The bike ramp entrance should have relatively flat angles
as shown so that bicyclists are not directed into the travel lane of motorized vehicles. The bike entrance ramp
should not be directed parallel to the bike lane.
The location of bike ramps and driveway aprons need to be spaced as not to conflict with each other. It is not
desirable for bicyclists and is a last resort to leave or re-enter the roadway by way of a driveway apron.
Design the bike ramps 4 feet wide between the roadway and the multi-use path such that they angle up (25 to
35 degrees) to the path where the bicycles exits the roadway, Figure 30.11. Angle down (25 to 35 degrees)
toward the roadway where the bicycles re-enter the roadway, Figure 30.11.
Figure 30.11 Bike Ramp Entrance & Exit
30.5.13.2 Pedestrian Facilities, Shared-Use Paths, and Roundabout Sidepaths
Isolated roundabouts and roundabouts in a series that are closely spaced, which is defined as a distance of
1000’ or less between the centers of any two consecutive roundabouts, have design criteria that is different than
other at-grade intersections. The following procedures include design guidance for these facilities near and
between roundabouts. See FDM 11-46-5 for additional information on Pedestrian Facilities. See FDM 11-46-15
and Wisconsin Bicycle Facility Design Handbook for additional information on standard shared use paths. See
FDM 11-46-1 on providing bicycle and pedestrian accommodations on projects.
In most urban and suburban areas sidewalks on both sides of the roadway are common and expected. These
sidewalks lead up to and transition into roundabout side path as these facilities approach a roundabout. This
typically is a 5 ft sidewalk and a 5 ft terrace, or if there are trees planted in the terrace the minimum terrace
width is 6-foot wide minimum.
When an existing or proposed sidewalk approaches either end of a roundabout, provide at least an 8 foot wide
roundabout side path with a terrace when sidepath use is anticipated to be low or medium around and between
the roundabout(s). The width of the sidepath (and terrace) remains consistent through the roundabout(s). When
the sidepath use is anticipated to be high (frequent passing of users), install a 10 foot wide sidepath with a
terrace. There are many reasons to anticipate high use such as parks close by, elementary and high schools,
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universities, gas/convenience stores, restaurants, etc. In an outlying district or rural area, there may be locations with on-road bicycle accommodations but without sidewalks (existing or proposed) because a Trans 75 omission for sidewalks applies (see FDM 11-46-1.3.1.4). In this case, 6-foot wide roundabout sidepaths are appropriate. Work with the Regional Bike and Pedestrian
Coordinator to determine the appropriate widths. When a shared-use path approaches the roundabout carry the shared-use path around the roundabout. The standard width is 10 feet with a 5 ft terrace, or if there are trees planted in the terrace the minimum terrace width is 6-foot wide minimum. For a series of closely spaced roundabouts, extend the roundabout side path or shared use path from the first bicycle exit ramp to the last bicycle entrance ramp, for the bicyclist to leave the roadway and travel through all roundabouts on the roundabout sidepath. Do not provide entrance ramps for bicyclists to re-enter the roadway between closely spaced roundabouts (1,000 feet or less between roundabout centers). However, provide exit ramps from the roadway to the sidepath prior to the upstream roundabout. When the distance between any two roundabouts is greater than 1,000 feet, center to center, then the roundabout side path may be discontinued beyond the last roundabout. Provide entrance ramps for bicyclists to re-enter the roadway downstream from each roundabout as well as exit ramps from the roadway to the sidepath. Provide sidewalk(s) between the roundabouts if there is sidewalk on the roundabout approaches, unless one of the Trans 75 exceptions applies (see FDM 11-46). A roundabout sidepath might not be built during the initial construction if the location meets an omission condition of the Trans 75 exceptions in FDM 11-46-1. If a roundabout sidepath around a roundabout is not installed with the initial roundabout construction, it is important to construct the appropriate platform by grading for future facilities (e.g in rural or outlaying area 5-foot terrace and a 6-foot width for the roundabout sidepath
around the roundabout) and provide pedestrian crossings in the splitter islands. Maintenance may not be required until the perimeter facilities are installed. 30.5.13.3 Roadway Width, Clear Roadway Width of Bridges, and Underpasses between Closely Spaced
Roundabouts
At a minimum, multi-lane roadways with a raised curb median between opposing roadways and between closely spaced roundabouts require a 2 foot median shoulder, two or more 12 foot lanes, and a 4 foot minimum outside shoulder, a 5-foot terrace adjacent to a shared-use path or roundabout sidepath. If there are trees planted in the terrace the minimum terrace width is 6-foot wide.
At a minimum, single lane roadways with a raised curb median between opposing roadways and between closely spaced roundabouts require 19 feet minimum from curb face to curb face. This typically allows for a 2 foot median shoulder, one 12 foot lane and a 5 foot minimum shoulder on the outside, followed by a 5-foot terrace and either a roundabout sidepath or a shared-use path. If there are trees planted in the terrace the minimum terrace width is 6-foot wide. A single lane roadway between opposing roadways and between closely spaced roundabouts without a raised curb median requires a minimum 32 feet from curb face to curb face.
If there is an overpass structure between two closely spaced roundabouts (1,000 feet or less between roundabout centers), and a roundabout sidepath is provided on around the outside of the roundabouts, then the roundabout sidepath is at least 2 ft wider on the structure (Figure 30.12). A roundabout sidepath will typically not have a barrier wall separating the path from the roadway. Vehicle travel speed between closely spaced roundabout is considered a low speed environment (40 mph or less) and bicycle travel is expected to be unidirectional thus barrier walls between the roadway and path are not required. When there is a barrier
proposed between the roadway and a roundabout sidepath, the sidepath is level with the roadway (not a raised sidewalk). See Figure 30.12 and FDM 11-35-1.6 and FDM 11-35 Attachment 1.1 pages 1 and 2. Section B-B shows a section view of a raised curb roundabout sidepath. A barrier between the roadway and roundabout
sidepath is unique and maybe a provision requested this requires WisDOT approval, including the Regional Bicycle and Pedestrian Coordinator.
When a shared-use path is provided around the outside of roundabouts the shared-use path standards on structures are followed. See FDM 11-46-15, FDM 11-35 Attachment 1.1 pages 1 and 2. Section B-B shows a section view of a raised curb shared-use path, and Section C-C shows a section view of the barrier wall between the roadway and the path. See FDM 11-35-1.6.3 for required separation distance between outside
travel lane and front face of barrier wall to determine the minimum barrier wall height. The roadway and structure width will depend on the median width, lane width, number of lanes, shoulder width, and path width requirements. For the STH system, the WisDOT minimum roadway width and clear roadway width of bridge from curb face to curb face, between two closely spaced roundabouts that are less than 1,000 feet apart, is: - 2 lane divided (each side) - 2’ median shoulder, 12’ lane, 5’ outside shoulder = 19’.
- 2 lane undivided - 4’ shoulder width, + 12’ lane +12’ lane + 4’ shoulder width = 32’, independent of
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ADT.
- 4 lane divided (each side) - 2’ median shoulder, + 12’ inside lane, + 12’ outside lane, + 4’ shoulder =
30’.
- 4 lane undivided - 4’ shoulder, 12’ outside lane, + 12’ inside lane +12 inside lane, + 12’ outside lane, +
4’ shoulder = 56’.
- 6 lane divided (each side) - 2’ median shoulder, + 2 inside lanes at 12’, + 12’ outside lane, + 4’
shoulder = 42’
The above widths provide a minimum roadway width between closely spaced roundabouts.
In an effort to reduce structure width the designer should consider a narrow raised median between the splitter
islands. A 4 foot raised curb median face to face will provide an 8 foot median measured from flange line to
flange line with 2 foot gutters just off the end of the structure. The distance between roundabouts should be
sufficient to allow for any curved curb and gutter portion that is formed at the ends of the splitter islands to
remain off the structure. The tangent narrow section in the middle between splitter islands could be 4 foot wide
face to face providing there are no signs or other road side elements in that area.
Under structures the roundabout sidepath and terrace widths are consistently provided through and between the
roundabouts. If there will be road signs, power poles, light poles or other fixtures installed along the roadside
then provide at least a 5 foot wide terrace between the curb face at the outside of the shoulder and the front of
the path. The cross-section under the structure provides at least the median shoulder width, lane width(s),
outside shoulder width and path width plus 2 ft if no obstructions are in the terrace. Follow shared-use path
standards for under structures.
The above minimum roadway widths between closely spaced roundabouts are not appropriate for rural highway
applications or where the distance between consecutive roundabouts is greater than 1000 feet. If existing or
proposed sidewalk approaches consecutive roundabouts that are not closely spaced (i.e. greater than 1,000
feet between roundabout centers), provide roundabout sidepath(s) around the roundabout(s) but not between
them - provide bike and pedestrian accommodations see FDM 11-46-1.
The roadway between the roundabouts transitions to a cross-section roadway width and clear roadway width of
bridges based on the design class of the roadway (see FDM 11-15-1, FDM 11-20-1, FDM 11-35-1.2, and FDM
11-46-1).
If bike or pedestrian facilities are omitted around or between roundabouts per Trans 75 discuss with the
Regional Bicycle and Pedestrian Coordinator the need to provide an 8-foot roundabout sidepath on or under the
structure. Structures have a longer life-span and even if a roundabout sidepath is not immediately included on a
structure it is necessary to consider constructing a wider substructure to allow widening of the superstructure in
the future to accommodate a roundabout sidepath. In such cases, the pedestrian refuge in the splitter islands
should still be constructed.
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Figure 30.12 Roundabout Sidepath
30.5.14 Splitter Islands
For WisDOT roundabout projects crosswalk alignment is not optional as shown on NCHRP 672 Exhibit 6-66.
The angled crosswalk produces shorter perpendicular crossing paths and discourages bicyclists from crossing
without stopping in the refuge area.
The splitter island minimum width within the pedestrian refuge area is 6 feet, desirable is 8 feet, (face of curb to
face of curb). The minimum crosswalk width in the splitter island is 7 feet, desirable is 10 feet. See NCHRP 672,
Exhibit 6-12 except design the curbing to have a continuous gutter through the crosswalk as shown on Figure
30.13.
In general, locate the pedestrian crossing one car length or approximately 20-25 feet upstream from the yield
line (2009 MUTCD, Figure 3C-1). FDM 11-26-30.5 provides additional guidance on pedestrian crossing
placement and design. This helps to reduce decision-making problems for drivers and avoids creating a queue
of vehicles waiting to enter the roundabout. However, for pedestrian safety the crossing should not be located
too far back from the yield line such that entering vehicle speeds are insufficiently reduced or exiting vehicles
are accelerating. It may be appropriate to design the pedestrian crossing at two or three car lengths from the
yield line on some multilane entries. Make the crossing perpendicular to the direction of traffic on multilane
entrances and exits to minimize pedestrian travel and exposure time as shown on Figure 30.13. On single-lane
roundabouts it may be appropriate to provide a crosswalk straight through the splitter island (See NCHRP 672, Exhibit 6-66).
Splitter islands can be crowned upward with a slope toward the center of the island area using between a 4 percent slope to as much as a 6:1 slope. This improves visibility of the splitter island for rural conditions. The
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maximum overall height above the top of the curb within the splitter island area should be approximately 18
inches from top of curb to the top of any concrete/asphaltic surface. Some islands may become quite wide near
the circulating roadway however limit the height to 18 inches. The approach nose separating the entering traffic
and the exiting traffic shall be a Concrete Median Sloped Nose, Type 1. This splitter island nose should be 6
foot face-to-face where the R4-7 (KEEP RIGHT) sign is located. The other noses at the edge of the circulatory
roadway and the splitter island shall be Concrete Median Sloped Nose, Type 2. Both nose types are shown in
SDD 11B2. Where there is a divided highway approaching the roundabout the approach nose is eliminated.
For additional information, see NCHRP 672, §6.4.1.
