Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-23-023 MARCH 2023

Reasons for Drivers Failing To Yield 

at Multi-Lane Roundabout Exits: 

Transportation Pooled Fund Study 

Final Report
FOREWORD

Roundabouts represent an important intersection control strategy for the Federal Highway 

Administration (FHWA) due to their ability to drastically reduce severe crashes. They are listed 

as one of FHWA’s Proven Safety Countermeasures. Since FHWA published its first roundabout 

informational guide in 2000, the estimated number of roundabouts in the United States has 

grown from fewer than a hundred to several thousand. 

As roundabouts become more common across a wide range of traffic conditions, roundabouts 

with more than one lane are seeing increased use. These multi-lane roundabouts have 

demonstrated good safety performance with respect to crashes involving injuries or fatalities, but 

some multi-lane roundabouts have experienced higher-than-desired property damage only (PDO) 

crashes, especially at exits. This final report details the work performed for a Transportation 

Pooled Fund project that investigates root causes and contributing factors for PDO crashes near 

multi-lane roundabout exits. The findings presented here provide guidance to practitioners and 

public agencies in support of proactive reductions in the potential for crashes at new multi-lane

roundabouts and possible countermeasures to reduce PDO crashes at existing sites.

Brian P. Cronin, P.E. 

Director, Office of Safety and Operations 

Research and Development 

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No. 

FHWA-HRT-23-023

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Reasons for Drivers Failing To Yield at Multi-Lane 

Roundabout Exits: Transportation Pooled Fund Study Final 

Report

5. Report Date

March 2023 

6. Performing Organization Code:

7. Author(s)

Coprincipal investigators: Alejandra Medina (Virginia Tech 

Transportation Institute) and Justin Bansen (Kittelson and 

Associates, Inc.)

Coauthors: Brian Williams, Alek Pochowski, Lee Rodegerdts, 

Jamie Markosian, Yingfeng Li, and Ronald Gibbons

8. Performing Organization Report No.

9. Performing Organization Name and Address

Virginia Tech Transportation Institute

3500 Transportation Research Plaza

Blacksburg, VA 24060

Kittelson & Associates, Inc.

100 M Street SE, Suite 190

Washington, DC 20003

10. Work Unit No.

TRAIS

11. Contract or Grant No.

DTFH6116D00041L 

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered

Technical Report; October 2018–March 2023

14. Sponsoring Agency Code

HRSO-10

15. Supplementary Notes

The Contracting Officer’s Representative was Wei Zhang, Ph.D., P.E. (HRDS-10; ORCID: 

0000-0002-5175-7527). Contributing agencies to this Transportation Pooled Fund study include the following: 

City of Hilliard (OH), Connecticut Department of Transportation (DOT), Florida DOT, Georgia DOT, New York 

State DOT, Washington State DOT, and Wisconsin DOT.

16. Abstract

The Federal Highway Administration (FHWA) has included roundabouts on a list of proven safety 

countermeasures since 2008. However, higher than expected numbers of property damage-only crashes occurring 

near multi-lane roundabout exits has emerged at some U.S. roundabouts. This report details a Transportation 

Pooled Fund study that investigates root causes and contributing factors for these crashes based on an evaluation of 

a sample of eight multi-lane sites. For each study site, the project team reviewed historical crash data; conducted 

field observations; collected drone video data; analyzed vehicular volumes and conflicts; and reviewed potential

influences from roundabout geometry, signing, and markings and the roadway network. Findings related to each of 

these topics are presented throughout this report to provide guidance that supports proactive reductions in the 

potential for these crashes at new multi-lane roundabouts and potential countermeasures to reduce these crashes for 

existing sites.

17. Key Words

Roundabout, intersection, safety, traffic control, 

crashes

18. Distribution Statement

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

148 

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.
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iii

TABLE OF CONTENTS

CHAPTER 1. BACKGROUND ................................................................................................... 1

CHAPTER 2. SITE SELECTION AND DATA COLLECTION ........................................... 7

Site Selection Process and Site Characteristics .................................................................... 7

Project Data Collection and Field Investigations .............................................................. 20

CHAPTER 3. SAFETY ANALYSIS ......................................................................................... 23

Crash Severity ....................................................................................................................... 25

Crash Type ........................................................................................................................... 27

Crashes Involving Failure To Yield ................................................................................. 33

Crashes Involving Left Turns From Incorrect Lanes ....................................................... 37

Crashes Involving Right Turns From Incorrect Lanes .................................................... 41

CHAPTER 4. CONFLICT ANALYSIS ................................................................................... 45

Event Types ........................................................................................................................... 45

Violation Types ..................................................................................................................... 45

Events by Location ............................................................................................................... 47

Events by Volume ................................................................................................................ 51

CHAPTER 5. DESIGN INFLUENCES .................................................................................... 53

Large Conflict Areas ............................................................................................................ 55

Approach Lanes Ending in Mandatory Turn Lanes ......................................................... 66

Alignment of Entry Lane Additions and Exit Merging .................................................... 68

Reverse Curves Through Exits ............................................................................................ 75

Small Diameters .................................................................................................................... 77

Entry Alignment ................................................................................................................... 78

Circulatory Roadway Striping ............................................................................................ 84

Yield Signs and Pavement Markings .................................................................................. 89

Advance Lane-Control Signs and Pavement Marking Arrows ....................................... 93

Circulatory Lane-Use Pavement Marking Arrows .......................................................... 94

CHAPTER 6. NETWORK INFLUENCES ........................................................................... 103

Interchange or Other Major Network Connection Proximity ...................................... 105

Adjacent Land Uses and Access ........................................................................................ 110

Close Intersection Spacing ................................................................................................. 115

Adjacent Traffic Signal Influences ................................................................................... 116

Downstream Lane Drops ................................................................................................... 119

CHAPTER 7. SUMMARY OF FINDINGS ........................................................................... 121

Summary of Key Findings ................................................................................................. 121

Analysis of Key Findings .................................................................................................... 123

Menus of Countermeasures .............................................................................................. 124

Lane Configurations ....................................................................................................... 125

Network Modifications ................................................................................................... 127

Approach Signing and Markings .................................................................................... 128

Entry Improvement Countermeasures ............................................................................ 129

Circulatory Roadway and Exit Countermeasures ........................................................... 130
iv

Geometric Countermeasures .......................................................................................... 132

ACKNOWLEDGEMENTS .................................................................................................... 135

REFERENCES ......................................................................................................................... 137
v 

LIST OF FIGURES

Figure 1. Photo. Example 2x1 roundabout configuration (Tampa, FL). ........................................ 2

Figure 2. Photo. Example 2x2 roundabout configuration (Lakeville, MN). .................................. 3

Figure 3. Photo. Entering vehicle failing to yield to both lanes. ................................................... 5

Figure 4. Photo. Left turn from incorrect lane. ............................................................................... 5

Figure 5. Photo. Right turn from incorrect lane. ............................................................................ 5

Figure 6. Photo. Carmel, IN—Aerial view of roundabout at 106th Street and Illinois Street. ..... 12

Figure 7. Photo. Oshkosh, WI—Aerial view of roundabout at Jackson Street (STH 76) and 

Murdock Avenue (US 45). .................................................................................................. 12

Figure 8. Photo. De Pere, WI—Aerial view of roundabout at Monroe Road (CTH GV) and 

Dickinson Road (CTH G). ................................................................................................. 13

Figure 9. Photo. Melbourne, FL—Aerial view of roundabout at Lake Andrew Drive and 

Wickham Road. ................................................................................................................... 13

Figure 10. Photo. Carmel, IN—Aerial view of roundabout at 116th Street and Illinois 

Street. .................................................................................................................................. 14

Figure 11. Photo. Malta, NY—Aerial view of roundabout at US 9, NY 67, and Dunning 

Street. .................................................................................................................................. 14

Figure 12. Photo. Slingerlands, NY—Aerial view of roundabout at New Scotland Road 

and Cherry Avenue. ............................................................................................................ 15

Figure 13. Photo. Hilliard, OH—Aerial view of roundabout at Main Street and Cemetery 

Road (as observed in May 2019). ....................................................................................... 15

Figure 14. Photo. Wales, WI—Aerial view of roundabout at South 83 (North Wales Road) 

at US 18 (West Summit Avenue). ....................................................................................... 16

Figure 15. Photo. Tethered drone collecting video data at Hilliard, OH site. ............................. 21

Figure 16. Photo. Drone operator and tethered cable management system. ................................ 21

Figure 17. Graph. Average yearly crashes per site by site type. .................................................. 25

Figure 18. Graph. Average number of each crash type over 3 yr. ............................................... 27

Figure 19. Graph. Percent distribution of crashes at control sites and problem sites by crash 

type over 3 yr. ..................................................................................................................... 28

Figure 20. Graph. Total crashes over 3 yr by intersection and crash type. .................................. 29

Figure 21. Graph. Stacked total crashes over 3 yr by intersection and crash type. ...................... 30

Figure 22. Graph. Percent of crashes over 3 yr by crash type per intersection. .......................... 31

Figure 23. Graph. Number of crashes over 3 yr involving failure to yield and sum of 

entering plus conflicting volume, by intersection approach. ............................................. 34

Figure 24. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

entering volume. ................................................................................................................. 35

Figure 25. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

conflicting volume. ............................................................................................................. 36

Figure 26. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

entering plus conflicting volume. ....................................................................................... 36

Figure 27. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes 

and average 1-h left turn volumes, by left-turn movement. ................................................ 38

Figure 28. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes by 

number of left turns as a percentage of entering volume. ................................................... 39
vi 

Figure 29. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes by 

average 1-h left-turn volumes. ........................................................................................... 40

Figure 30. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and average 1-h right-turn volumes, by right-turn movement. ........................................... 42

Figure 31. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and number of right turns as a percentage of entering volume, by right-turn 

movement. ........................................................................................................................... 43

Figure 32. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and average 1-h right-turn volumes, by right-turn movement. ........................................... 44

Figure 33. Illustration. ‘Categories of Violations.’ ....................................................................... 46

Figure 34. Graph. Average hourly events by entering volume. ................................................... 51

Figure 35. Graph. Average hourly conflicts by entering volume. ................................................ 52

Figure 36. Photo. Melbourne—conflict areas. ............................................................................. 56

Figure 37. Photo. Melbourne—Failure-to-yield crash patterns associated with large conflict 

areas. ................................................................................................................................... 57

Figure 38. Melbourne—Crash patterns for right turns from incorrect lanes associated with 

large conflict areas. ............................................................................................................ 58

Figure 39. Photo. Slingerlands—conflict areas. ........................................................................... 59

Figure 40. Photo. Malta—conflict areas. ...................................................................................... 60

Figure 41. Photo. Carmel 106th—conflict areas. ........................................................................ 61

Figure 42. Photo. Carmel 116th—conflict areas. ........................................................................ 62

Figure 43. Photo. De Pere—conflict areas. .................................................................................. 63

Figure 44. Photo. Hilliard—conflict areas. ................................................................................... 64

Figure 45. Photo. Wales—conflict areas. ..................................................................................... 65

Figure 46. Photo. Oshkosh—conflict areas. ................................................................................. 66

Figure 47. Photo. Carmel 106th—Westbound through from incorrect exclusive left-turn 

lane. ..................................................................................................................................... 67

Figure 48. Carmel 106th—Observed near-crash from westbound through from incorrect 

exclusive left-turn lane (image facing south). ..................................................................... 67

Figure 49. Photo. Malta—Exclusive eastbound left-turn lane. .................................................... 68

Figure 50. Photo. Hilliard—Lane additions and lane merging. ................................................... 69

Figure 51. Photo. Wales—Westbound lane addition (plan view). .............................................. 70

Figure 52. Photo. Wales—Westbound lane addition (ground-level view). ................................. 71

Figure 53. Photo. Malta—Westbound lane addition. .................................................................. 72

Figure 54. Photo. Slingerlands—Entry lane additions. ................................................................ 73

Figure 55. Photo. De Pere—Flared lane additions and lane merging. ......................................... 74

Figure 56. Photo. Melbourne—Lane changing through exit reverse curves (plan view). ........... 75

Figure 57. Photo. Melbourne—Lane changing through exit reverse curves (ground view). ....... 76

Figure 58. Photo. Melbourne—Exit reverse curves influencing left turns from the incorrect 

lane. ..................................................................................................................................... 76

Figure 59. Photo. Carmel 116th—Lane changing through exit reverse curves. .......................... 77

Figure 60. Photo. Oshkosh—Photo from drone facing southeast. ............................................... 78

Figure 61. Photo. Melbourne—Entry lane alignment. ................................................................. 79

Figure 62. Photo. Carmel 106th—Eastbound lane straddling within circulatory roadway 

due to entry lane alignment. ................................................................................................ 80

Figure 63. Photo. Carmel 106th—Entry lane alignment. ............................................................. 80
vii 

Figure 64. Photo. Carmel 116th—Entry lane alignment. ............................................................. 81

Figure 65. Photo. Malta—Entry lane alignment. ......................................................................... 82

Figure 66. Photo. Wales—Observed lane-straddling conflict. ..................................................... 83

Figure 67. Photo. Wales—Entry lane alignment. ......................................................................... 84

Figure 68. Photo. Carmel 116th—Concentric lane-line markings. ............................................. 85

Figure 69. Photo. Slingerlands—Spiral pavement markings. ...................................................... 86

Figure 70. Photo. Malta—Spiral pavement markings. ................................................................ 87

Figure 71. Photo. Wales—Geometric spiral and spiral markings. .............................................. 88

Figure 72. Photo. Wales—Vehicles navigating the northbound left-turn spiral. ......................... 88

Figure 73. Photo. Carmel 116th yield signs only on each side of entrance. ................................ 89

Figure 74. Photo. Slingerlands —Yield signs with a one-way sign. ............................................ 90

Figure 75. Photo. Slingerlands—Yield signs with a one-way sign and ‘TO ALL LANES 

IN CIRCLE’ supplemental plaques. ................................................................................... 90

Figure 76. Photo. De Pere—Yield signs with a one-way sign and a ‘TO TRAFFIC FROM 

LEFT’ supplemental plaque. ............................................................................................... 90

Figure 77. Photo. Hilliard yield signs with ‘TO BOTH LANES IN ROUNDABOUT’ 

supplemental plaques. ........................................................................................................ 91

Figure 78. Photo. Wales entrance line and ‘YIELD’ word markings. ......................................... 92

Figure 79. Photo. Carmel 106th entrance line and yield lines. .................................................... 92

Figure 80. Photo. Melbourne entrance line and parallel yield line. ............................................. 93

Figure 81. Photo. Melbourne side-mounted lane-use sign and single set of fishhook 

markings. ............................................................................................................................. 93

Figure 82. Photo. Hilliard side-mounted lane-use sign and two sets of fishhook markings. ....... 94

Figure 83. Photo. Oshkosh overhead lane-use signs and two sets of standard arrow 

markings. ............................................................................................................................. 94

Figure 84. Photo. Carmel 116th—Fishhook arrows within the circulatory roadway. ................. 95

Figure 85. Photo. Melbourne—Circulatory lane-use pavement arrows. ...................................... 96

Figure 86. Photo. Oshkosh—Circulatory lane-use pavement arrows . ......................................... 97

Figure 87. Photo. Hilliard—Observed circulatory lane-use pavement arrows. ........................... 98

Figure 88. Photo. Hilliard—Current circulatory lane-use pavement arrows. ............................... 98

Figure 89. Photo. Wales—Circulatory lane-use pavement arrows. ............................................. 99

Figure 90. Photo. Malta—Circulatory lane-use pavement arrows and ‘ONLY’ markings. ....... 100

Figure 91. Photo. Slingerlands—Circulatory lane-use pavement arrows and ‘ONLY’ 

markings. ........................................................................................................................... 101

Figure 92. Photo. Carmel 106th—Influences of adjacent interchange ramp. ............................ 106

Figure 93. Photo. Carmel 116th—Examples of possible system influences .............................. 106

Figure 94. Photo. Melbourne—Roadway network and adjacent signalized intersections. ....... 108

Figure 95. Photo. Malta—Roadway network and adjacent land uses. ....................................... 109

Figure 96. Photo. Hilliard—Left turn from the incorrect lane for trips to school. ..................... 111

Figure 97. Photo. Melbourne—Improper right turns to access places of worship. .................... 112

Figure 98. Photo. Carmel 116th—Left turn from incorrect lane after departing adjacent 

driveway. .......................................................................................................................... 113

Figure 99. Photo. Oshkosh—U-turn from incorrect lane after departing an adjacent 

driveway. ........................................................................................................................... 114

Figure 100. Photo. Malta—Observed U-turn-related lane changing. ........................................ 114

Figure 101. Photo. Hilliard roadway network and surrounding development patterns. ............ 115
viii

Figure 102. Photo. Hilliard—Northbound vehicle queues during peak a.m. hours. .................. 116

Figure 103. Photo. Melbourne—Platooned westbound arrivals (top) resulting in 

southbound queueing (left). .............................................................................................. 117

Figure 104. Photo. Melbourne—Queues from the adjacent signal extending into the 

roundabout. ....................................................................................................................... 118

Figure 105. Photo. De Pere—Platooned vehicle arrivals at southbound entry and crash 

patterns caused by left turn from incorrect lane. ............................................................... 118
ix

LIST OF TABLES

Table 1. Long list of candidate sites. ............................................................................................ 10

Table 2. Control site characteristics. ............................................................................................ 11

Table 3. Problem site characteristics. .......................................................................................... 11

Table 4. Control site lane configurations. .................................................................................... 17

Table 5. Problem site lane configurations. ................................................................................... 17

Table 6. Control site signing characteristics. ............................................................................... 18

Table 7. Problem site signing characteristics. .............................................................................. 18

Table 8. Control site pavement marking characteristics. ............................................................. 19

Table 9. Problem site pavement marking characteristics. ............................................................ 19

Table 10. Field investigation and video data collection dates. .................................................... 22

Table 11. Total crashes at reviewed control sites during selected 3 yr periods. .......................... 23

Table 12. Total crashes at reviewed problem sites during selected 3 yr periods. ........................ 24

Table 13. Reviewed crashes at all sites. ....................................................................................... 24

Table 14. Total crashes by crash severity at control sites. ........................................................... 26

Table 15. Total crashes by crash severity at problem sites. ......................................................... 26

Table 16. Total crashes by crash severity at all sites. ................................................................... 26

Table 17. Crashes by crash type and site type per intersection for control sites. ......................... 32

Table 18. Crashes by crash type and site type per intersection for problem sites. ....................... 32

Table 19. Number of events by event type and roundabout location. ......................................... 47

Table 20. Average hourly events by event type and site type. .................................................... 47

Table 21. Average hourly events by violation type and roundabout location. ............................ 48

Table 22. Average hourly events by violation type and site type. ............................................... 49

Table 23. Average hourly conflicts by violation type and roundabout location. ......................... 50

Table 24. Rate of conflicts per hour by violation type and site type. ........................................... 51

Table 25. Geometric design influences for control sites. ............................................................. 54