Figure 30.13 Typical Splitter Island
30.5.15 Intersection Sight Distance (ISD) and Length of Conflicting Leg of Sight Triangle
See NCHRP Report 672 starting on page 6-63 for guidance on Intersection Sight Distance (ISD) for roundabout
approaches. The basis for ISD in NCHRP Report 672 is providing the critical headway time gap (tc) for entering
the roundabout. The critical headway time gap (tc) for entering the roundabout is based on the amount of time
required for a vehicle to safely enter the conflicting stream. If the perceived available headway time gap is less
than tc then most drivers will slow down or stop and wait for an acceptable gap. The critical headway time gap
will possibly change over time. WisDOT has revised this time gap per FDM 11-26-20.4.5 however at this time
WisDOT will use the critical headway time gap (tc) equal to 5 seconds as stated in NCHRP Report 672 for
intersection sight distance. This is less than the 6.5 second required by the 2000 FHWA Roundabout Guide, but
greater than the previous FDM requirement of 4.5 seconds. Table 30.7 shows computed distance for various
speeds based on a critical headway time gap (tc) = 5.0.
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Table 30.7 Roundabout Intersection Sight Distance
Conflicting Approach Speed
(mph)
* Computed Distance (ft) for tc = 5.0s
10
74
15
110
20
147
25
184
30
221
*distance in feet = speed (mph) multiplied by time (seconds) multiplied by a factor of 1.468.
The “clear sight window” requirements for critical headway time gap (tc) are shown on Exhibit 6-58 of NCHRP
Report 672. Use an eye height above the roadway surface of 3.5 feet for passenger cars and 7.6 feet for trucks
in establishing sight lines through a clear sight window. Use an object height above the roadway surface of 3.5
feet.
Figure 30.14a shows “Normal ISD” for a roundabout approach; Figure 30.14b shows “Minimum ISD” for a
roundabout approach. Use the following guidance when designing the ISD “clear sight window” for a roundabout
approach:
- [Normal ISD & Minimum ISD - driver’s eye position on approach] Set the initial position of the
driver’s eye at 50 feet behind the yield line, as depicted on Exhibit 6-58 of NCHRP Report 672, and as
shown in Figure 30.14a and b or the vehicle approaching on Leg 2.
- [Normal ISD & Minimum ISD - to circulating roadway] Provide ISD based on [tc=5.0 seconds x
“circulating speed X factor”] for the circulating stream distance d2, as depicted on Exhibit 6-58 of
NCHRP Report 672, and shown on Figure 30.14a and b as the distance from point 2 to point 4. For
example, if the circulating speed is 20 mph, the distance between point 2 and point 4, per Table 30.8,
is 147 feet.
- [Normal ISD - to adjacent leg to the left] Provide ISD based on [tc=5.0 seconds x “fastest path
speed X factor”] for the entering stream distance d1, as depicted on Exhibit 6-58 of NCHRP Report
672, and shown on Figure 30.14a as the distance from point 1 to point 4. For example, if the “fastest
path speed” is 25 mph, the distance between point 1 and point 4, per Table 30.8, is 184 feet.
- [Minimum ISD - to adjacent leg to the left] It may not be possible to provide “Normal ISD” at some
approaches because of a sight obstruction whose removal would cause unacceptable impacts. For
these locations, provide ISD to at least 50-feet behind the yield line of the adjacent leg to the left - as
shown on Figure 30.14b. The resulting reduced entering stream distance d3 from point 3 to point 4 is
less than [tc=5.0 seconds x “fastest path speed X factor”]. However, it is unlikely that all vehicles will be
traveling at the “fastest path speed” between points 3 and 4 because some drivers will slow down or
stop behind the yield line if there is an unacceptable gap.
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Legend
d1 Entering Stream Distance
d2 Circulating Stream Distance
d3 Reduced Entering Stream Distance starting at least 50-feet behind the Leg 1 yield line
ISD Clear Sight Window for vehicle on Leg 2
Figure 30.14 Example of Roundabout ISD Clear Sight Window
(Leg 2 ISD shown - other legs are similar)
Designer experience and judgment is needed to balance the impacts where ISD is severely restricted or where
excess ISD is available. More is not better when it comes to Intersection Sight Distance for roundabouts.
Research on sight distance has determined that excessive intersection sight distance results in a higher
frequency of crashes because excessive forward visibility at entry or visibility between adjacent entries can
result in approach and entry speeds greater than desirable for intersection geometry
Consider limiting visibility by the use of selective landscaping. This refers to landscaping or a visual block down
the side road or median to restrict visibility between adjacent entries, as well as the forward visibility through the
central island. Limiting visibility in this way helps encourage drivers to slow down on the roundabout approach,
which provides a safer environment for both drivers and pedestrians.
Forward visibility for the driver entering to have sight of the circulatory roadway ahead of the driver’s entering
path can also be checked but is generally accounted for by ensuring sight to the left of circulating vehicle
upstream (see Figure 30.14b for vehicle along path d2).
30.5.16 Angles of Visibility
The intersection angle between consecutive entries must not be overly acute in order to allow drivers to
comfortably turn their heads to the left to view oncoming traffic from the immediate upstream entry. The
intersection angle between consecutive entries, and the angle of visibility to the left for all entries, should
conform to the same design guidelines as for conventional intersections. Based on guidance for designing for
older drivers and pedestrians, the recommended angle for visibility to the left at entry is 90° ±15°. NCHRP 672
Exhibit 6-62 illustrates an example of a visibility angle for a roundabout entry at a ramp terminal.
Designers should also be aware of the visibility angle for conditions when the entering traffic does not yield, i.e.
drivers looking left upstream of the yield line when not needing to yield or stop, a common condition for off-peak
traffic conditions. The view to the left is then executed when the driver is well upstream of the roundabout entry
unlike what NCHRP 672 Exhibit 6-62 shows. Thus visibility angles must also be checked for non-yielding driving
conditions from a distance upstream of the point of entry. The designer is cautioned not to provide generous
sight to the left as this can contribute to failure to yield conflicts and collisions.
For additional information, see NCHRP 672, §6.7.4.
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30.5.17 Right Turn Lanes
Right turn lanes should only be used when capacity needs dictate or when other geometric layouts fail to
provide acceptable traffic operations or accommodations for the design vehicle. The decision to use right turn
lanes should take into account pedestrian and right-of-way constraints. Choosing the proper alternative is
dictated by the volume of right turns and the available space. See NCHRP 672, §6.8.6 for additional information.
Three alternatives exist to provide for heavy right turn demand:
30.5.17.1 Free Flow Right Turn Lane (Figure 30.15 and NCHRP 672 Exhibit 6-72)
Free flow bypass lanes allow vehicles to bypass the roundabout and then merge into the exiting stream of
traffic. A high right-turn demand when coupled with other approaching traffic may indicate the need for a full
bypass lane in order to avoid a wider, faster entry. Roadway right-turn free-flow lanes are not recommended for
pedestrians and bicyclists and should be avoided, if possible, in high ped/bike use areas. If free flow right turn
lanes are used keep vehicle speeds slow by using a small right turn radius.
30.5.17.2 Partial Bypass Right Turn Lane (Figure 30.15b or c and NCHRP 672 Exhibit 6-73)
A partial bypass lane with a curbed vane island requires approaching vehicles to yield to traffic leaving the
adjacent exit. This alternative ‘snags’ the right turner from making a through movement while preserving good
sight to the left for circulating/exiting traffic. Generally an intersection angle of 70 degrees or higher is desirable.
Dual partial right turn bypass lanes with a curbed vane island may also be an appropriate alternative to
accommodate heavy right turn demand, especially at interchange ramp terminals. Dual partial bypass lanes
maybe problematic for pedestrians and should only be used at locations where there is not a crosswalk in close
proximity on the exit receiving the dual right turning vehicles. Pedestrians may have a hard time seeing a vehicle
turning right from the left lane of the dual right turn entry.
When designing dual partial right turns, special attention is required to ensure that vehicles in both of the right
turning lanes have adequate sight of vehicles in the circulatory roadway. Speed of vehicles in the right turn
lanes also need to be well controlled. Use a smaller entry radius to help reinforce that vehicles exiting the
roundabout have the right of way. This will also minimize the potential for rear end crashes associated with
larger right turn radii. Similar to the guidance provided for a Case 1 design, allow the design vehicle to encroach
into adjacent lanes on the entry and exit while making the right turn.
30.5.17.3 Exclusive Right Turn Lane (Figure 30.15a and NCHRP 672 Exhibit 6-74)
Exclusive right turn lanes with or without a painted gore help to keep the overall roundabout layout compact
while accommodating the heavy right turning movement. An exclusive right turn lane should be ‘snagged’ from
making a through movement while preserving good sight to the left for circulating/exiting traffic.
(a) No Bypass Lane SB Movement
(b) Partial Bypass Lane SB Movement
(c) Dual Partial Bypass Lane SB Movement
Figure 30.15 Right Turn Bypass Lanes
(See also NCHRP 672 Exhibit 6-74)
30.5.18 Vehicle Path Overlap and Methods to Avoid Path Overlap
Designing multilane roundabouts is significantly more complex than single-lane roundabouts due to the
additional conflicts present with multiple traffic streams entering, circulating and exiting the roundabout in
adjacent lanes. The natural path of a vehicle is the path it will take based on the speed and orientation imposed
by the roundabout geometry. While the fastest path assumes a vehicle will intentionally cut across the lane
markings to maximize speed, the natural path assumes there are other vehicles present and all vehicles will
attempt to stay within the proper lane.
Designers should determine the natural path by assuming the vehicles stay within their lane up to the yield
point. At the yield point, the vehicle will maintain its natural trajectory into the circulatory roadway. The vehicle
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will then continue into the circulatory roadway and exit with no sudden changes in curvature or speed. If the
roundabout geometry tends to lead vehicles into the wrong lane, this can result in operational or safety
deficiencies.
Path overlap occurs when the natural paths of vehicles in adjacent lanes overlap or cross one another. It occurs
most commonly at entries, where the geometry of the right-hand lane tends to lead vehicles into the left-hand
circulatory lane. However, vehicle path overlap can also occur at exits, where the exit geometry or pavement
marking of the exit tends to lead vehicles from the left-hand lane into the right-hand exit lane. Figure 30.16
illustrates an example of entry path overlap at a multilane roundabout where the left lane geometry directs the
approaching vehicle into the central island, while the right lane geometry directs the approaching vehicle toward
the inside circulatory lane, thus creating entry path overlap.
Figure 30.16 Entry Path Overlap
30.5.18.1 Method for Checking Path Overlap
Figure 30.17 provides a method for checking entry and exit path overlap. To avoid path overlap the desirable
tangent length is 40-ft to 50-ft or two car lengths for the entry path tangent and 40-ft and greater for exit path
tangent. The minimum tangent length to avoid entry and exit path overlap is 26-ft or one car length.
As a rule of thumb path overlap can be avoided if there is typically 5 feet between the face of the central island
curb and the extension of the face of curb on the splitter island, see Figure 30.17.
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Figure 30.17 Method for checking path overlap
30.5.18.2 Design Method to Avoid Path Overlap
Figure 30.18 shows the preferred method to avoid path overlap in multilane entries. Start with an inner entry
curve designed so when the edge of the splitter island curve is extended across the circulatory roadway the line
is tangent to the central island as shown. Once the lane geometry is determined to avoid path overlap then
design the adjacent lane(s). The small radius entry curve will vary depending on the approach geometry and the
fastest speed path but will typically range from 65-110 feet. A large-radius (greater than 150 feet) curve is then
fitted between the entry curve and the outside edge of the circulatory roadway.
The primary objective of this design technique is to locate the entry curve at the optimal placement so that the
projection of the inside entry lane at the yield point forms a line tangent to the central island. This inner curve
design concept is essential for multilane design and is recommended for single lane entries as well. Figure
30.18 illustrates the result of proper entry design.
The location of the entry curve directly affects path overlap. If it is located too close to the circulatory roadway, it can result in path overlap. However, if it is located too far away from the circulatory roadway, it can result in drivers accelerating to the yield point. For additional information, see NCHRP 672, §6.4.3.
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Figure 30.18 Multilane Entry Design
30.5.19 Approach Design
The primary safety concern in high-speed context is clarity of the driving situation, that is, to make drivers aware
of the roundabout with ample distance to comfortably decelerate to the appropriate speed. Therefore designs
should follow these principles:
- Provide the desirable stopping sight distance of the entry point based on approach operating speed.
- Align approach roadways and set vertical profiles to make the central island conspicuous.
- Splitter islands should extend upstream of the yield line to the point at which entering drivers are
expected to begin decelerating - a minimum length of 200 feet is recommended.
- Approach curves should be gentle, become successively smaller and should be sized based on the
design speed and expected speed change.
- Tangents should be used between reverse curves.
- Use landscaping on extended splitter islands and roadside to create a tunnel effect.
- Provide illumination in transition to the roundabout.
- Use signs and pavement marking effectively to advise of the appropriate speed and path for drivers.
The consequences of an inconspicuous central island and/or splitter islands is mainly loss of control crashes as
motorists unfamiliar with the roundabout are not given sufficient visual information to elicit a change in speed
and path. See Figure 30.19.
30.5.20 Vertical Design
Super elevation of curves on approaches to roundabouts is counterproductive to the objective of transitional
speed reduction. Design super elevation on approaches based on the low-speed urban street criteria outlined in
FDM 11-10-5.3.2. Speed for the curve being designed is based on its distance from the yield line and the
deceleration length determined from AASTHO Figure 2-25.
Example: For a posted speed of 55 mph with deceleration to 0 mph, the distance is approximately 410 feet.
Curves prior to 410 feet should be designed for 55 mph; curves within 410 feet should be based on a prorated
estimated speed based on distance from the yield line.
30.5.20.1 Approaches/Departures (Intersection Legs)
The most critical vertical design area of the roundabout is the portion of roadway from the approach end of the
splitter island to the circulatory roadway. This area requires special attention by the designer to ensure that the
user is able to safely enter the circulatory roadway, especially for OSOW vehicles. This area usually requires
pavement warping or cross-slope transitions to provide an appropriate cross-slope transition rate through the
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entire transition area and within the circulatory roadway.
Entry grade profiles (approximately 2 car lengths from the ICD) are not to exceed 3%, with 2% being the
desirable maximum. It is desirable to match the exit grades and the entry grades. Adjustments to the circulatory
roadway cross-slope may be required to meet these criteria, but should be balanced with the effects on the
circulatory roadway.
30.5.20.2 Circulatory Roadway
Roundabouts typically should be constructed on relatively flat or rolling terrain with an approach grade that is
desirably less than 3%, but not greater than 5%. Grades approaching 4% and steeper terrain may require
greater transitions to provide an appropriate grade through the intersection. The profile grades along the central
island should generally not exceed 4%, (3% or less is desirable).
- Single-lane Roundabout - crown the roundabout circulating roadway with a 2% cross-slope with
approximately 2/3 width sloping toward the central island and 1/3 width sloping outward.
- Multilane Roundabout - Same crown guidance applies where possible. However, when considering
factors such as paver screed width, contraction joint location for concrete pavement, pavement
marking location, and the total width of the circulatory roadway, it may be a challenge to comply with
the 2/3 sloping inward and 1/3 sloping outward. Therefore another alternative (independent of material
type) on dual lane roundabouts is to slope the inside lane, or left lane, toward the central island and
slope the outside lane (typically wider lane) to the outside. This alternative will allow the contraction
join on concrete pavement to generally coincide with the lane line pavement marking and allow
asphalt pavement roundabouts to be similar in design. On triple lane roundabouts it may be possible
to slope the two inside lanes toward the central island and slope the outside lane to the outside.
The crown vertical design feature provides good drivability, keeps water from draining across the circulating
roadway which is particularly important in a northern climate with freeze-thaw cycles, and provides a smooth
transition in/out of the approaches and departures. This ‘crown’ also reduces the probability of load shifting or
truck over turning.
The preferred truck apron slope is between one and two percent toward the circulatory roadway. Greater than
one percent slope should not be used on OSOW routes. However, it may vary between 1 and 2 percent when
justified on other routes.
30.5.21 Curbing
30.5.21.1 Approach Curbs
Low speed approaches should incorporate 6-inch vertical face curbs, on both sides of the roadway. The
purpose of the vertical face curbs is to control the fastest speed paths at the roundabout entrances and exits.
On the OSOW network a 4-inch mountable curb and gutter may be used in limited situations to better
accommodate truck tires that may have to go over the curb or the splitter island.
High speed approaches to roundabouts usually occur where there is a rural cross-section. This rural crosssection for undivided highways will have shoulders without curb on the outside. When the highway is divided
there will be shoulders on the inside, sometimes with sloped curbs, the outside will have shoulders typically
without curb leading up to the roundabout. High speed approach design will require a transition section to the
roundabout where the shoulders will narrow and vertical curb will be introduced. See Figure 30.19 for an
example of the high-speed approach layout.
In rural areas the pavement marked gore and the curbs serve to alert the driver approaching a roundabout of
the changing conditions and that a speed reduction is expected. Driver awareness that conditions are changing
is accomplished through a combination of roadway curvature, channelization, lighting, landscaping, and signing.
Figure 30.19 shows the layout of the gore area for the beginning of the splitter island and the curb and gutter
layout as the driver approaches the yield line. The pavement marked gore area transitions into a raised curb
median nose (Type 1) followed by a 4-inch sloping curb and gutter for a short distance. The curb transitions in
two ways as it approaches the roundabout. At the nose the curb face is offset 4 to 6 feet from the driving lane, or
has a 4 to 6 foot shoulder on the left side of the approach. The shoulder narrows (according to the minimum
shifting taper shown in FDM 11-25 Attachment 2.2 as the vehicle is anticipated to decelerate to 40 mph. When
the vehicle speed is anticipated to be 40 mph the 4-inch sloped curb and gutter transitions into a 6-inch vertical
curb and gutter. Both curb and gutter types should have a 24-inch gutter, therefore the flow line and gutter
flange are consistent. Total curb length starting from the yield line should be the deceleration distance required
to reduce from the approach speed to the fastest path design speed (R1).
Example: The posted speed is 55 mph, and decelerating to approximately 20 mph produces a desirable total
raised curb length distance of approximately 350 feet for the splitter island side of the roadway. Approximately
250 feet of that 350 feet is 4-inch sloped face curb and gutter and approximately 100 feet is 6-inch vertical face
curb and gutter (may be 6-inch sloped face on OSOW network, or 4-inch sloped in limited situations). At a
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posted speed of 40 mph and decelerating to 20 mph produces a desirable total raised curb length of
approximately 200 feet and all of the length is 6-inch vertical face curb and gutter (may be 6-inch sloped face on
OSOW network, or 4-inch sloped in limited situations). Deceleration distance guidance can be found in the 2011
AASHTO GDHS, Exhibit 2-25, page 2-35. Use the posted speed as the AASHTO design speed. Differing
approach conditions may produce different deceleration distances.
For the roundabout approach the minimum length of vertical face curb on the right side of the travel way should
be the greater of; 25 feet prior to the bike ramp or 100 feet prior to the yield line (may be 6-inch sloped face on
OSOW network, or 4-inch sloped). The vertical face curb installation will enforce the fastest speed path
geometry.
The curb on the right side at the exit from the roundabout needs to be long enough to control exit speed and
generally should be the greater of: 25 feet past the bike ramp or 100 feet past the exit measured from the ICD.
Consider drainage in the area of the curb/gutter by providing a flume or inlet structure.
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Figure 30.19 High-Speed Roundabout Approach
30.5.21.2 Curb and Gutter Separating the Circulatory Roadway from the Truck Apron
Use Type R or T curb and gutter, 4-inch sloped, between the circulating roadway and the truck apron shown in
SDD 8D1. Use a Type T inlet casting on the drainage structure, as shown in SDD 8A5. This curb and gutter is
gentle to large truck tires, but should be unfriendly for SUVs and autos to traverse. When the circulatory
roadway is concrete it shall be tied to the gutter flange with tie-bars, but not to the truck apron. When the
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circulatory roadway is asphalt, the apron shall be tied to the gutter flange with tie-bars. See FDM 14-10-35 for
pavement related topics.
30.5.21.3 Curb at the Inside of the Truck Apron or Edge nearest the Central Island
This curb shall be a reverse-slope 18-inch curb and gutter. The adjacent pavement will be a concrete truck
apron. There may be situations when this inside curb could be deleted, but this is rare and should be addressed
in the DSR.
30.5.22 Spirals
A spiral system involves a series of lane gains and lane drops around the circulatory roadway to lead drivers
into the appropriate lane for their desired exit. Spirals guide drivers that enter the roundabout on the inside lane
to shift to the outside lane at the appropriate location within the circulatory roadway to exit from the outside lane,
unless there are dual lefts then the two inside lanes could be shifted. The spiral is designed to prevent vehicles
from becoming trapped on the inside lane and then drivers making a quick lane change to exit all while
maximizing the use of the circulating space and reducing potential conflicts between adjacent vehicles. Spirals
can also accommodate for heavily biased turning movements. Spirals should only be considered where the
circulatory roadway has sufficient width to provide two or more lanes of traffic and where the geometry and
traffic volumes are determined to warrant the use of spirals. Circulatory roadway spirals require considerable
engineering judgment to design and locate properly, although they are intended to guide drivers, they may be
confusing to properly understand and not always intuitive to the driver. Small compact two lane circles do not
function as well with spiral designs because the lengths of arcs are too short to guide drivers to ‘spiral out’. In
such cases speed reduction occurs in the circulatory roadway where the spiral often begins. Drivers are more
likely to turn tight across the spiral rather than follow it to the next outside lane. Spirals can be very effective on
larger circles where the spiraling curves are longer, intuitive to drivers and more easily detectable.
A spiral should be developed from the central island by curb and gutter until a full lane width is available.
Observations of previously installed spiral crosshatch pavement markings without a ‘hard surface’ indicate that
some drivers ignore the pavement markings, which increases the potential for vehicle conflict in the circulatory
roadway.
An example of a curbed spiral is shown in Figure 30.20. This spiral is used to shift the westbound left turn to the
outside lane. The spiral is used because the southbound exit is only a single lane exit and the southbound
entrance allows dual left turns. To exit without conflict, the westbound left turn needs to be spiraled to the
outside lane. Without the spiral, the left turn would be trapped on the inside lane and would do a U-turn or have
to crossover lanes.
Figure 30.20 Spiral
30.5.23 Entry Angle, phi
Phi is not discussed in detail in NCHRP 672. This angle is not a controlling design parameter but instead a
gauge of sight to the left and ease of entry to the right. This affects both capacity and safety at the intersection.
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The typical range for the Phi angle is between 20 and 30-degrees with 25-degrees or greater being the optimal,
although there are designs that operate safely and efficiently with a Phi angle as low as 16 degrees. Designers
may find it difficult to attain Phi angle values in the desirable range, but provided that the fast path speeds are
relatively low, the Phi angle is not a controlling criterion.
There are three situations or design conditions in which Phi can be measured. They are:
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1. Condition 1: Phi = 2 where the distance between the left sides of an entry and the next exit are
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NOT more than approximately 100 feet. In Condition 1, the acute angle is denoted as 2 PHI in which
the actual value must be divided by two to obtain Phi (see Figure 30.21, Method 1).
2. Condition 2: Phi = Phi if the distance between the left sides of an entry and the next exit are more
than approximately 100 feet (see Figure 30.22, Method 2).
3. Condition 3: Applicable when an adjacent exit does not exist or an exit located at such a distance
or obtuse angle to render the circulatory roadway a dominating factor of an entry (such as in a “3-leg”
intersection). Used at “T” intersections or where the adjacent entrance and exit lane(s) are far apart
(see Figure 30.22, Method 2).