Table 26. Summary of design influences for problem sites. ........................................................ 54

Table 27. Traffic control influences for control sites. ................................................................. 55

Table 28. Traffic control influences for problem sites. ............................................................... 55

Table 29. Summary of network influences for control sites. ..................................................... 104

Table 30. Summary of the network influences for problem sites. ............................................. 105

Table 31. Menu of potential lane configuration changes. .......................................................... 126

Table 32. Menu of potential network modifications. ................................................................. 127

Table 33. Menu of potential signing and marking treatments. .................................................. 128

Table 34. Menu of potential entry treatments. ........................................................................... 129

Table 35. Menu of circulatory roadway and exit treatments with medium effectiveness 

potential. ........................................................................................................................... 130

Table 36. Menu of circulatory roadway and exit treatments with low effectiveness 

potential. ........................................................................................................................... 131

Table 37. Menu of geometric treatments with high effectiveness potential. ............................. 132

Table 38. Menu of geometric treatments with medium-to-high effectiveness potential. .......... 133
x 

LIST OF ABBREVIATIONS

A Incapacitating/Serious Injury (crash type)

AADT average annual daily traffic

B Nonincapacitating/Visible Injury (crash type) 

C Possible Injury/Complaint (crash type)

CTH County Trunk Highway 

EB Eastbound 

FHWA Federal Highway Administration

ICD inscribed circle diameter

K Fatal (crash type)

MUTCD Manual on Uniform Traffic Control Devices

NB Northbound 

NCHRP National Cooperative Highway Research Program 

O No Injury (crash type) 

PDO property damage only 

SB Southbound 

STH State Trunk Highway

TPF Transportation Pooled Fund 

WB Westbound 
1 

CHAPTER 1. BACKGROUND

The Federal Highway Administration (FHWA) has included roundabouts on a list of proven 

safety countermeasures since 2008.(1) However, higher-than-expected numbers of property 

damage only (PDO) crashes occurring near multi-lane roundabout exits have emerged at some 

U.S. roundabouts. This report details a Transportation Pooled Fund (TPF) study that investigates 

root causes and contributing factors for these crashes based on an evaluation of a sample of 

multi-lane sites. For each study site, the project team reviewed historical crash data; conducted 

field observations; collected drone video data; analyzed vehicular volumes and conflicts; and 

reviewed potential influences from roundabout geometry, signing and markings, and the 

roadway network. Findings related to each of these topics are presented throughout this report to 

provide guidance that supports proactive reductions in the potential for these crashes at new 

multi-lane roundabouts and potential countermeasures to reduce these crashes for existing sites. 

Studies of U.S. roundabouts dating back to the 1990s indicate that converting signalized or 

stop-controlled intersections into roundabouts of varying sizes and types leads to strong 

reductions in crashes. In 2007, based on aggregated data from 55 sites, the National Cooperative 

Highway Research Program (NCHRP) identified a 76-percent reduction in crashes involving 

severe injuries and fatalities and a 35-percent reduction in total crashes after signalized or 

stop-controlled intersections were converted to roundabouts. The 55 sites included a range of 

settings (urban and rural), number of legs (3 and 4), number of lanes (single or multi-lane), and 

prior traffic control (two-way stop, all-way stop, and traffic signal). In general, single-lane 

roundabouts demonstrated better reductions in both total and severe crashes after conversion than 

multi-lane roundabouts. At multi-lane roundabouts, instances of injury crashes were typically 

low, but higher numbers of PDO crashes were often observed. A greater number of conflict 

points at multi-lane roundabouts is one factor that will vary depending on number of lanes and 

complexity of lane configurations.(2)

The following terminology is used in this report to describe different multi-lane roundabout lane 

configurations and conflict area complexities: 

• A 2x1 configuration refers to a conflict area with two entering lanes and one conflicting 

circulating lane. This configuration reflects the mainline entry of a partial two-lane 

roundabout, as illustrated in figure 1. 

• A 1x2 configuration refers to a single-lane entry with two conflicting circulating lanes. 

This configuration reflects the minor road entry of a partial two-lane roundabout, as 

illustrated in figure 1. 

• A 2x2 configuration reflects two entering lanes with two conflicting circulating lanes, as 

illustrated in figure 2. 

As illustrated in figure 1, 2x1 configurations with a mix of single-lane and two-lane entries result 

in fewer conflict points in the critical conflict areas near the entries and exits as compared to 2x2 

configurations, illustrated in figure 2. Introduction of exclusive or dual left-turn movements has 

the potential to further increase the intersection complexity, requiring design details to support 
2 

appropriate navigation. Various elements of the geometric design, signing, and pavement 

markings also have the potential to influence driver behavior and crash trends. 

Previous research completed as part of NCHRP Report 888: Development of Roundabout Crash 

Prediction Models and Methods developed crash prediction models for multi-lane roundabouts at 

the intersection level and at a design level for individual legs.(3)

 However, the nature of the 

modeling in the report established possible correlations between geometric parameters and crash 

experience; it did not go into the level of detail needed to assess potential causes. 

Original map: © 2013 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 14, 2013. 28° 00′ 12.37′′ N, 82° 24′ 50.57′′ W. Elevation 59 ft. Eye altitude 670 ft.

Figure 1. Photo. Example 2x1 roundabout configuration (Tampa, FL). 
3 

Original map: © 2021 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: August 12, 2021. 44° 40′ 53.36′′ N and 93° 16′ 40.49′′ W. Elevation 1085 ft. 

Eye altitude 1,700 ft.

Figure 2. Photo. Example 2x2 roundabout configuration (Lakeville, MN). 

As roundabouts have become more prevalent in the United States, roundabout design guidance 

has been developed and updated as part of an evolution in roundabout implementation. This 

evolution includes the development of the original publication in 2000 of the FHWA report 

Roundabouts: An Informational Guide.

(4)

 This guide was replaced in 2010 by NCHRP Report 

672: Roundabouts: An Informational Guide: Second Edition.

(5) At the time of this writing, a new 

national roundabout guide is under development as part of NCHRP Project 03-130. Some 

individual States and agencies have also released their own design guidance to supplement the 

national document. State agencies participating in this TPF study, including Florida, Georgia, 

New York, Washington, and Wisconsin, each have either stand-alone roundabout guides or 

roundabout content incorporated into State design manuals. 

Through the various iterations of State and National guidebooks, multi-lane roundabout design 

guidance has evolved in significant ways since 2000. Geometrically, additional emphasis is now 

placed on entry and exit vehicle path alignments, a range of strategies have been introduced for 

designing for trucks, and enhanced guidance has been introduced for multimodal design 

elements. Some agencies have also shifted their preferences toward the use of approach 

alignments that offset the centerline of the leg to the left of the roundabout center. This strategy 
4 

results in more deflection and speed control through the entries and less curvature through the 

exits.

In the 2000 FHWA informational guide, no lane markings were recommended within the 

circulatory roadway.

(4) However, use of lane markings was later introduced into the 2009 

Manual on Uniform Traffic Control Devices (MUTCD) and has become a standard component 

of roundabout design.(6) Various patterns of lane markings within the circulatory roadway have 

been implemented, along with different styles of arrows on the approach signs and markings. 

These changes in practice, among others, reflect a goal of continuing to maximize safety 

performance at multi-lane roundabouts. However, crashes near roundabout exits remain a 

common type, particularly at 2x2 roundabout configurations. 

There are several violation and conflict types that may be contributing to crashes near 

roundabout exits. Example violations and conflicts that may be attributable to design 

components include:

• Failure of entering vehicles to yield to vehicles in both circulating lanes (figure 3).

• Left-turn movement from the incorrect outside lane (figure 4).

• Right-turn movement from the incorrect inside lane (figure 5).

A variety of factors or combinations of factors may contribute to these violations. A recent study 

by the Minnesota Department of Transportation discussed in the report titled Evaluation of 

Safety and Mobility of Two-Lane Roundabouts is one example of previous efforts to evaluate 

driver behavior and violation rates.(7)

 The study looked at violations at four multi-lane 

roundabouts over an extended period, including before/after analysis for effectiveness of 

improvements. The study identified the benefits of lane markings and overhead signing, 

particularly on reducing instances of improper left turns or right turns. However, the study was 

not able to produce any additional insights into the nature of the problem or potential solutions 

related to entering vehicles failing to yield to both circulating lanes. All sites featured geometric 

designs with offset-left configurations and tangential or near-tangential exits.
5 

Source: FHWA. 

Figure 3. Photo. Entering vehicle failing to yield to both lanes. 

Source: FHWA.

Figure 4. Photo. Left turn from incorrect lane.

Source: FHWA.

Figure 5. Photo. Right turn from incorrect lane.

7 

CHAPTER 2. SITE SELECTION AND DATA COLLECTION

Candidate study sites were identified and screened following a tiered framework and 

coordination with the project panel. This chapter summarizes criteria considered during each tier 

of the screening process and a summary of the characteristics of the final selected sites, including 

geometric features, signing, and pavement markings. Further discussion is provided regarding 

data sources and field data collected as part of the project investigations.

SITE SELECTION PROCESS AND SITE CHARACTERISTICS

The team selected sites to provide a range of geometric, signing, marking, and traffic conditions 

to capture potential key factors contributing to crash patterns occurring near multi-lane exits. The 

study project scope included a total of eight study sites, divided into five “problem sites” and 

three “control sites.” Problem sites were defined for this project as multi-lane roundabouts that 

experience high frequencies of failure-to-yield crashes or exit-circulating crashes. Control sites 

were defined as multi-lane roundabouts with similar geometric and traffic design features as the 

problem sites but lower crash frequencies. This definition of problem sites and control sites is 

used throughout the remainder of the report. 