The two methods of measuring Phi are described below in Figure 30.21 and Figure 30.22. Method 1 phi is measured by dividing the entry and exit radii into three segments. The midpoint of the lane for each segment is best fit with a curve that extends to the face of curb of the splitter island extended. Begin line (a-b) and (c-d) at the intersection of the best fit arc and face of curb of the splitter island extended. Line (a-b) and (c-d) are then projected tangent from the best fit arc towards the circulating roadway, the angle formed by the intersection of the two lines is twice the value of Phi see Figure 30.21. Figure 30.21 Method 1 Phi Measurement
Method 2 Phi is measured by dividing the entry radii into three segments. The midpoint of the lane for each
segment is best fit with a curve that extends to the face of curb of the splitter island extended. Begin line (a-b) at
the intersection of the best fit arc and face of curb of the splitter island extended. Line (a-b) is then projected
tangent from the best fit arc towards the circulating roadway. Begin line (c-d) at the intersection of line (a-b) and
the arc located at the center of the circulating roadway. Line (c-d) is then projected tangent from the arc located
in the center of the circulating roadway. The angle formed by the intersection of (a-b) and (c-d) is Phi.
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Figure 30.22 Method 2 Phi Measurement
30.5.24 Clear Zone
Clear zone guidance for roundabout installations requires consideration of the approach speeds, fastest path
speeds, adjacent side slopes leading into and through the roundabout, and average daily traffic on the facility.
The guidance for the determination of clear zone is provided in the current AASHTO Roadside Design Manual
and FDM 11-15, Attachments 9 and 10.
The vehicle speed approaching an intersection and the speed allowed through an intersection, along with the
ADT and side slopes, will determined the required clear zone. A traffic signal controlled intersection allows
vehicles to go through the intersection at the posted speed, does not require the vehicle to reduce speed as it
approaches the intersection, and therefore the clear zone is maintained through the intersection. A stop sign
controlled intersection located in a high speed rural condition will require less clear zone as the vehicle slows
down to stop. As the approaching vehicle reduces speed it may be appropriate and desirable to reduce the
corresponding clear zone. The designer has the responsibility to balance the need for clear zone and right-of
way acquisition.
The yield condition for a roundabout and the fastest path design speed approaching and traveling through the
roundabout are similar to the stop sign controlled intersection. The horizontal geometrics leading to and through
the roundabout intersection requires the vehicle to slow down leading to the approach and through the
roundabout. The approaching speed transition distance for a roundabout is determined by the posted highway
speed and the deceleration needed to enter the roundabout in accordance with the fastest speed path
calculation, R1 value. FDM 11-26-30.5.21.1 and Figure 30.19 show how to determine the roundabout approach
layout for high-speed highways. The design speed to use for clear zone around the perimeter of the roundabout
is the average of the entry speed (R1) and the circulating path speed (R2) values. The maximum average entry
speed (R1) and circulating speed (R2) for any type of roundabout is approximately 25-30 mph. The average fast
path,
of approximately 25-30mph will produce a clear zone between 7 and 18 feet depending on ADT.
The exit ramps from an interchange are also considered to be low speed in close proximity of the approach to
the roundabout. In an urban environment lateral clearance is typically used rather than clear zone to determine
the minimum distance to fixed objects such as power poles, light poles, fire hydrants, trees etc. In a rural
environment it is typical to use a clear zone based on the design speed, ADT and slopes. The side slopes
adjacent to a roundabout are generally quite flat to accommodate a small terrace and a shared-use path around
the perimeter. When the shared-use path is not installed at the time of the roundabout the area should be
graded such that at some time in the future the path could be installed. The side slopes in the approach area
having an approach speed of 40mph or less and the perimeter of the roundabout, outside of the shared-use
path, should be 4:1 (recoverable slope) but may be steeper depending on meeting the clear zone requirement
and local impacts.
Central island clear zone is considered to be within a low speed environment therefore needs to meet the lateral
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clearance for urban streets, typically 2 feet back from the face of curb. Having stated this WisDOT believes
there are precautions, which are dependent upon the approach speed that need to factor into the central island
landscaping design. See FDM 11-26-40, for additional guidance on central island landscaping.
30.5.25 Coloring and Stamping Concrete
The truck apron shall be reddish colored, concrete conforming to Standard Spec 405.
For shared-use paths that are colored use a reddish colored concrete pavement. Colored pavement materials
are a community and designer agreed upon preference. Do not stamp pedestrian areas that will result in an
uneven surface as this may aggravate back injuries or violate other ADA considerations. Refer to TGM 3-2-18,
2009 MUTCD Section 3B.18, and Wisconsin MUTCD Section 3B.18, for additional information on marking
crosswalks and use of reflective materials. The colored concrete pavement could be used for terrace areas and
may be stamped, if not a walking surface, but stamping must be specified in the special provisions. Colored or
uncolored concrete in the terrace adjacent to the corner radii where there is the possibility of truck off-tracking
shall be 8-inch thickness or thicker depending on anticipated loading.
See FDM 14-10-35 for additional information relating to colored concrete, pavement design, tie bar location,
dowel bar location, contraction joint layout, and other pavement guidance.
30.6 Plan Preparation
30.6.1 Plan Preparation Considerations
The overall concept of roundabout plan preparation is similar to other intersection types (see example plan
sheets in FDM 11-26-50). The designer should provide the following plan information when designing
roundabouts. At a minimum, roundabout plans should include the following plan details:
- Layout details for any alignments utilized for the roundabout
- Layout details for any crosswalks and bike ramps if utilized
- Elevation at low points, high points, island noses, and 25 foot intervals within circulatory roadway
- Provide one 1”=40’ scale plan sheet for each concrete roundabout in the plans (1”=20’ scale is
preferred if it will fit on one sheet). Plan sheet will be used by the contractor to prepare the concrete
transverse joint details. This plan sheet must show all curb and gutter lines, longitudinal joint lines,
proposed pavement marking lane lines, surface utilities such as manhole covers, valve box covers,
and inlet covers in the concrete circulatory roadway and concrete truck apron.
- Storm sewer plans
- Landscaping and erosion control plans
- Permanent signing plans
- Lighting plans
- Pavement marking, and pavement marking-layout plans
30.6.2 Alignment Plans
When considering the location of alignments the designer should consider their usefulness in generating crosssections, profiles, layout details, and ease of use during construction layout. Alignments along both flange lines
of the splitter islands are required. The designer should also consider additional alignments for the following
locations:
- Along the curb and gutter flange line located between the truck apron and the circulatory roadway
- Along the curb and gutter flange lines at locations where the width is varying from the main alignments
(usual from bike ramp to bike ramp)
- Along the curb and gutter flange lines for both sides of right turn bypass lanes
- Along the back of sidewalks or shared use paths where the distance from the back of curb varies
- On OSOW routes: Along the inside of the central island and along the back of additional pavement
placed outside the entry/exit curbs
30.6.3 Profile Information
The designer should consider placing profiles on all of the alignments mentioned above. Some general
guidelines for creation of the profiles are:
- It is ideal from a drivability and safety perspective to design and construct the circular component of
the roundabout in one plane (planar) with one low point and one high point around the circle.
- Once the circulatory roadway profile is established, the approach and exit leg profiles can be adjusted
to match the outside edge of the circulatory roadway.
- Varying of cross-slopes may be done on the circulating lane(s), but the variance from 2% should
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generally be minimized where possible except where OSOW profile and grading design governs (see
FDM 11-26-30.6.1). Varying of cross-slopes may require the approach and exit profiles to be modified.
- The designer should also complete a profile on the outside edges to verify a smooth transition from the
approach roadway, roundabout and exit roadway. The designer may have to adjust profiles or crossslopes on the approach, in the roundabout or on the roundabout exit if there are major kinks in the
profile.
30.6.4 Typical Sections
At a minimum, roundabout plans should include typical sections at the following:
- Approaches and exits to the roundabout
- Within the splitter island
- Within the central island
The plans should include a sufficient number of cross-sections through the roundabout to allow for accurate
construction of the roundabout.
30.7 References
[1] Roundabout Design Guidelines, Ourston Roundabout Engineering, page 36 and 37
[2] NCHRP 672, Roundabouts: An Informational Guide, Second Edition, 2010
[3] Joint Roundabout Truck Study, Minnesota DOT and Wisconsin DOT, June 2012
FDM 11-26-35 Signing and Pavement Marking
March 4, 2013
35.1 Signing
The overall concept for roundabout signing is similar to general intersection signing. Proper regulatory control,
advance warning, and directional guidance are required to provide positive guidance to roadway users. Locate
signs where roadway users can easily see them when they need the information in advance of the condition.
Sign location should be checked so they are not in conflict with vehicle turning movements, the swept path of
vehicles with a long overhang, or vehicle navigation on the OSOW Freight Network. Signs should never obscure
pedestrians, motorcyclists or bicyclists. Signing needs differ for urban and rural applications and for different
categories of roundabouts. On connecting highways coordinate sign selection with the Region Traffic Section
and local agency to maintain consistency on the facility.
The signing and pavement marking can get complex on roundabout projects. To assist project managers and
contractors, the designer should use a minimum of 40 scale drawings for signing and pavement marking plan
sheets.
The Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD), the Wisconsin Manual on
Uniform Traffic Control Devices, Wisconsin DOT Traffic Guidelines Manual and appropriate sign plate details
govern the design and placement of signs. To the extent possible, this text follows the principles outlined in the
2009 MUTCD and the Wisconsin MUTCD supplement to the 2009 MUTCD.
35.1.1 Regulatory Signs
A number of regulatory signs are appropriate for roundabouts and are described below and shown in Figure
35.1.
1. Install a YIELD sign (R1-2) on both the left (in splitter island) and the right side of all approaches,
single lane and multilane entrances, to the roundabout. Attention should be given to ensure that the
left side YIELD sign and right side YIELD sign are mounted at the same height. Place a note on the
signing plans directing the contractor to make sure the YIELD signs are at the same mounting height
(7’ - 3”± to bottom of YIELD signs. During the first six months of operation of the roundabout, install
18” x 18” orange flags on top of the YIELD signs to emphasize the yield movement. Install a ONE
WAY sign, R6-2R, under the left side yield sign on all approaches, single and multilane entrances, to
the roundabout to establish the direction of traffic flow within the roundabout. Install a TO TRAFFIC
FROM LEFT sign, R1-54, under the right side yield sign on all approaches, single and multilane
entrances, to the roundabout to reinforce the yielding required at a roundabout.
2. A chevron sign (series of 4 chevrons, R6-4b) shall be used in the central island opposite the entrances
in combination with the ONE WAY sign (R6-1R). The mounting height to the bottom of the Chevron
sign is 48-inches, measured from the surface of the truck apron to the bottom of sign. Specify the four
(4) foot mounting height from the surface of the truck apron in the Miscellaneous Quantities.
3. Install a ONE WAY sign, R6-1R, in the central island opposite each entrance and mounted above the
chevron sign (R6-4b) to emphasize the direction of travel within the circulatory roadway.
4. Install a KEEP RIGHT sign (R4-7) at the nose of raised curb splitter islands. The mounting height of
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the R4-7 ranges from 5-feet to 7-feet to the bottom of the sign. In urban areas where pedestrians or
bicyclists are expected to use the crosswalk it is recommended to use the 7-foot mounting. The Down
Arrow, W12-1R, may be used but is less desirable for consistency and driver expectancy but may be
mounted 2-feet to the bottom of the sign. Attention should be given to the location of the KEEP RIGHT
sign and light poles on the right side to ensure that conflicts do not occur with larger width vehicles.
This is especially critical with single lane entry roundabouts.
Lane use signs such as the R3-8 sign are not used for single-lane entries. For multilane entries consult the
Regional Traffic Engineer for sign placement. Roundabout operation will dictate which R3-8 sign is installed.
Figure 35.1 Regulatory Signs
* The R3-8 sign is modified to show the placement of a dot under the left arrow, which graphically helps depict
the presence of a roundabout. Use the dot under the left arrow, only for the left most lane.