Candidate multi-lane roundabout sites were identified and reviewed through a two-tier screening 

process based on geometric and traffic control device configuration. The following criteria were 

established by the project team for candidate sites:

• Intersection configuration:

o Preference for 2x2 entry/exit roundabout configuration, which provides four 

conflict areas, each with two entering and two conflicting circulating lanes. 

o Preference for four legs.

o Priority for sites where geometric design is without clear deficiencies and 

inscribed circle diameters (ICDs) are within the typical range for a two-lane 

roundabout (140–200 ft). 

• Age of roundabout: Minimum of 2 yr in operation to provide sufficient crash history.

• Average annual daily traffic (AADT): Daily traffic volumes within the typical range for a 

multi-lane roundabout configuration (minimum of 20,000 daily vehicles). 

• Crash history: 

o Data available on annual crashes and crash patterns.

o Access to individual crash reports or similar crash details sufficient to review 

the crash location and type. 
8 

• Location: 

o Preference for sites in States contributing to the TPF project, although site 

screening was not limited to those States.

o Consideration of the ability to cluster sites to provide efficiency in data 

collection and allow for comparison between similar driver populations.

o Consideration of proximity to airports, railroads, or similar areas that would 

limit the use of drones for data collection.

The team compiled an initial long list of potential multi-lane roundabout sites based on multiple 

sources. All sites included on the long list featured one or more 2x2 conflict areas (two entering 

lanes with two conflicting circulating lanes). The database of sites generated as part of 

NCHRP Report 888, Development of Roundabout Crash Prediction Models and Methods

provided geometric, crash, and volume data for 10 multi-lane sites with 2x2 configurations 

across 5 States: Florida, Minnesota, New York, Washington, and Wisconsin.(3)

 Additional sites 

were added based on a review of a national roundabout database.(8) Possible multi-lane

roundabout sites and data were also contributed by TPF member agencies. 

Using the data sources mentioned previously, the team compiled an initial long list of 47 multilane roundabouts across the United States for consideration in this study. For each site in the 

long list, the team compiled information related to intersection configuration, age, AADT, and 

crash history. The team also gathered the following information related to roundabout geometric 

considerations, signing, markings, and context: 

• Roundabout design case to accommodate heavy vehicles, as described in several State 

DOT roundabout guides, including Wisconsin DOT:(9)

o Case 1: Roundabouts “designed to allow trucks to encroach into adjacent lanes as 

they approach, enter, circulate, and exit the intersection.” 

o Case 2: Roundabouts “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.”

o Case 3: Roundabouts “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.” 

• Key dimensions, including ICD, entry and exit widths, and circulating width. 

• Pavement marking scheme and style (i.e., fishhook versus standard arrows, circulatory 

roadway lane marking patterns, use of gore striping through entries, etc.). 
9 

• Signing scheme and placement (i.e., side-mounted versus overhead-mounted). 

• Site context (i.e., urban, suburban, rural). 

The team ranked the sites in the long list into three tiers based on the following characteristics:

• Tier 1: Sites observed to have the following: 

o Full 2x2 configuration with four legs.

o AADT greater than 20,000 vehicles per day. 

o High or low numbers of historical crashes that clearly fit into a “problem site” 

or “control site” category. 

• Tier 2: Sites observed to meet most of the Tier 1 criteria but that have a possible 

deficiency in one category, such as the following: 

o Four legs but not a full 2x2 configuration (e.g., use of an exclusive left-turn 

lane that results in one leg having a single-lane exit).

o AADT less than 20,000 vehicles per day. 

o Geometric characteristics substantially different from the guidance presented 

in NCHRP Report 672.

(5)

o Historical crash totals in the middle of the range.

• Tier 3: Sites observed to have the following characteristics:

o Three approach legs or only one 2x2 conflict area. 

o AADT of less than 20,000 vehicles per day. 

o Geometric characteristics substantially different from the guidance presented 

in NCHRP Report 672. 

o Signing or marking deficiencies possibly influencing crash history. 
10 

Table 1 summarizes the long list of candidate sites by State and tier. 

Table 1. Long list of candidate sites.

State Number of 

Sites 

Screened

Number of 

Sites in 

Tier 1

Number of 

Sites in 

Tier 2

Number of 

Sites in 

Tier 3

Florida 9 1 3 5 

Illinois 1 1 — — 

Indiana 9 8 1 — 

Michigan 1 1 — — 

Minnesota 2 2 — — 

New York 4 — 3 1 

Ohio 6 3 2 1 

Texas 4 3 — 1 

Washington 1 1 — — 

Wisconsin 10 4 6 — 

Total 47 24 15 8 

—No data.

Based on desktop screening efforts for each of the long-list sites and discussions with Technical 

Advisory Panel members from TPF-member agencies, the team narrowed down the long list to a 

short-list of 25 sites from which the final 8 study roundabout sites would be selected. Alternate 

sites were also identified in case chosen sites had to be disqualified later for some reason. The 

distribution of the 25 short-listed sites was as follows:

• One site in Florida. 

• One site in Illinois. 

• Seven sites in Indiana. 

• One site in Michigan. 

• Two sites in Minnesota. 

• Four sites in New York. 

• One site in Ohio. 

• One site in Washington. 

• Seven sites in Wisconsin. 

From the short list, the team selected five problem sites and three control sites. Two primary 

clusters of sites were selected to facilitate comparisons within driver populations and 

considerations for streamlining data collection, as follows: 

1. Three sites in Indiana and Ohio: 2 in Carmel, IN, and 1 in Hilliard, OH.

2. Three sites in Wisconsin. 

The remaining two study sites were selected in Florida and New York to provide geographic 

diversity among the TPF participants. Table 2 summarizes the characteristics of the three 

selected control sites. 
11 

Table 2. Control site characteristics.

Intersection Location 

Lane 

Config. 

ICD 

(ft)

Lane-Use 

Signing

Pavement 

Marking AADT

Crash 

History

106th St. and 

Illinois St. Carmel, IN

2x2; 

single lane 

exit, one 

leg

165 Side-mounted Fishhook 

arrows

N/A1 44 crashes 

(2016–2019) 

Jackson St. 

and Murdock 

Ave.

Oshkosh, WI 2x2 135 Overhead 

mounted 

Standard 

arrows 28,450 39 crashes (2017–2019) 

Monroe Rd. 

and 

Dickinson 

Rd.

De Pere, WI 2x2 184 Side-mounted Standard 22,050 23 crashes 

(2017–2019) 

1

AADT was not available at all sites at time of data collection/screening. 

Config. = configuration.

Table 3 summarizes the characteristics of the five selected problem sites and one alternate site.

Table 3. Problem site characteristics.

Intersection Location 

Lane 

Config. 

ICD 

(ft)

Lane-Use 

Signing

Pavement 

Marking AADT

Crash 

History

Wickham Rd. 

and Lake 

Andrew Blvd. 

Melbourne, 

FL 2x2 200 Side-mounted Fishhook arrows 25,000 182 crashes (2017–2019) 

116th St. and 

Illinois St. Carmel, IN 2x2 177 Side-mounted Fishhook arrows

N/A1 192 crashes 

(2016–2019) 

US 9/NY SR 67 

and Dunning 

St.

Malta, NY

2x2; 

single 

lane exit, 

two legs 

165 Side-mounted 

Fishhook 

arrows, 

gore 

striping

26,500 116 crashes 

(2016–2019) 

New Scotland 

Rd. and Cherry 

Ave. 

(alternate site)

Slingerlands, 

NY

2x2; 

single 

lane exit, 

two legs 

198 Side-mounted 

Fishhook 

arrows; 

gore 

striping

30,000 126 crashes 

(2016–2019) 

Main St. and 

Cemetery Rd. Hilliard, OH 2x2 159 Side-mounted Fishhook arrows 32,000 231 crashes (2014–2016) 

South 83 

(North Wales 

Rd.) and US 18 

(West Summit 

Ave.) 

Wales, WI

2x2; 

Single 

lane exit, 

one leg 

152 Overhead and 

side-mounted 

Standard 

arrows, 

gore 

striping

34,000 104 crashes 

(2017–2019) 

1

AADT was not available at all sites at time of data collection/screening. 

Aerial views illustrating each of the study sites are provided in figure 6 through figure 14. 
12 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 10, 2018. 39° 56′ 29.71′′ N and 86° 09′ 40.36′′ W. 

Elevation 821 ft. Eye altitude 1,752 ft.

Figure 6. Photo. Carmel, IN—Aerial view of roundabout at 106th Street and Illinois Street.

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: April 13, 2015. 44° 02′ 21.52′′ N and 88° 32′ 33.17′′ W. Elevation 760 ft. 

Eye altitude 1,539 ft.

STH = State Trunk Highway.

Figure 7. Photo. Oshkosh, WI—Aerial view of roundabout at Jackson Street (STH 76) and 

Murdock Avenue (US 45).
13 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: May 17, 2018. 44° 25′ 57.57′′ N and 88° 00′ 58.87′′ W. Elevation 598 ft. 

Eye altitude 1,485 ft.

CTH = County Trunk Highway. 

Figure 8. Photo. De Pere, WI—Aerial view of roundabout at Monroe Road (CTH GV) and 

Dickinson Road (CTH G).

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 9. Photo. Melbourne, FL—Aerial view of roundabout at Lake Andrew Drive and 

Wickham Road.
14 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 10. Photo. Carmel, IN—Aerial view of roundabout at 116th Street and Illinois 

Street.

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft.

Eye altitude 1,415 ft.

Figure 11. Photo. Malta, NY—Aerial view of roundabout at US 9, NY 67, and Dunning 

Street. 
15 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 38′ 13.51′′ N and 73° 51′ 21.50′′ W. Elevation 217 ft. 

Eye altitude 1,205 ft.

Figure 12. Photo. Slingerlands, NY—Aerial view of roundabout at New Scotland Road and 

Cherry Avenue. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 48.28′′ N and 83° 09′ 40.58′′ W. Elevation 937 ft. 

Eye altitude 1,935 ft.

Figure 13. Photo. Hilliard, OH—Aerial view of roundabout at Main Street and Cemetery 

Road (as observed in May 2019). 
16 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 43.21′′ N and 88° 23′ 02.71′′ W. Elevation 987 ft. 

Eye altitude 2,450 ft.

Figure 14. Photo. Wales, WI—Aerial view of roundabout at South 83 (North Wales Road) 

at US 18 (West Summit Avenue).

Additional information related to the study’s site lane configurations is summarized in table 4

and table 5. 
17 

Table 4. Control site lane configurations.

Intersection

SB Entry 

Lane 

Config.

NB Entry 

Lane 

Config.

WB 

Entry 

Lane 

Config.

EB Entry 

Lane 

Config.

Spiral Within 

Circulatory 

Roadway

Entries 

With Lane 

Additions

Single Lane 

Exits or 

Exit-Lane 

Merging

Carmel 106th LT/TR LT/TR L/LTR LT/TR Painted EB Single lane 

WB exit

Oshkosh LT/TR LT/TR LT/TR LT/TR — — Lane drop 

WB exit

De Pere LT/TR LT/TR LT/TR LT/TR — EB and WB

Lane merge 

EB and WB 

exits

—Not applicable.

EB = eastbound; NB= northbound; SB = southbound; WB = westbound.

L = exclusive left-turn lane; LT = shared left through lane; LTR = shared left through right lane; R = exclusive 

right-turn lane; TR = shared through right lane; RT bypass = right-turn bypass. 

Table 5. Problem site lane configurations.

Intersection

SB Entry 

Lane 

Config.

NB Entry 

Lane 

Config.

WB 

Entry 

Lane 

Config.

EB Entry 

Lane 

Config.

Spiral Within 

Circulatory 

Roadway

Entries 

With Lane 

Additions

Single Lane 

Exits or Exit 

Lane 

Merging

Melbourne LT/TR/R LT/TR/R LT/TR/R LT/TR — 

RT bypass 

lanes on NB, 

SB, and WB 

entries

— 

Carmel 116th LT/TR LT/TR LT/TR LT/TR — — — 

Malta LT/TR LT/TR L/TR L/TR Painted WB

Single lane 

EB and WB 

exits

Slingerlands LT/TR LT/TR L/LTR L/LTR Painted EB and WB

Single lane 

EB and WB 

exits

Hilliard LT/TR LT/TR LT/TR LT/TR — SB and WB

Lane merge 

on NB and 

EB exits

Wales LT/TR LT/TR L/TR LT/TR

Physical spiral 

built into 

central island 

curbline

WB Single lane 

WB exit

—Not applicable.
18 

Information regarding signing and pavement marking characteristics is summarized in table 6

through table 9. Table 6 summarizes signing characteristics at the selected control sites.

Table 6. Control site signing characteristics.

Intersection

Intersection 

Warning Sign 

Type 

(Speed Plaque 

Used?)

Advance Guide 

Signs

Lane-Use 

Signs Per 

Approach

Lane-Use Sign

Placement and

Arrow Type

(Standard or 

Fishhook)

Yield Sign 

Placement

(One Side, 

Both Sides)

Carmel 106th Circular (N) None 1 sign Right side 

(fishhook) Both

Oshkosh W2-6 (Y) Route assemblies 1 sign Overhead 

(standard) Both

De Pere Circular (Y) Route assemblies 1 sign Right Side 

(fishhook) Both

N = no; Y = yes.

W2-6 = Standard MUTCD Circular Intersection symbol sign.(6)

Table 7 summarizes signing characteristics at the selected problem sites.

Table 7. Problem site signing characteristics.

Intersection

Intersection 

Warning Sign 

Type 

(Speed Plaque

Used?)

Advance Guide 

Signs 

Lane-Use 

Signs Per 

Approach

Lane-Use Sign 

Placement and 

Arrow Type

(Standard or 

Fishhook)

Yield Sign 

Placement

(One Side, 

Both Sides)

Melbourne — Diagrammatic 1 set

EB and WB left 

side; NB and SB 

both sides 

(fishhook)

Both

Carmel 116th Circular (Y) — 1 sign Right side 

(fishhook) Both

Malta W2-6 (Y) Destination and 

route assemblies 1 sign Right side (fishhook) Both

Slingerlands W2-6 (Y) Destination and 

route assemblies 1 sign Right side (fishhook) Both

Hilliard W2-6 (Y) Route 

assemblies 1 sign Right side (fishhook) Both

Wales Circular (Y) Destination and 

route assemblies 2 sets

Overhead plus 

side mounted on 

both sides of 

approach 

(standard) 

Both

—No sign provided
19 

Table 8 summarizes information regarding pavement markings at the selected control sites.

Table 8. Control site pavement marking characteristics.

Intersection

Approach 

Lane-Use 

Arrow Type

Sets of Lane-Use 

Arrow Markings 

Per Approach

Yield Line, 

Yield Word 

Marking, or 

Both

Circulatory 

Roadway Lane 

Marking Style

Carmel 106th 

Fishhook 1 set EB; 2 sets all 

other approaches 

Yield line Combination of 

solid and dotted 

lane lines

Oshkosh Standard 2 sets Yield word 

marking

Strong dash

De Pere Standard 1 set Yield word 

marking

Strong dash

Table 9 summarizes information regarding pavement markings at the selected control sites.

Table 9. Problem site pavement marking characteristics.

Intersection

Approach 

Lane-Use 

Arrow Type

Sets of Lane-Use 

Arrow Markings 

Per Approach

Yield Line, 

Yield Word 

Marking, or 

Both

Circulatory 

Roadway Lane 

Marking Style

Melbourne 

Fishhook 1 set Yield line Combination of 

solid and dotted 

lane lines

Carmel 116th Fishhook 2 sets Yield line Dotted

Malta

Fishhook 2 sets Both Combination of 

solid and dotted 

lane lines

Slingerlands 

Fishhook 1 set EB; 2 sets on 

all other entries

Both Combination of 

solid and dotted 

lane lines

Hilliard Fishhook 1 set NB; 2 sets all 

other approaches 

Yield line Strong dash

Wales Standard 3 sets SB; 2 sets all 

other approaches 

Yield word 

marking

Strong dash
20 

PROJECT DATA COLLECTION AND FIELD INVESTIGATIONS

For each of the study sites, the team coordinated with local agency partners to obtain key 

historical data, including the following:

• Historical AADT volume information and peak period intersection turning movement 

counts, if available.

• Historical crash data for 3 yr, with police reports or other narrative information sufficient 

to identify the crash type and location. 

• Background information, including previous intersection studies, if applicable. 

The team completed desktop reviews and analyses of each site to create an inventory as follows:

• Key dimensions and geometric characteristics.

• Crash locations and types using police report narratives for each individual crash. 

• Summary graphics and tables of key crash trends by crash location.

• Crashes by severity, times of day, months of the year, and weather conditions. The team 

used time-of-day crash data in the selection of periods for further field investigations and 

analysis.

• Fastest path speeds for each site according to NCHRP Report 672 procedures based on 

scaled aerial photography.(5)

• Inventory of signs and pavement markings on each roundabout approach and within the 

roundabout. 

Information gleaned during the desktop screening process informed the data collection plan for 

field investigations. Based on available traffic volume and crash time-of-day data, the team 

identified 8 h throughout the day for video data collection to capture peak volume and crash 

periods as well as off-peak periods for comparison. Additionally, the team completed a full day 

of observations at each site over the course of the morning, midday, afternoon, and evening peak 

periods. During these sessions, the team recorded qualitative observations of traffic operations 

and driver behavior and times and locations for any observed vehicle conflicts and violations. 

Additionally, the team verified site geometric characteristics, including key dimensions and 

signing and pavement markings. 

During field data collection, the team used a tethered drone to record 8 h of video footage for 

each site. The tether provides the drone with continuous power to allow for extended flight 

times. Continuous data collection for up to 4 h at a time was completed in the afternoon and 

evening hours at each site. Figure 15 shows the tethered drone in the process of collecting video 

data at the Hilliard site.
21 

© 2019 Quality Counts, LLC. 

Figure 15. Photo. Tethered drone collecting video data at Hilliard, OH site.

Figure 16 shows the drone operator and tethered cable management device, which maintains 

appropriate cable tensioning and spools excess cable.

Source: FHWA.

Figure 16. Photo. Drone operator and tethered cable management system. 

Table 10 summarizes the dates for data collection at each site and the total hours of video data 

collected. Site observations and drone video data collection were completed on the same day to 

allow ground-level field observations to match with aerial drone footage to be analyzed later. At 

most sites, the collection of video data was completed on a single day. However, the Malta and 

Melbourne sites had data collection split up over multiple days due to adverse weather 

conditions. 
22 

Table 10. Field investigation and video data collection dates. 

Intersection Field Investigation Date Hours of Video Data 

Collected

Carmel 116th May 13, 2019 8 

Hilliard May 14, 2019 8 

Carmel 106th May 15, 2019 8 

Malta October 29–October 30, 2019 8 

Slingerlands October 30, 2019 3 

Melbourne September 15–September 17, 2020 8 

De Pere October 19, 2020 8 

Oshkosh October 20, 2020 8 

Wales October 21, 2020 8 

The Slingerlands site was the alternate New York site and not intended as part of primary data 

collection. The study team completed field observations and gathered supplemental video there 

due to flexibility in travel schedule and its close proximity to Malta, the chosen New York site. 