35.1.2 Warning Signs
A number of warning signs are appropriate for roundabouts and are described below and shown in Figure 35.2.
The amount of warning a motorist needs is related to site-specific intersection conditions and the vehicular
speeds on approach roadways. The applicable sections of the MUTCD govern the specific placement of
warning signs.
1. Install a circular intersection sign (“chasing arrows”, W2-6) on each approach in advance of the
roundabout. Below the W2-6 sign, install an advisory speed plate (W13-1). Rural roundabouts have a
typical advisory speed is 20 mph, urban roundabouts have a typical advisory speed of 15 mph. Check
with the Regional Traffic Engineer before assigning an advisory speed. The speed given on the
advisory speed plate should be no greater than the design speed of the circulatory roadway. Advisory
speeds are posted in multiples of 5 mph. For conventional highways with posted approach speeds of
45 mph or greater or 3 or more approach lanes, use size 3 W2-6, and W13-1 signs and double up the
placement of the W2-6, and W13-1 signs. For expressways, use size 4 W2-6, and W13-1 signs and
double up the placement of the W2-6, and W13-1 signs. Coordinate with the Region Traffic Section on
the proper sign sizes and type of roadway (conventional highway or expressway). For closely spaced
roundabouts, these signs may be omitted, see FDM 11-26-35.1.6 below for guidance as to when these
signs may be omitted.
2. Use a YIELD AHEAD sign (W3-2) on each approach to a roundabout if the approach speed is 45 mph
or greater. If the approach speed is less than 45 mph, the YIELD AHEAD (W3-2) would only be
needed if the yield sign is not readily visible for a sufficient distance per Table 35.1 (Minimum Visibility
Distance). For closely spaced roundabouts, this sign may also be omitted, see FDM 11-26-35.1.6 for
guidance as to when these signs should be omitted.
3. The usage of the pedestrian crossing sign assembly is optional per the 2009 MUTCD and is generally
used if the visibility of the pedestrian crossing is poor. The designer needs to coordinate the usage of
pedestrian crossing signs with the Region Traffic Section. In general, rural roundabouts will not have
pedestrian accommodations and therefore would not require signing. For closely spaced roundabouts,
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the pedestrian crossing sign assemblies may be omitted, see FDM 11-26-35.1.6 below for guidance
as to when these signs may be omitted, When used, If used, the pedestrian crossing sign assembly
shall be placed at the actual pedestrian crossing as well as in advance for locations where the posted
speed is 45 mph or greater. If there is a school crossing at the roundabout, the school warning sign
assembly with arrow (S1-1 and WF16-7L) is required at the crosswalk location. In addition, install the
school warning sign, AHEAD plaque and FINES HIGHER plaque (S1-1, WF16-9P and R2-6P) in
advance of the school crosswalk assembly. Install the pedestrian crossing sign (W11-2 and W16-7L)
or school crossing sign assembly (S1-1 and WF16-7L) just in front of the crosswalk for approaching
traffic at entries and exits. School crossing signs are required if there are any school pedestrians. If the
crosswalk at a roundabout is not considered to be part of the intersection and is instead considered a
marked mid-block crossing, pedestrian crossing signs are required.
The Combination Bike/Pedestrian Crossing sign (W11-15 and W16-7L) may be used in lieu of the
pedestrian crossing sign assembly if there are recreational trails crossing the roundabout, where the
primary trail users are bicyclists and pedestrians. The TRAIL CROSSING word message sign (W11
15A and W16-7L) may be used in lieu of the pedestrian crossing sign assembly if there are multi-use
recreational trails crossing the roundabout. The usage of these signs is optional per the 2009 MUTCD
and the designer is encouraged to coordinate the usage of these signs with the Region Traffic Section.
Placement criteria for these signs are the same as that of the pedestrian crossing signs mentioned
above.
4. A bicycle sign may be needed to designate the exit to the bike path (D11-1a and M7-2, federal sign
plate).
Locate pedestrian crossing signs in such a way to not obstruct the approaching driver’s view of the YIELD sign
or pedestrians standing at the crosswalk.
Flashing beacons may be used above some warning signs as a long-term awareness technique for areas with
approach speeds of 45 mph or higher.
Figure 35.2 Warning Signs
35.1.3 Guide Signs
Guide signs provide drivers with needed navigational information. They are particularly needed at roundabouts
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since circular travel may disorient unfamiliar drivers. Overhead guide signs should be considered at multilane
roundabout approaches to guide motorists into the proper travel lane in order to navigate the roundabout
properly and help avoid lane changing within the roundabout. A number of guide signs are appropriate for
roundabouts and are described below.
35.1.3.1 Intersection Destination/Direction Signs
Use intersection destination/direction style signs in all single lane approach roundabouts for rural locations and in urban/suburban areas where space allows and is appropriate. The diagrammatic style guide sign is preferred over the text style sign (D1 series sign); examples of both are shown in Figure 35.3. The circular shape in a diagrammatic guide sign provides an important visual cue to all users of the roundabout. Diagrammatic guide signs are preferred because they reinforce the form and shape of the approaching intersection and make it clear to the driver how they are expected to navigate the intersection. If lack of terrace space or longitudinal location spacing are issues, use a text style sign or overhead diagrammatic guide sign.
Use 4 1/2” lower case / 6” upper case letters with 18” Interstate, U.S. and State route shields and 15” County route shields for ground mounted signs in urban and rural areas where posted speed is less than 45 mph, and 2 or less approach lanes. Use 6” lower case / 8” upper case letters with 24” Interstate, U.S. and State route shields and 20” County route shields for signs in urban and rural areas if the signs are overhead, posted speeds are 45 mph or greater or there are 3 or more approach lanes. In general, the lettering height rule of thumb is to provide approximately 1-inch in letter height for each 40-foot of distance from the sign. All capital letters are harder to read than the first letter capitalized with the following letters small case. Cardinal directions shall be all capital letters with the first letter slightly larger.
The arrow direction conventions for the text signs follow the same convention as that for conventional intersections as shown in the 2009 MUTCD, §2D.37. The ahead destination is on top, the left destination in the middle and the right destination on the bottom. The curved-stem arrow (D1-1d signs) shown in the 2009 MUTCD, §2D.38 shall not be used.
Occasionally, Specific Information Signs (SIS - GAS, FOOD, LODGING, CAMPING or ATTRACTIONS) may need to be included on roundabout approaches. The arrow direction convention and placement of SIS signs follows the 2009 MUTCD, §2J.09.
Sample dimensioned details on the designs of diagrammatic signs, including the arrow and shaft dimensions
are shown on the Bureau of Traffic Operations A11-12 sign plate. Intersection destination signs may not be necessary at local street roundabouts or in urban settings where there are no significant destinations and the majority of users are familiar with the site.
Figure 35.3 Destination Signs
35.1.3.2 Overhead Lane Guide Signs
In general, overhead lane guide signs are encouraged at roundabouts with multiple approach lanes. By giving
destination guidance to the motorist in advance, the motorist will be able to be in the correct lane at the
roundabout approach and be discouraged from making a lane change within the roundabout. Qualifying criteria
for overhead lane guide signs would include two or more approach lanes, higher vehicle ADT’s, lane splits
approaching roundabouts, dual turn lanes, if the major route is turning, closely spaced roundabouts, narrow
terrace widths, unfamiliarity of drivers, and lane drops within the roundabout. Since these are lane use guide
signs, they would have an up arrow. A sign is placed over each travel lane (see multilane layout example in
Attachment 35.4) and the arrow is typically placed over the center of the lane. Coordinate sign designs with the
Region Traffic Operations section and the Bureau of Traffic Operations Traffic Design unit. If overhead guide
signs are used on an approach, then the circular diagrammatic guide sign may not be needed. The circular
diagrammatic guide sign is good for showing destinations and directions, however it does not depict proper lane
assignments like the overhead lane guide signs do.
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There may be situations in urban, multilane roundabout approaches where the overhead lane guide signs (Type
I) may not be feasible, (space constraints). Options for the overhead guide signs are shown in Attachment
35.32. Region Traffic Section approval is required to use these options.
The 2009 MUTCD allows the usage of combination lane-use / destination overhead guide signs (D15-1 and
D15-2). The advantage of these types of overhead signs is that they show both the route/destination with the
regulatory lane-use arrows, thus eliminating the need for additional installations of the ground mounted
regulatory lane control signs. It should be noted that these signs shall not be used for lanes that have optional
movements. They shall be used only for lanes that have an exclusive ahead, left or right turn movement. If the
roundabout approach has a lane that has an optional movement, then all signs on the approach should be the
Overhead Lane guide signs with separately mounted regulatory lane control signs. If the roundabout is part of a
closely-spaced corridor of roundabouts, (i.e., ramp terminals), the design of all Overhead Lane guide signs in
each direction shall match. If designs are mixed along the same direction, motorists may become confused by
the change in location of lane control information. Refer to Attachment 35.5 for further design guidance in
addition to consulting the Regional Traffic Engineer.
Use 8” lower case / 10.67” upper case letters with 24” Interstate, U.S. and State route shields and 20” County
route shields for all overhead signs. For situations with overhead structure loading limitations or on approaches
with posted speeds of 35 mph or less, 6” lower case / 8” upper case letters with 18” Interstate, U.S. and State
route shields and 15” County route shields may be used. Use a dot with the left arrow to designate the
roundabout. The dot shall only be used to depict the left-most lane of the approach. Use an ONLY plaque over
thru lanes that become turn lanes. The ONLY plaque is optional elsewhere. Consult the Regional Traffic
Engineer for further design guidance.
Sample details of overhead lane guide signs are shown in Figure 35.4. Additional dimensioned details on the
designs of diagrammatic signs, including the arrow and shaft dimensions are shown on the Bureau of Traffic
Operations A11-13 sign plate.
Generally use overhead sign supports, not sign bridge trusses. See FDM 11-55-20 for overhead sign support
design guidance.
Figure 35.4 Overhead Lane Guide Signs
35.1.3.3 Exit Guide Signs - In Splitter Island
Exit guide signs reduce the potential for disorientation. Use them to designate the destinations of each exit from
the roundabout. These signs are conventional intersection direction signs (D1 series signs). Exit guide signs
with route shields should have the shield incorporated into the sign with cardinal direction and arrow. If the same
route marker is used in more than one direction, the route shield should be accompanied with the cardinal
direction. The arrow is slanted up and to the right. At freeway ramp situations utilize the route continuation with
exit on the exit guide sign. Letter heights for signs are 4 1/2” lower case / 6” upper case with 12” route shields.
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Signs are placed in the splitter island facing the circulating traffic. The mounting height is to be a minimum of 60
inches from the ground to the bottom of the sign. Specify the revised mounting height in the special provisions.
Sample details of exit guide signs are shown in Figure 35.5. Additional dimensioned details on the designs of
the exit guide signs are shown on the Bureau of Traffic Operations A11-14 sign plate.
Figure 35.5 Exit Signs
35.1.3.4 Junction Assemblies
As with traditional intersections, consider using junction assembly consisting of either a “JCT” (M2-1) auxiliary
sign with the appropriate route markers or a junction (J1-1) assembly in advance of the roundabout.
35.1.3.5 Route Confirmation Signs
For roundabouts involving the intersection of one or more numbered routes, install confirmation assemblies
(J4’s) directly after the roundabout exit to reassure drivers that they have selected the correct exit at the
roundabout. Locate confirmation assemblies no more than 500 feet beyond the intersection in urban or rural
areas. If possible, locate the assembly’s close enough to the intersection so drivers in the circulatory roadway
can see them.
35.1.4 Urban Signing Considerations
Urban intersections tend to exhibit lower speeds. Consequently, the designer can, on a case-specific basis,
consider using fewer and smaller signs in urban settings than in rural settings. However, include some indication
of street names in the form of exit guide signs or standard street name signs. Also review proposed signing to
ensure that sign clutter will not reduce its effectiveness. Avoid sign clutter by prioritizing signing and eliminating
or relocating lower priority signs.