Thus, even though only 3 h of video were captured at Slingerlands, site observations and crash 

data for the site are incorporated throughout the rest of this report.

For each site, the team reduced 4 h of video data, generally into four 1-h blocks, for further 

analysis and comparison of hourly data across sites. The team completed video data reduction 

and analysis in two parts, as follows: 

• Lane-by-lane volume reduction and analysis: The team logged passenger cars, heavy 

vehicles, pedestrians, and bicycle users in 1-min intervals to provide information 

regarding volume profiles throughout each hour. The team logged vehicular turning 

movements (left, through, right, and U-turn) on a lane-by-lane basis corresponding to the 

lane the vehicle was in when it crossed the yield line entering the roundabout. The laneby-lane breakdown of vehicle data allowed for further analysis of lane utilization and 

frequency of vehicles making turning movements from incorrect lanes. Summarized 

volume information is incorporated into chapters 3 through 6. 

• Conflict analysis: On an approach-by-approach basis, the team reviewed video data and 

logged timestamps and details corresponding to various violation types. Violations were 

further categorized by severity including violation only, potential conflict, incident, nearcrash, and crash. Further information regarding the violation types recorded and analysis 

results is presented in chapter 4. 

The team prepared individual field investigation reports for each site to document findings from 

the desktop screening activities, field observations, and follow-up volume and conflict analyses. 

The field investigation reports provide additional detail regarding site-specific observations as 

well as more detailed volume and conflict analysis results.
23 

CHAPTER 3. SAFETY ANALYSIS

The project team conducted a safety analysis of the control and problem sites as one component 

of investigating the root causes of crashes at multi-lane roundabouts. The team studied three 

consecutive years of crash history at each identified site. This analysis included review of 

detailed crash reports to verify information such as crash types and locations. The team 

aggregated crash data for the control and problem sites by crash frequency, severity, and type to 

allow for comparisons across sites. For selected crash types, the team performed supplemental 

analysis to review approach-level crash trends based on observed entering and circulating 

volume levels. At each site, the team verified that adjustments had not been made to the 

roundabout during the period covering 3 yr of reviewed crash data through the date of field 

observations. 

Table 11 shows the total crashes at each of the reviewed control sites the during selected 3 yr 

periods. 

Table 11. Total crashes at reviewed control sites during selected 3 yr periods.

Intersection

Site 

Code

Total 

Crashes Period of Reviewed Crashes

106th St./North Illinois St., 

Carmel, IN

Carmel 

106th 44 August 1, 2016–July 31, 2019 

Murdock Ave./Jackson St., 

Oshkosh, WI

Oshkosh 39 January 1, 2017–December 31, 2019 

Monroe Rd. (CTH 

GV)/Dickinson Rd. (CTH G), De 

Pere, WI

De Pere

23 January 1, 2017–December 31, 2019 

Total crashes at all control sites 

over 3 yr

— 106 —

Average crashes per control site

over 3 yr

— 35.3 —

Average crashes per control site

per year

— 11.8 —

—No data.

Table 12 shows the total crashes at each of the reviewed problem sites the during selected 3 yr 

periods. 
24 

Table 12. Total crashes at reviewed problem sites during selected 3 yr periods.

Intersection Site Code

Total 

Crashes Period of Reviewed Crashes

Main St./Cemetery Rd., 

Hilliard, OH

Hilliard 231 January 1, 2014–December 31, 2016 

116th St./North Illinois St., 

Carmel, IN

Carmel 116th 192 October 1, 2016–September 30, 2019 

North Wickham Rd./Lake 

Andrew Dr., Melbourne, FL

Melbourne 182 January 1, 2017–December 31, 2019 

New Scotland Rd./Cherry 

Ave., Slingerlands, NY

Slingerlands 126 May 1, 2016–April 30, 2019 

US 9/NY SR 67/Dunning St., 

Malta, NY

Malta 116 May 1, 2016–April 30, 2019 

North Wales Rd. (STH 

83)/West Summit Ave. (US 

18), Wales, WI

Wales

104 January 1, 2017–December 31, 2019 

Total crashes at all problem 

sites over 3 yr

— 951 — 

Average crashes per problem 

site over 3 yr

— 158.5 — 

Average crashes per problem 

site per year

— 52.8 — 

—No data.

Table 13 summarizes crashes across all the sites. As shown, the 3 control sites experienced a 

total of 106 crashes over 3 yr, an average of 35.3 crashes per control site over 3 yr. The 6 

problem sites experienced a total of 951 crashes over 3 yr, an average of 158.5 crashes per 

problem site over 3 yr. These totals represent approximately 4.5 times more crashes per problem 

site than control site.

Table 13. Reviewed crashes at all sites.

Intersection Type Control Sites Problem Sites All Sites

Total crashes over 3 yr 106 951 1,057 

Average crashes per site over 3 yr 35.3 158.5 117.4 

Average crashes per site per year 11.8 52.8 39.1 
25 

Figure 17 displays the average total crashes by site type.

Source: FHWA.

Figure 17. Graph. Average yearly crashes per site by site type.

The team further investigated the crashes at the control and problem sites by performing a deeper 

examination of the crash severity and type at each type of intersection. The results of this 

investigation are provided in the next two sections. 

CRASH SEVERITY

Using the Federal KABCO crash severity scale, the severity of the crashes at each intersection 

was reviewed. For the review, the following common definitions were used to define each level 

of crash severity:

• K—Fatal. 

• A—Incapacitating/Serious Injury. 

• B—Nonincapacitating/Visible Injury. 

• C—Possible Injury/Complaint. 

• O—No Injury. 

At five sites (Oshkosh, De Pere, Wales, Hilliard, and Melbourne), the crash severities were 

provided in a format directly compatible with the five crash severity levels associated with the 

KABCO scale. At the remaining four sites, the numbers of fatal (K), incapacitating/serious injury 

(A), and no injury (O) crashes were known, but the numbers of nonincapacitating/visible injury 

(B) and possible injury/complaint (C) crashes were not consistently known. Thus, the team 

reviewed the crash descriptions and police reports associated with all unspecified B and C 

crashes at these four sites to classify them as B crashes or C crashes in cases where the 

description clearly indicated the crash severity. However, 21 crashes among the 4 sites were not 

able to be classified as definitively either B crashes or C crashes, and these crashes were left as 

unspecified “B or C” crashes.

 
26 

Table 14 summarizes the total crashes by crash severity at the control sites.

Table 14. Total crashes by crash severity at control sites.

Intersection K A B

B or 

C C O

Total 

Crashes

Carmel 106th 0 0 0 2 0 42 44

Oshkosh 0 0 2 0 3 34 39

De Pere 0 0 1 0 2 20 23

Total crashes at all control sites 0 0 3 2 5 96 106 

Average crashes per control site 0 0 1 0.7 1.7 32 35.3

Table 15 summarizes the total crashes by crash severity at the problem sites.

Table 15. Total crashes by crash severity at problem sites.

Intersection K A B

B or 

C C O

Total 

Crashes

Hilliard 0 0 7 0 16 208 231

Carmel 116th 0 0 0 3 2 187 192

Melbourne 0 0 8 0 11 163 182

Slingerlands 0 1 0 7 10 108 126

Malta 0 0 0 9 1 106 116

Wales 0 0 3 0 6 95 104

Total crashes at all problem sites 0 1 18 19 46 867 951

Average crashes per problem site 0 0.2 3 3.2 7.7 144.5 158.5

Table 16 summarizes the total crashes by severity across all sites.

Table 16. Total crashes by crash severity at all sites.

Site Group K A B

B or 

C C O

Total 

Crashes

Total crashes at all control sites 0 0 3 2 5 96 106 

Average crashes per control site 0 0 1 0.7 1.7 32 35.3

Total crashes at all problem sites 0 1 18 19 46 867 951

Average crashes per problem site 0 0.2 3 3.2 7.7 144.5 158.5

Total crashes at all sites 0 1 21 21 51 963 1,057

Average crashes at all sites 0 0.1 2.3 2.3 5.7 107 117.4

As shown in table 16, there were no fatal crashes at any of the sites reviewed, and there was one 

incapacitating/serious injury crash at the problem site in Slingerlands. Crashes involving no 

injury were the predominant crash type at both the control and problem sites, representing 
27 

90.6 percent (96 of 106) of the crashes at the control sites and 91.2 percent (867 of 951) of the 

crashes at the problem sites. While both the control and problem sites had similar percentages of 

crashes at each crash severity level, because the problem sites had approximately 4.5 times more 

overall crashes per site, the numbers of B, unspecified B or C, and C crashes were higher at the 

problem sites than at the control sites. The average numbers of B crashes were three times higher 

at the problem sites than at the control sites (three crashes per problem site versus one crash per 

control site). The average of unspecified B or C crashes was 4.8 times higher at the problem sites 

than at the control sites (3.2 crashes per problem site versus 0.7 crashes per control site). The 

average of C crashes was 4.6 times higher at the problem sites than at the control sites (7.7 

crashes per problem site versus 1.7 crashes per control site).

CRASH TYPE

The team reviewed crashes by type to identify trends related to crash type and intersection 

characteristics. The five most common crash types identified for further review are as follows:

a. Failure to yield. 

b. Left turn from incorrect lane.

c. Improper entry/circulating/exit lane change.

d. Rear-end. 

e. Right turn from incorrect lane.