There are sometimes situations with multilane approach urban roundabouts where the right-of-way is tight and
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there is no physical room for typical overhead sign structures. There may be aesthetic considerations for
multilane approach urban roundabouts where large overhead guide signs may not fit in. Scaled-down versions
of overhead guide signs or J-assemblies may be utilized for these situations that may show route assembly
panels instead of large guide signs as shown in Attachment 35.4.
35.1.5 Rural and Suburban Signing Considerations
Route guidance emphasizes destinations and numbered routes rather than street names. The exit guide sign
needs to be visible (but discrete) from within the roundabout and much smaller than the typical rural shields and
lettering size. Six inch upper case and 4-1/2 inch lower case lettering height is the maximum needed.
35.1.6 Closely-spaced Multiple Roundabouts
Often times multiple roundabouts may be installed in close proximity to each other (roundabouts 1,000 feet
apart center to center, or less). This can often happen at interchange ramp terminals and roundabouts beyond
ramp terminals at frontage roads. Multiple roundabouts in close proximity to each other can cause signing
challenges due to longitudinal space constraints between the roundabouts. As a result, some signing may be
eliminated between the roundabouts. Visibility distance is based on stopping sight distance of vehicles. The
roundabout warning assembly signs (W2-6, W2-6P and W13-1), pedestrian warning signs (W11-2, W11-15,
W11-15A, W16-9P and W16-7L/R)and YIELD AHEAD (W3-2) may be eliminated between roundabouts if the
visibility distance between the roundabouts exceed the minimum visibility distance shown in Table 35.1. Other
signs may be eliminated with consultation with the Region Traffic Section. The roundabout warning assembly
signs and YIELD AHEAD would continue to be placed at the approaches to the first roundabouts in the series.
Table 35.1 Minimum Visibility Distance*
Posted or 85th
Percentile Speed
Minimum Visibility
Distance
25 mph
280 ft
30 mph
335 ft
35 mph
390 ft
40 mph
445 ft
45 mph
500 ft
50 mph
555 ft
55 mph
610 ft
* Minimum Visibility Distances are from Section 2C.36 of the Wisconsin Supplement to the 2009 MUTCD
35.1.7 Roundabouts in Close Proximity to Railroad Crossings
Railroad crossings in close proximity to roundabouts can present additional signing challenges due to safety
concerns involving railroad crossings and the installations of additional signs in spaces already containing
numerous of signs. Because each railroad crossing is unique, roundabout designers need to contact the Bureau
of Traffic Operations Traffic Design unit and the appropriate Region Traffic Operations section for the proper
signing and marking layout if the railroad crossing is 1000 feet or less from the roundabout.
35.1.8 Wrong Way Movements in Roundabouts
There is a potential for wrong way movements at roundabouts, especially roundabouts that are new in an area.
The typical signing applications include the usage of a chevron sign (series of 4 chevrons, W1-8a) in the central
island with a One Way sign (R6-1R sign) mounted above it. In addition, a One Way sign is mounted below the
left side YIELD sign. If wrong way movement problems persist, there are some signing options that can be
employed:
- Oversize ONE WAY (R6-1R) sign in the central island, above the chevron sign
- DO NOT ENTER (R5-1) signs mounted in the circular island to face potential wrong way traffic
- DO NOT ENTER (R5-1) and NO RIGHT TURN (R3-1) signs is required for roundabouts at ramps per
TGM 2-15-12 mounted on the outside radius of roundabout as shown in the detail in 2-15-12
35.1.9 Wide Turning Trucks in Roundabouts
As large trucks maneuver a multilane roundabout, often times they need to encroach into the adjacent travel
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lanes. In many multilane roundabouts, this happens by design. Occasionally there may be issues resulting from
large trucks encroaching into the adjacent travel lanes as they make the turn. For these problem areas, it may
be necessary to warn the motorist that the large trucks will encroach into the adjacent travel lanes in the
roundabout circle. The WATCH FOR WIDE TURNING TRUCKS (W8-73) sign may be installed on the
roundabout approaches for multilane roundabouts exhibiting these problems
35.1.10 Short Term Awareness Techniques
Some of the following bullet items are listed as short-term awareness techniques and others are mitigation
considerations after field problems have been identified. In either situation contact the Region Traffic Engineer
for guidance. Do not expect traffic control devices to accomplish what the geometric design cannot.
- Provide portable changeable message signs.
- Install orange flags on top of the YIELD signs during the first six months of operation.
35.1.11 Maintenance of Signs
For roundabouts on the STH System with county highway approaches and/or local road approaches, it is
recommended that early in the design process, a Maintenance Agreement needs to be developed. By having
the Maintenance Agreement developed early in the design process, the county or local unit of government will
clearly have knowledge of what they are to maintain.
Some particular items that should be included in the Maintenance Agreement would include:
- Specific signs that WisDOT would maintain and what the locals/county would maintain. This would also
include signposts.
- Specific overhead sign supports (if any), that WisDOT would maintain and what the locals would
maintain.
- Recommended inspection frequencies for overhead sign supports that the locals would maintain.
Further guidance on the maintenance of signs for roundabouts is included in the Traffic Guidelines Manual,
policy 2-15-52.
35.1.12 Signing Installation for OSOW Vehicle Routes
Careful attention must be given to signs that are installed for roundabouts on OSOW vehicle routes. Periodically
signs and posts may have to be temporarily removed to accommodate the vehicles to pass through the
roundabout and turn properly.
For roundabouts on OSOW routes, install tubular steel sign post assemblies or a comparable system (approved
by the Project Engineer) for the following signs:
1. Left side YIELD (R1-2) - ONE WAY (R6-2R) sign assembly
2. Right side YIELD (R1-2) - TO TRAFFIC FROM LEFT (R1-54) sign assembly
3. Exit Guide signs (D1 series) in the splitter islands
4. PEDESTRIAN CROSSING (W11-2m W16-7R or similar) sign assemblies at the intersection
crosswalks
5. Roundabout chevron bank (R6-4b) and ONE WAY (R6-1R) sign assembly in the circular island
6. Any signs located on the median island separating a right turn lane from the through lane(s)
7. Any additional signs on the outer portion of the roundabout circle
Install tubular steel sign post assemblies in accordance with Standard Spec 634.3.2. To help prevent bending of
the anchor tube and potential puncturing of vehicle tires, place the top of the 2 1/4” x 2 1/4” anchor level with the
top of the 18” diameter PVC box-out (which is at ground level). The box-out is typically filled with gravel or dirt
which will require about 2” of it to be removed in order to access the corner bolt when removing/reinstalling the
post. The designer will need to ensure that notes are placed on the permanent signing plan to notify contractors
of the required height of the top of the anchor system.
35.2 Pavement Marking
Pavement marking is needed on single and multilane roundabouts. The more complex the roundabout and the
higher the volume, the greater the need for proper pavement marking. Pavement marking must be closely
evaluated when designing a roundabout. Pavement marking is part of a “whole system” to consider, meaning
that various design concepts from geometric design, to signing, and pavement marking should complement
each other.
Typical pavement marking for roundabouts consists of delineating the entries, exits, bike lane accommodations
(only on approaches and exits), and marking the circulatory roadway. Single lane roundabouts need no lane
arrows or circulatory roadway pavement marking, except for edge line marking. Attachment 35.1 shows various
combinations of common roundabout lane configurations, including full and partial right-turn bypass situations.
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In order for roundabout markings to be effective and sustainable, they must:
- Be integrated with and preferably designed at the same time as the roundabout geometry
- Be configured to guide proper usage of the roundabout
- Help the motorist identify the correct lane as early as possible by the use of lane arrows on multilane
approaches and circulatory roadways
- Be designed and implemented collaboratively between Regional Traffic Operations and project
development staff with expertise in roundabouts and knowledge of maintenance considerations
- Use grooved in skid resistant durable materials
- Preformed thermoplastic for truck traffic and interior roundabout especially for arrows, words,
crosswalks and lines 8-inches or wider
- Preformed plastic may be used for channelizing 8 inch line approach marking.
- Special provisions for such markings can be found at:
https://trust.dot.state.wi.us/extntgtwy/dtid_bho/extranet/manuals/hottopics/index.shtm in the pavement
marking section
Markings not covered in this policy shall follow practices established by standard detail drawings or require the
approval of the Regional Traffic Engineer in collaboration with others who have knowledge of the design of
roundabouts. On connecting highways, (local jurisdiction), coordinate pavement marking with the Regional
Traffic Engineer and the local agency to maintain consistency on the facility.
It is just as important to make sure field layout and pavement marking application on the circulatory pavement is
located and positioned correctly. A pavement marking layout detail showing the exact locations is required on all
multilane roundabouts. Consider wheel tracking when developing the pavement marking layout detail.
Proper pavement marking within the circulatory roadway will help prevent left turns from the outer lane and thus
reduce exit crashes. Complex lane configurations should be reviewed by an experienced roundabout designer
and the Regional Traffic Engineer.
35.2.1 Approach Markings
1. Centerline marking on the approach to the splitter island may require a minimum of 500-foot segment
no passing barrier line as shown in SDD 15C18 “Median Island Marking”. Refer to Attachment 35.1
item Z.
2. Lane lines on the approach shall be 4 inches wide. The markings are at the standard spacing of 12.5
ft segment, 37.5-ft gap, unless an even segment of 12-ft segment, 12-ft gap is needed. Start when
flare widens to 9.5 feet for each lane. Match the width of line extended. Refer to items T and Da in
Attachment 35.1.
3. A lane line on the approach shall be 4 inches wide when it separates two through lanes. The line shall
be solid for a length of 50 feet in advance of the Point of Curve (P.C.) or as far as possible in advance
of the P.C. to allow minimum marked lane widths of 9.5 feet, whichever is shorter. Refer to items B
and I in Attachment 35.1.
4. When an approach lane is a turn only lane, the channelizing line shall be 8-inches wide and solid. A
R3-8 series Lane Control sign shall be placed for this type of approach. Refer to FDM 11-26-35.1.1.
The line shall be solid for a length of 50 feet in advance of the P.C., or as far as possible in advance of
the P.C. to allow minimum marked lane widths of 9.5 feet, whichever is shorter. Refer to item B in
Attachment 35.1.
5. When the left approach lane is a dropped lane/ exclusive turn lane, the approach dotted marking shall
be 8-inches wide with 3-ft segment, 9-ft gap. Consult with the Regional Traffic Engineer on the start of
this marking. Refer to item Db in Attachment 35.1.
6. The painted median splitter island marking on the approach shall be double yellow with 12-inch yellow
diagonal marking. The diagonal marking is optional if the island is less than 6-ft wide. When required,
the diagonals shall be spaced every 25-ft if the length is longer than 50-ft; spaced every 10-ft if the
length is 50-ft or less. Refer to items J and K in Attachment 35.1.
7. Lane separation markings (truck gores) shall be outlined by 8-inch white lines. Refer to item V in
attachment 35.1. When the separation is greater than 6-ft, 12-inch white chevrons shall be placed and
spaced every 25-ft if the length is longer than 50-ft; spaced every 10-ft if the length is 50-ft or less. The
chevrons are optional when the painted island is less than 6-ft in width. The point of the chevron shall
‘point’ upstream. Refer to item U in Attachment 35.1.
8. The edge line marking on the circle end of the splitter island will be white. Refer to Attachment 35.1
which shows the breakpoint from 8-inch white to 4-Inch yellow markings 5 feet in advance of the
curb/splitter island P.C (items M and N of the Special Case in Attachment 35.1). Refer to TGM 3-10-1
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and consult the Regional Traffic Engineer for further placement guidance of the yellow edge line
upstream of the roundabout.
When the yellow edge line marking is used to narrow the width of an entry or exit, 12-inch yellow
diagonal markings should be placed. When used, the diagonals shall be spaced every 25-ft if the
length is longer than 50-ft; spaced every 10-ft if the length is 50-ft or less. Refer to item QQ of the
Special Case in Attachment 35.1.
9. When two or more lanes approach a roundabout, lane use arrows shall be marked in each lane to
denote proper lane usage. Full complement of signing shall be installed as shown in Figure 35.1,
Regulatory Signs. Refer to item W in Attachment 35.1. Lane use arrows should not be used on singlelane approaches. Left turn arrows with the oval (Type 2R or Type 3R) shall only be placed in the left
most lane. Refer to SDD 15C7d for typical detail of a dot with left pavement marking arrow. The fishhook arrow shall not be used.
In addition to approach lane lines, appropriate lane arrows encourage balanced lane use, which
improves capacity and safety. Left turn arrows are important on multilane approaches, since traffic
otherwise has a bias towards the right-most lane. Place arrows to show the movements for each lane,
and to indicate permitted dual right or left turns. Place the arrows at or just before the point where the
channelizing or lane line begins or when the road widens to allow minimum lane widths of 9.5 feet.
This is intended as a visual cue to the motorist to select an appropriate lane for entering the
roundabout. Refer to SDD 15C 8-14e and Regional Traffic Engineer for guidance for multiple sets of
arrows.
10. Crosswalk markings should be placed such that vehicles approaching the roundabout are not likely to
stop on the crosswalk. A distance of 20 to 25 feet per stored vehicle back from the yield point is
typically appropriate. Refer to items Ha and Hb in Attachment 35.1 as well as crosswalk policy in TGM
3-2-18.
11. The word, “YIELD” placed prior to the dotted edge line extension is encouraged as an educational tool
initially as part of a project. It should typically be used on multilane approaches when necessary as a
tool for enforcement or where there are unusual geometrics, visibility problems, or crashes caused by
motorists failing to yield. An example is an approach with a high volume of through traffic that was not
required to stop or yield prior to the construction of the roundabout, especially where there is no side
road leg 90 degrees to the right. When used on a multilane approach, the “YIELD” word should be
placed in each approach lane. After initial placement, this marking should only be maintained as
necessary based upon crash data. Refer to item S in Attachment 35.1 and SDD 15C7-12b, Yield
Markings.
12. Dotted Edge Line Extensions shall be 18-inch-wide dotted white at 2-ft segment, 2-ft gap. Place
markings to avoid conflict between the entering vehicle and internal roundabout traffic, this is the point
where entering traffic must yield. Refer to item A in Attachment 35.1.
Approach and entry pavement markings consist of lane line, channelization marking, dotted edge line extension
marking (yield line) and symbol markings. Consider high durability markings on the approaches. Refer to
Attachment 35.2 for approved pavement marking materials and their locations at a roundabout. Consult with the
Regional Traffic Engineer before determining final pavement marking materials.
35.2.2 Circulatory Roadway Marking
13. Lane lines within the roundabout shall be 4-inch or 8-inch width, with a 6-ft segment, 3-ft gap marking
cycle. These lines shall be the same width as the lines they extend. Lane lines in the circle can have a
spiral effect and together with proper lane assignment guide motorists through the roundabout to the
appropriate exit eliminating the need to change lanes. Refer to item C in Attachment 35.1. For
longevity, place the markings to avoid wheel paths of the intersecting traffic.
14. When used, dotted line markings shall be the same width of the lane lines and 1-ft segment, 3-ft gap
marking cycle. Refer to item E in Attachment 35.1.
15. When two lanes are allowed to proceed around the circle, Lane use arrows shall be marked in each
lane within the roundabout adjacent to each splitter island to denote proper lane usage. Arrows placed
within the circulatory roadway shall not include the oval. Refer to item X in Attachment 35.1.
35.2.3 Exit Marking
16. Chevron markings at the exit point adjacent to the splitter island shall be 12-inch white with 10-ft
spacing where needed or appropriate. Refer to item O of the Special Case in Attachment 35.1.
Chevron makings on the exit and/or on the approach should be avoided, if possible, as they do not
provide the speed control and directional guidance as curb and gutter. Consult experienced
roundabout designers and the Regional Traffic Engineer before implementing.
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17. Do not paint the noses of the splitter island yellow (where the splitter island meets the circulatory
roadway, unless there is a documented crash problem). Yellow nose paint is intended to separate
opposing directions of traffic such as the approach nose.
35.2.4 Bicycle Marking
18. When required, bike lane markings should be placed as per Figure 35.6.
Bike lane marking within the circulatory roadway is not permitted on any roundabouts. Refer to Figure 35.6 for
Bike Lane markings on roundabout approaches.
Figure 35.6 Bike Lane Roundabout Marking
35.2.5 Maintenance of Pavement Marking
For roundabouts on the STH System with county highway approaches and/or local road approaches, it is
recommended that early in the design process no later than the time of the design study report, a Maintenance
Agreement be developed. By having the Maintenance Agreement developed early in the design process, the
county or local unit of government will clearly have knowledge of what they are to maintain. Refer to TGM 3-3-1
for additional roundabout pavement marking guidance and policy.
Sample Signing Layout for a Multilane Roundabout
Attachment 35.4
Attachment 35.5
Sample Signing Plan for Roundabout Ramp Terminals
LIST OF ATTACHMENTS
Attachment 35.1
Example Pavement Markings for Typical Designs
Attachment 35.2
Roundabout Pavement Marking Bid Item and Product Type
Attachment 35.3
DM 11-26 Attachment 35.3 Sample Signing Layout for Single-lane Roundabout
Attachment 35.4
Sample Signing Layout for a Multilane Roundabout
Attachment 35.5
Sample Signing Plan for Roundabout Ramp Terminals
FDM 11-26-40 Landscaping and Maintenance
March 4, 2013
Illumination has been moved to TGM 11-11-1.
40.1 Landscaping
Landscape elements are vital to the proper operation of a roundabout and needs to be in place when the
roundabout is opened to traffic. The purposes of landscape elements in the roundabout are to:
- Make the central island conspicuous to drivers as they approach the roundabout
- Clearly indicate to drivers that they cannot pass straight through the intersection. Restrict the ability to
view traffic from across the roundabout through mounding of the earth and plantings. This will lead to
slower entering speeds, which increases safety.
- Require motorist’s to focus toward on-coming traffic from the left
- Help break headlight glare
- Discourage pedestrian traffic through the central island
- Help blind and visually impaired pedestrians locate sidewalks and crosswalks
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- Improve and complement the aesthetics of the area
When designing landscaping for a roundabout it is important to:
- Consider maintenance requirements early in the program stages of development
- Develop a formal municipal agreement describing the landscaping and maintenance requirements for
roundabouts elements early in the scoping process and prior to design of the facility.
- Maintain adequate sight distances
- Avoid obscuring the view to signs
- Minimize fixed objects such as trees, poles, or guard rail
- Apply the guidance below relative to approach speeds and the permissible use of fixed objects such
as trees, poles, non-hazard walls, non-hazard rocks/boulders, or guard rail
Landscape the central island by mounding the earth and providing planting. Refer to Figure 40.1 for the general
layout of the central island. The clear zone and lateral clearance requirements for roundabouts are provided in
FDM 11-26-30.5.24. The truck apron is not part of this clear zone distance. The clear zone for the central island
is considered to begin at the inside curb adjacent to the central island landscaping.
The combination of the earth mound and plantings in the central island shall provide a visual blocking such that
drivers will not be able to see through the roundabout central island. The central island area is considered a low
speed environment, however errant vehicles occasionally end up in the central island or crossing the central
island.
The approach highway speed is an indicator of the probability of an errant vehicle entering the central island.
Therefore, when the posted speed on any approaching leg to the roundabout is greater than 30 mph the
following items are prohibited within the central island:
- Hazardous material - such as concrete, stone, or wood walls
- Fixed objects - including trees having a mature diameter greater than 4-inches
Where the approaching leg to a roundabout has a posted speed of 30 mph or less there may be objects that
appear to be hazardous such as walls or rocks, but they are to be constructed with materials and in a manner
that is not hazardous to errant vehicles. It is important to minimize the consequences of an errant vehicle that
may impact a wall or rocks/boulders. The inner portion of the central island is typically most vulnerable to
drivers/vehicles that for some reason leave the roadway and enter the central island at a high impact angle. If in
the event that a driver is driving too fast to negotiate a curved approach to a roundabout , or otherwise
distracted and/or is not aware of the upcoming roundabout the impact angle entering the central island typically
will be much greater than 25 degrees and outside the realm of roadside design. The consequence of hitting a fixed object at an angle greater than 25 degrees is severe. Minimize the consequence of hitting a wall or boulders by following these guidelines:
1. Do not allow any walls in the central island with cast in-place or reinforced concrete or natural
boulders.
2. Construct any walls with light-weight, Styrofoam type, artificial bricks/blocks typically used in
landscaping and boulders with chicken wire and stucco. No mortar or reinforcing between the
bricks/bocks. Minimize the wall thickness while maintaining stability.
3. If light-weight walls are desired for aesthetic reasons then construct at a height 20-inch or lower. This
will tend to keep flying debris at a lower level as not to penetrate a windshield, or impact other
vehicles.
4. Do not allow fill material in back of the light-weight brick/block wall for approximately 2 feet. Then at
ground level begin to slope the earth up and away from the non-hazardous wall at a 6:1 slope or
flatter.
Design the slope of the central island with a minimum grade of 4% and a maximum of 6:1 sloping upward
toward the center of the circle. The earth surface in the central island area forms an earth mound that is a
minimum of 3.5-feet to a maximum of 6-feet in height, measured from the circulating roadway surface at the
curb flange. As an absolute minimum, keep the outside 6 feet of the central island free from landscape features
to provide a minimum level of roadside safety, snow storage, and unobstructed sight distance. In some
situations this central island area may need to maintain a low profile beyond 6-feet to allow OSOW vehicle loads
to pass over the central island without the axles passing over the central island,( i.e. 165-foot girder, wind
turbine parts).
Avoid items in the central island that may be considered an attractive nuisance that may encourage passersby
to go to the central island for pictures, or other objects that might distract drivers from the driving task.
When reasonable, consider a frost proof water supply (small hand hydrant, not fire hydrant) and electrical supply
to the central island. The water supply should be considered for long term use not just to establish plant material
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during the two-year surveillance and care period. When planning utilities such as water and/or electricity in the central island, they must be discussed with the local unit of government as to need, proximity to the site and who would pay operating costs after installation. Cost agreements shall be included in the project agreement for water and electric costs and agreed to prior to design of the roundabout. Do not install street furniture in the central island as it may attract pedestrian traffic, such as benches, decorative statues, community welcome signs, monuments or large fixed hazardous landscaping objects (walls, rocks, etc.).
Landscape design elements for municipalities/communities that are in excess of Department standards may wish to seek funding through Community Sensitive Design (CSD) or enhancement funds. 40.2.1 Landscape Design
Landscape design is an important aspect of roundabout operation. Before starting the landscape design first
determine the maintaining authority and comply with the intersection sight distance as described in FDM 11-26
30.5.15. More flexibility is allowed on projects that are not maintained by WisDOT. Low-to-the-ground landscape plantings in the splitter islands and approaches can both benefit public safety and enhance the visual quality of the intersection and the community. In general, unless the splitter islands are very
long or wide they should not contain trees, planters, or light poles.
Landscape plantings on the approaches to the roundabout can enhance safety by making the intersection more
conspicuous and by countering the perception of a high-speed through traffic movement. Avoid landscaping
within 50 feet in advance of the yield point. Plantings in the splitter islands (where appropriate) and on the right
and left side of the approaches (except within 50 feet of the yield point) can help to create a funneling effect and
induce a decrease in speeds approaching the roundabout. Low profile landscaping in the corner radii can help
to channelize pedestrians to the crosswalk areas and discourage pedestrian crossings to the central island.