Figure 18 displays the average number of crashes by crash type per site, by site type. As shown, 

crashes involving failure to yield and crashes involving left turns from incorrect lanes account 

for the majority of crashes across all sites (51.9 percent of the control site crashes and 68 percent 

of the problem site crashes).

Source: FHWA.

Figure 18. Graph. Average number of each crash type over 3 yr. 

 
28 

Figure 19 compares the distribution of crashes at problem versus control sites by crash type. At 

problem sites, failure-to-yield crashes represent 35 percent of total crashes, compared to 

20.8 percent at problem sites. At control sites, crashes due to lane changes, right turns from 

incorrect lanes, and “other” types reflect a higher proportion of the total crashes compared to 

problem sites. The proportion of crashes due to left turns from incorrect lanes and rear-ending 

are comparable at control and problem sites.

Source: FHWA.

Figure 19. Graph. Percent distribution of crashes at control sites and problem sites by 

crash type over 3 yr.

 
29 

The team broke down the number of crashes by type by intersection, as seen in figure 20. 

Source: FHWA.

Figure 20. Graph. Total crashes over 3 yr by intersection and crash type. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
30 

Figure 21 shows the same information in a stacked format.

Source: FHWA.

Figure 21. Graph. Stacked total crashes over 3 yr by intersection and crash type. 

Figure 22 shows the same information as the percentage of crashes by crash type at each

intersection. Failure-to-yield crashes make up about half of the total number of all crash types at 

the Hilliard, Malta, and Wales intersections. In contrast, crashes involving left turns from 

incorrect lanes surpass all other crash types at Carmel 116th and comprise most of the crashes at 

Melbourne. The numbers of crashes involving failure to yield, left turns from incorrect lanes, and 

rear-ending are all approximately the same at Slingerlands.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
31 

Source: FHWA.

Figure 22. Graph. Percent of crashes over 3 yr by crash type per intersection.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
32 

Table 17 summarizes the crashes by crash type and site type per intersection for the control sites.

Table 17. Crashes by crash type and site type per intersection for control sites.

Intersection

Failure 

To Yield

Left Turn 

From 

Incorrect 

Lane

Improper 

Entry/ 

Circulating/ 

Exit Lane 

Change Rear-End Other

Right 

Turn 

From 

Incorrect 

Lane

Total 

Crashes

Carmel 106th 10

(22.7%)

9

(20.5%)

6

(13.6%)

4

(9.1%)

5

(11.4%)

10

(22.7%)

44

(100%)

Oshkosh 9

(23.1%)

10

(25.6%)

7

(17.9%)

6

(15.4%)

5

(12.8%)

2

(5.1%)

39

(100%)

De Pere 3

(13%)

14

(60.9%)

3

(13%)

0

(0%)

3

(13%)

0

(0%)

23

(100%)

Average number 

of crashes by site 

and percent of 

total crashes at 

control sites

7.3

(20.8%) 

11

(31.1%) 

5.3

(15.1%) 

3.3

(9.4%) 

4.3

(12.3%) 

4 

(11.3%) 

35.3

(100%) 

Table 18 summarizes the crashes by crash type and site type per intersection for the problem 

sites.

Table 18. Crashes by crash type and site type per intersection for problem sites.

Intersection

Failure 

To Yield

Left Turn 

From 

Incorrect 

Lane

Improper 

Entry/ 

Circulating/ 

Exit Lane 

Change Rear-End Other

Right 

Turn 

From 

Incorrect 

Lane

Total 

Crashes

Hilliard 114

(49.4%)

43

(18.6%)

0

(0%)

0

(0%)

47

(20.3%)

27

(11.7%)

231

(100%)

Carmel 116th 34

(17.7%)

121

(63%)

3

(1.6%)

10

(5.2%)

11

(5.7%)

13

(6.8%)

192

(100%)

Melbourne 39

(21.4%)

73

(40.1%)

33

(18.1%)

17

(9.3%)

2

(1.1%)

18

(9.9%)

182

(100%)

Slingerlands 34

(27%)

32

(25.4%)

15

(11.9%)

42

(33.3%)

3

(2.4%)

0

(0%)

126

(100%)

Malta 57

(49.1%)

27

(23.3%)

13

(11.2%)

17

(14.7%)

2

(1.7%)

0

(0%)

116

(100%)

Wales 55

(52.9%)

18

(17.3%)

21

(20.2%)

5

(4.8%)

2

(1.9%)

3

(2.9%)

104

(100%)

Average number 

of crashes by site 

and percent of 

total crashes at 

problem sites

55.5

(35%)

52.3

(33%)

14.2

(8.9%)

15.2

(9.6%)

11.2

(7%)

10.2

(6.4%)

158.5

(100%)
33 

As shown in table 17 and table 18, the number of crashes involving failure to yield comprises an 

average of 20.8 percent of all crashes at the control sites, compared to 35 percent of all crashes at 

the problem sites. However, the average of crashes involving left turns from incorrect lanes 

comprises a similar proportion of crashes at control sites (31.1 percent) compared to problem 

sites (33 percent). The average proportion of crashes involving improper entry, circulating, or 

exit lane changes is higher at the control sites (15.1 percent) compared to the problem sites (8.9 

percent). Meanwhile, the average proportion of rear-end crashes is similar at the control sites 

(9.4 percent) and the problem sites (9.6 percent). The average number of crashes involving right 

turns from incorrect lanes is higher at the control sites (11.3 percent) compared to the problem 

sites (6.4 percent).

Three crash types (failure to yield, left turn from incorrect lane, and right turn from incorrect 

lane) were identified for further review. These crash types were targeted because of their 

relationship to exit crashes and the likelihood of interventions reducing the numbers of these 

types of crashes. These three crash types are explored further in the subsequent sections.

Crashes Involving Failure To Yield

As shown in table 17, the problem sites experienced 55.5 crashes involving failure to yield on 

average over 3 yr, but the control sites experienced 7.3 crashes of this type over 3 yr. The 

number of crashes involving failure to yield at the problem sites ranged from 34 crashes over 

3 yr at Melbourne to 114 crashes over 3 yr at Hilliard.

The project team investigated possible explanations for the higher-than-desired number of 

failure-to-yield crashes at the problem sites. When looking at the entering and conflicting 

volumes, the control sites generally had lower volumes, both entering and conflicting, than the 

problem sites. These data emerged as one potential explanation for the number of failure-to-yield 

crashes. Figure 23 shows the number of failure-to-yield crashes by intersection approach and the 

1-h average of the entering and conflicting volume sum on the same approach. As shown, the 1-h 

average sum of the entering and conflicting volumes is relatively uniform and consistently 

between 600 vehicles per hour and 1,200 vehicles per hour for the control sites. However, the 

problem sites have 1-h average sums of entering and conflicting volumes ranging between 

900 vehicles per hour and just over 2,100 vehicles per hour, with less uniformity across the 

intersection approaches.
34 

Source: FHWA.

Figure 23. Graph. Number of crashes over 3 yr involving failure to yield and sum of 

entering plus conflicting volume, by intersection approach.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
35 

To further investigate potential relationships between traffic volume and number of failure-toyield crashes, the project team produced three plots, as follows: 

• The average 1-h entering volume versus the number of failure-to-yield crashes by 

intersection approach, shown in figure 24. 

• The average 1-h conflicting volume versus the number of failure-to-yield crashes by 

intersection approach, shown in figure 25. 

• The average 1-h entering plus conflicting volume versus the number of failure-to-yield

crashes by intersection approach, shown in figure 26. 

No notable relationship is apparent between any of the three volume-metric combinations and 

failure-to-yield crashes, especially for the problem sites, due to the large spread in data.

Source: FHWA.

Figure 24. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

entering volume. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
36 

Source: FHWA.

Figure 25. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

conflicting volume. 

Source: FHWA.

Figure 26. Graph. Number of crashes over 3 yr involving failure to yield by average 1-h 

entering plus conflicting volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
37 

The problem sites, in aggregate, had over seven times as many failure-to-yield crashes as the 

control sites, but these crashes at the problem sites varied by approach. On the low end, the 

southbound approach at Slingerlands had one crash over 3 yr involving failure to yield. 

Conversely, on the high end, the westbound approach at Hilliard had 35 crashes over 3 yr 

involving failure to yield. To draw a further distinction between the approaches with higher 

numbers of these types of crashes, the project team investigated the intersection entries with 

more than 15 failure-to-yield crashes over 3 yr, which included the following 8 (out of 24 total) 

entries:

• All four entries at Hilliard.

• The southbound entry at Melbourne. 

• The eastbound entry at Slingerlands. 

• The eastbound and westbound entries at Malta. 

These eight entries were adjacent to two conflicting lanes that each allowed for a through exiting 

movement. This configuration requires the outside lane to exit the roundabout because vehicles 

in the inside circulating lane are given the option of exiting or continuing to circulate. These 

eight entries generally had a large imbalance in lane use, with high percentages of vehicles using 

the right/outside lane. For instance, 85.4 percent of the volume was in the right/outside lane at 

the eastbound entry at Slingerlands. 

Crashes Involving Left Turns From Incorrect Lanes

As shown in table 17, the problem sites experienced an average of 52.3 crashes over 3 yr 

involving left turns from incorrect lanes, compared to 11 crashes over 3 yr of this type at the 

control sites. The total number of crashes involving left turns from incorrect lanes at the problem 

sites ranged from 18 crashes at Wales to 116 crashes at Carmel 116th. The project team 

investigated possible explanations for these crashes. When looking at the left-turn volumes, the 

control sites generally had lower volumes than the problem sites, and these data emerged as one 

potential explanation for these types of crashes.