40.2.1.1 Owned, Operated, and Maintained by WisDOT
The goal for State-owned and maintained roundabouts is to achieve a landscape design that enhances the safety in the area of the central island and splitter islands with little or no landscape maintenance required over time. Landscape design elements should minimize areas of mulch and the planted vegetation that requires maintenance. Low maintenance planting plans for roundabout landscapes are required. Vegetation approved for use by the department requires minimum maintenance and has been demonstrated to tolerate highway site conditions. The central island earth berm may be planted with trees and shrubs and/or a prairie grass mixture that doesn’t require mowing. Plant materials approved for use by the Department, including trees and shrubs listed in FDM 27-25 Attachment 1.3 are approved for use on roundabouts owned, operated and maintained by the Department. Certain native grasses are also approved at roundabouts and are included in the grasses portion of the “Table of Native Seed Mixtures” in Standard Spec 630. Locations of plant materials shall be selected for salt tolerance and be located to allow for sufficient snow
storage in the winter. Snow removal operations typically radiate out from the central island. Plant materials shall not be placed so as to impede snow removal practices.
The uses of pre-emergent herbicides are recommended for use in plant bed and “hardscape” areas. Follow label instructions provided on the product container for use and application procedures.
Contact the Highway Maintenance and Roadside Management Section in the Bureau of Highway Operations for additional landscape design guidance. Page 83
Figure 40.1 Low-Maintenance Central Island Landscaping
40.2.1.2 Owned by WisDOT but Maintained by Others
Landscape design requests in excess of FDM 11-26-40.2.1.1 will be considered only upon receipt of a formal,
signed project agreement prior to design of the facility and are the sole responsibility of the requesting
municipality. These agreements are to be obtained in the planning stages of the project.
40.2.1.3 Local Roads and Connecting Streets
Landscape design costs in excess of department standards described in FDM 11-26-40.2.1.1 on local roads and
connecting streets are the sole responsibility of the municipality.
40.3.2 Landscape Maintenance
Maintenance responsibilities for roundabouts will vary by ownership. Roundabouts are located on the local road
system, on connecting state highways, and state highways.
40.3.2.1 Owned, Operated, and Maintained by WisDOT
All maintenance costs and operations of roundabout landscaping owned, operated and maintained by the
department are the responsibility of the department, except as provided below. Landscape design elements and
guidance have been outlined to minimize maintenance and operational costs to the department. Plants shown
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on the approved list have been selected to best meet these needs, FDM 27-25 Attachment 1.3. FDM 11-26-30
and Figure 40.1 provide detailed layout dimensions of the area to be planted within the central island area.
Only those landscape maintenance operations necessary to maintain the safe operation of the department
roundabout will be undertaken.
40.3.2.2 Owned by WisDOT but Maintained by Others
Municipalities often request special landscaping. Landscape requests in excess of requirements contained in
FDM 11-26-40.2.1.1 are the responsibility of the requesting municipality. Such requests will be considered only
upon receipt of a formal, signed municipal agreement approved by the department prior to the design of those
roundabouts. This procedure shall be completed early in the planning stages of project development.
40.3.2.3 Local Roads and Connecting Streets
Maintenance and operating costs of roundabouts located on local roads and connecting streets are the
responsibility of the local government.
40.4 Shared-Use Path Maintenance
For urban, suburban, outlaying and rural locations for roundabouts, a roundabout sidepath or shared-use path is
provided accordingly; see FDM 11-26-30.5.13. Facilities may be omitted if conditions are met as defined in
Trans 75 exception process see FDM 11-46-1. Appropriate cost share policies apply and maintenance
agreements with the local unit of government are required, unless refusal to maintain omission conditions are
met see FDM 11-46-1. If conditions are met to omit facilities, grading for future facilities apply as detailed in
FDM 11-26-30.5.13 and cut-through crossing are to be provided in splitter islands. The cost of the path
installation and maintenance after the original roadway improvement is the responsibility of the local unit of
government. There have been situations where land uses change, the local government leaders change, and/or
attitudes about such improvements change, or that pedestrian or bicycle volume increase over time, and later
there is a strong desire to install the path.
FDM 11-26-45 Work Zone Traffic Control
March 4, 2013
45.1 Work Zone Traffic Control
During construction, traffic control by police and/or construction personnel (i.e. flagging) may be needed. Space
channelizing devices so that motorists, bicyclists, and pedestrians have a clear indication of the required travel
path and turning radii. This may require closer spacing than the MUTCD would otherwise specify. SDD15D21
and SDD15D31 show example device spacing at turning radii and curve transitions. Evaluate traffic control
needs for each roundabout installation on a site-specific basis until the Department develops the expertise in
roundabout construction to provide guidance.
45.1.1 Pavement Markings
Because of the confusion of a work area and the change in traffic patterns, pavement markings must clearly
show the intended travel path. Misleading pavement markings shall be removed or covered in accordance with
the Wisconsin Standard Specifications. As new pavement courses are placed consider specifying in the plans
that splitter island delineation and broken white lines on the outside edge of the circulatory roadway be marked
the same day the pavement course is placed according to Wisconsin Standard Specifications. When pavement
markings are not practical, or misleading markings cannot be adequately deactivated, use closely spaced
channelizing devices to define both edges of the travel path. When possible, pavement markings used within the
work zones should be the same layout type and dimension as those to be used in the final layout. Additional
pavement markings may be necessary to avoid confusion from changing traffic patterns used in staging.
45.1.2 Signing
Construction signing for a roundabout should conform to the MUTCD and the Standard Detail Drawings. Provide
all necessary signing for the efficient movement of traffic through the work area, including pre-construction
signing advising the public of the planned construction, and any regulatory and warning signs necessary for the
movement of traffic outside of the immediate work area. The permanent roundabout signing may be installed,
where practicable, during the first construction stage so that it is available when the roundabout is operable, but
these signs must be covered until they are needed. Consider using portable changeable message signs when
traffic patterns change.
45.1.3 Lighting
Illuminate the temporary construction area through the intersection where possible. Consider adjacent lighting
conditions, traffic volumes during the evening when the roundabout is illuminated, and mixture of use such as
pedestrians and trucks.
Page 85
45.1.4 Construction Staging
The Transportation Management Plan, FDM 11-50-5, will consider detouring traffic away from the intersection
during construction of the project. A detour will significantly reduce the construction time and cost, increase the
safety of the construction personnel and will provide for an overall better finished product
It is desirable to complete construction as soon as possible to minimize the time the public is faced with an
unfinished layout or where the traffic priority may not be obvious. If possible, all work, including the installation of
splitter islands and pavement marking, should be done before the roundabout is open to traffic.
If it is not possible to detour all approaches, detour as many approaches as possible. Carefully consider
construction staging during the design of the roundabout if it must be built under traffic. Minimize the number of
stages if at all possible. Staging should accommodate the design vehicle and maintain sightlines.
Prior to the work that would change the traffic patterns to that of a roundabout, certain peripheral items may be
completed including permanent signing (covered), lighting, and some pavement markings that reflect actual
conditions. These items, if installed prior to the construction of the central island and splitter islands, would
expedite the opening of the roundabout and provide additional safety during construction.
As is the case with any construction project, install appropriate traffic control devices as detailed in the project
plans and the Standard Specifications. This traffic control shall remain in place as long as it applies and be
removed when it no longer applies to the condition. Maintain consistent traffic control; do not change between
stop and yield control multiple times during construction.
Stage the construction as follows unless a different staging plan is approved during design:
- Install and cover proposed signing
- Remove or mask pavement markings that do not conform to the intended travel path
- Construct outside widening if applicable
- Reconstruct approaches if applicable
- Construct splitter islands and delineate the central island. Uncover the signs at this point and operate
the intersection as a roundabout
- Finish construction of the central island
If it is necessary to leave a roundabout in an uncompleted state overnight, construct the splitter islands before
the central island. Any portion of the roundabout that is not completed must be marked, delineated, and signed
in such a way as to clearly outline the intended travel path. Remove or mask pavement markings that do not
conform to the intended travel path. Consider adding temporary lighting if the roundabout will be used by traffic
in an unfinished state overnight, or install the permanent lighting that is in operational condition.
45.1.5 Public Education
The Transportation Management Plan, FDM 11-50-5, will advise the public whenever there is a change in traffic
patterns. Education and driver awareness campaigns are especially important for a roundabout because a
roundabout will be new to most motorists. The Regional Communication Manager coordination through both
design and construction is typically vital to the success of a project. Provide brochures on how to drive, walk and
bicycle through a roundabout. The following are some specific suggestions to help alleviate initial driver
confusion:
- Hold public information meetings prior to construction
- Prepare news releases/handouts detailing what the motorist can expect before, during, and after
construction
- Consider the creation of a project website, flash animation graphics, traffic simulation recording ( such
as Paramics, etc.) or the use of social media before and during construction
- Install portable changeable message signs or fixed message during construction and before
construction begins. Advise drivers of anticipated changes in traffic patterns for about one week prior
to the implementation of the new pattern.
- Use Wisconsin 511, news media (and Highway Advisory Radio, if available) to broadcast current
status of traffic patterns and changes during construction. Also, if appropriate, establish a web site, to
post up-to-date traffic and construction information.
FDM 11-26-50 Design Aides
March 4, 2013
50.1 Example Plan Sheets
Several example plan sheets of the above information have been provided as an aide to the designer when
completing roundabout plans. The plan sheets provided are examples and should only be used as guidance.
FDM 11-26-50.1.pdf is a .pdf of the various plan sheets. The PDF attached has bookmarks for the various plan
Page 86
sheets as noted above to assist you in viewing the sheets.
- Project Overview
- Typical Section
- Construction Details
- Pavement Elevation (Concrete)
- Pavement Elevation (Asphalt)
- Erosion Control
- Storm Sewer
- Landscaping
- Permanent Signing
- Lighting
- Pavement Marking
- Construction Staging
- Plan and Profile
- Cross-Sections
50.2 Creating Roundabout Fastest Paths (B-spline Curves) and Using AutoTurn software
Spline curves can be created in both AutoCAD and MicroStation. In AutoCAD, they are called polylines and in
MicroStation they are called B-spline curves. Instructions for creating roundabout fasted paths B-spline in AutoCAD 3D is in Attachment 50.1, and for creating roundabout fasted paths B-spline in Microstation Version 8 is in Attachment 50.2. Instructions for using AutoTurn software in AutoCAD Civil 3D and MicroStation is in Attachment 50.3. 50.3 OSOW Vehicle Inventory Evaluation Overview
Use AutoTurn or AutoTrack software for OSOW horizontal evaluation and AutoTurn Pro for low clearance evaluation (DST lowboy). Refer to these links for videos and assistance in using these tools.
This is the link to the AutoTurn Pro tutorial videos:
ftp://ftp.dot.wi.gov/dtsd/bpd/methods/ground-clearance-training
The vehicles in WisDOT’s OSOW library that have rear steering capabilities:
- 55 Meter Wind Blade
- 165' Beam
- Wind Tower Section, 78’L x 14.7’W
The easiest of these three is the Wind Tower Section 78’L x 14.7’W because the rear steering is linked to the
front. So all you have to do is just drive the vehicle and the rear steers itself. The Wind Tower Upper-Mid
Section is not set up as a rear steer vehicle.
The 55 Meter Wind blade and the 165' Beam are a little more complicated because they have rear steering that
is completely independent of what the front axle is doing. For those, when you initiate a swept path command,
you will see a check box called "Override Angle". You need to check that box to control the steering of the rear
axles. In AutoCAD Civil 3D, the rear steering is then controlled by holding the Ctrl key and using the wheel on
your mouse as you move through the swept path.
Page 87
Figure 50.1 Check Box for Override Angle
LIST OF ATTACHMENTS
Attachment 50.1
Creating Roundabout Fastest Paths (Spline Curves) in AutoCAD Civil 3D
Attachment 50.2
Creating Roundabout Fastest Paths (Spline Curves) in Microstation Version 8i
Attachment 50.3
Guide for Using AutoTURN in AutoCAD Civil 3D and MicroStation Version 8i
Page 88

Wisconsin Department of Transportation Roundabout Guide

Transcription

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