Figure 27 shows the number of crashes involving left turns from incorrect lanes by left-turn 

movement and the 1-h average of the left-turn volumes for the same movement. As shown, the 

1-h average of the left-turn volumes is relatively uniform and consistently under 200 for the 

control sites. However, the problem sites have 1-h average left-turn volumes ranging between 9 

vehicles per hour and 526 vehicles per hour, with less uniformity across the left-turn movements. 

The entry with nine left turns (the southbound left turn at Slingerlands) reflected only 1 h of data. 

This approach had 25 crashes involving left turns from incorrect lanes, which may be due to 

higher left-turn volumes during other hours. In comparison, the entry with 526 left turns (the 

westbound left turn at Slingerlands) had zero crashes involving left turns from incorrect lanes. 
38 

Source: FHWA.

Figure 27. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes 

and average 1-h left turn volumes, by left-turn movement.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

  

 

 

 

 

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
39 

To further investigate potential relationships between left-turn volumes and the number of 

crashes involving left turns from incorrect lanes, the team produced two plots, as follows: 

• The number of crashes over 3 yr involving left turns from incorrect lanes versus the 

number of left turns as a percentage of entering volume is shown in figure 28. 

• The number of crashes over 3 yr involving left turns from incorrect lanes versus the 

average 1-h left-turn volume is shown in figure 29. 

Neither of these plots supported a strong relationship between left-turn movement volumes and 

the number of left-turn crashes.

Source: FHWA.

Figure 28. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes by 

number of left turns as a percentage of entering volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
40 

Source: FHWA.

Figure 29. Graph. Number of crashes over 3 yr involving left turns from incorrect lanes by 

average 1-h left-turn volumes.

The problem sites, in aggregate, had over four times as many crashes involving left turns from 

incorrect lanes as the control sites, but crash numbers varied considerably among problem sites.

Seven of the 24 left-turn movements had zero crashes over 3 yr associated with left turns from 

incorrect lanes. Conversely, on the high end, three approaches had over 40 crashes of this type 

over 3 yr, including 42 crashes over 3 yr at the westbound left-turn movement at Melbourne, and 

48 and 47 crashes over 3 yr on the westbound and southbound left turns, respectively, at Carmel 

116th. 

The 6 left-turn movements (out of a total of 24) with more than 20 crashes over 3 yr involving 

left turns from incorrect lanes are as follows: 

• The westbound and southbound left-turn movements at Carmel 116th (48 and 47 crashes 

over 3 yr, respectively). 

• The westbound left-turn movement at Melbourne (42 crashes over 3 yr). 

• The northbound left-turn movement at Hilliard (33 crashes over 3 yr).

• The southbound left-turn movement at Slingerlands (25 crashes over 3 yr).

• The northbound left-turn movement at Malta (21 crashes over 3 yr).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
41 

Crashes Involving Right Turns From Incorrect Lanes

As shown in table 17, the problem sites experienced an average of 10.2 crashes over 3 yr 

involving right turns from incorrect lanes, compared to 4 crashes of this type over 3 yr at the 

control sites. The number of crashes of this type at the problem sites ranged from zero crashes 

over 3 yr at Slingerlands and Malta to 27 crashes over 3 yr at Hilliard. 

The project team investigated possible explanations for these types of crashes at the problem 

sites by investigating potential links between right-turn volumes and the number of crashes 

involving right turns from incorrect lanes. Figure 30 shows the number of crashes involving right 

turns from incorrect lanes by right-turn movement and the 1-h average of the right-turn volumes 

on the same movement. 

When looking at the right-turn volumes, except for the northbound right-turn movement at 

Carmel 106th, the average 1-h volumes at the control sites were all lower than 160 vehicles per 

hour. In contrast, the problem sites had eight approaches, over six sites, with average 1-h rightturn volumes exceeding 160 vehicles per hour. The problem sites generally had more individual 

entries with higher right-turn movement volumes and a wider range in average 1-h right-turn 

movement volumes. However, the project team did not find any clear trends between the 

right-turn movement volumes and the number of crashes involving right turns from incorrect 

lanes.
42 

Source: FHWA.

Figure 30. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and average 1-h right-turn volumes, by right-turn movement.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
43 

The project team also generated two plots, as follows: 

• The number of crashes over 3 yr involving right turns from incorrect lanes versus the 

number of right turns as a percentage of entering volume is shown in figure 31. 

• The number of crashes over 3 yr involving right turns from incorrect lanes versus the 

average 1-h right-turn volume is shown in figure 32. 

Neither of these plots supported a strong relationship between right-turn movement volumes and 

the number of right-turn crashes.

Source: FHWA.

Figure 31. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and number of right turns as a percentage of entering volume, by right-turn movement. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
44 

Source: FHWA.

Figure 32. Graph. Number of crashes over 3 yr involving right turns from incorrect lanes 

and average 1-h right-turn volumes, by right-turn movement. 

While the problem sites, in aggregate, had twice as many crashes over 3 yr involving right turns 

from incorrect lanes as the control sites, the problem sites had a range of crashes of this type. 

Thirteen of the 24 right-turn movements had zero crashes of this type over 3 yr. Conversely, on 

the high end, 5 approaches had 8 or more crashes of this type over 3 yr, including 13 crashes 

over 3 yr at the westbound right-turn movement at Melbourne.

The 5 right-turn movements (out of a total of 24) with 8 or more crashes over 3 yr involving 

right turns from incorrect lanes are as follows: 

• The westbound right-turn movement at Melbourne (13 crashes over 3 yr).

• The northbound right-turn movement at Carmel 116th (11 crashes over 3 yr). 

• The southbound, northbound, and westbound right-turn movements at Hilliard (11, 8, and 

8 crashes over 3 yr, respectively).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
45 

CHAPTER 4. CONFLICT ANALYSIS

The project team reviewed video footage collected by the tethered drone and used the footage to 

track violations and conflicts at each roundabout. The team logged information about each event, 

including the time of the event, where the offending vehicle entered and exited the roundabout, 

where the event occurred, and what violation was committed, among others. 

EVENT TYPES

The project team categorized events into five types based on their severity. These types included 

the following:

• Violation—An event in which a single vehicle violated a rule while traveling through the 

roundabout, and no other vehicles were affected. 

• Potential conflict—A violation event that had the potential to create a conflict due to the 

proximity of another vehicle (i.e., another vehicle was within approximately two car 

lengths of the violating vehicle at the time of the violation).

• Conflict: Incident—A conflict between two or more vehicles in which at least one vehicle 

had to brake or steer to avoid another vehicle. 

• Conflict: Near-crash—A conflict between two or more vehicles in which at least one 

vehicle had to perform a hard or abrupt braking or steering maneuver to narrowly avoid 

colliding with another vehicle. 

• Conflict: Crash—A conflict in which two or more vehicles collided with each other.

VIOLATION TYPES

The project team categorized 15 violation types based on definitions by Richfield and Hourdos, 

which are illustrated in figure 33.

(10) Additionally, an “other” category was added as a catch-all 

for violations that did not fit into these categories. Each violation type is listed as follows:

• Yield violations: 

a. Failure to yield to vehicle in inner lane. 

b. Failure to yield to vehicle in outer lane. 

c. Failure to yield to vehicles in both lanes. 

• Lane change violations:

a. Entrance lane—entering the inside circulating lane from the outside entry lane or vice 

versa. 

b. Exit lane—exiting from the inside circulating lane into the outside exit lane or vice 

versa. 

c. Occupying or straddling both lanes. 

d. Cutting lanes when going straight through. 
46 

• Turn violations: 

e. Turning right from the inner lane. 

f. Turning left from the outer lane. 

g. Turning more than 270 degrees from the outer lane. 

• Wrong way and stop violations:

d. Wrong-way violation—entering the roundabout against traffic. 

e. Wrong-way violation—exiting the roundabout through entrance lanes. 

f. Stop violation—vehicle in circle stopping to yield to vehicles entering roundabout. 

g. Stop violation—general unjustified stopping. 

• Other violations: 

a. Other.

© 2013 Richfield and Hourdos. Modified by FHWA to remove color. 

Figure 33. Illustration. ‘Categories of Violations.’(10)
47 

EVENTS BY LOCATION

Table 19 shows the total number of events by type for each roundabout. While roundabouts with 

the highest number of each event type fell within the problem group, roundabouts in the control 

group still had high numbers of violations. The Carmel 106th roundabout also had a relatively 

high number of conflicts, although still fewer than its counterpart, Carmel 116th. 

Table 19. Number of events by event type and roundabout location.

Site Type

Roundabout 

Location Violations 

Potential 

Conflicts 

Conflicts:

Incidents 

Conflicts:

NearCrashes Total 

Control Carmel 106th 1,185 26 16 3 1,230 

Control Oshkosh 898 12 8 1 919 

Control De Pere 419 14 2 435 

Problem Melbourne 1,247 63 16 4 1,330 

Problem Malta 1,289 15 12 1 1,317 

Problem Carmel 116th 769 23 21 6 819 

Problem Wales 582 17 5 2 606 

Problem Hilliard 168 17 14 6 205 

Problem Slingerlands 155 37 8 1 201 

The team extracted data and completed analysis for 4 h of video for each roundabout except for 

the New York roundabouts, which had a total of 4 h between the two locations (3 h at Malta and 

1 h at Slingerlands). In total, the team evaluated 12 h of video for control sites and 20 h for 

problem sites. Table 20 shows the average hourly events for each event type for control and 

problem sites. Problem sites had higher rates of conflicts and potential conflicts, but the rate of 

violations was similar between the two groups. 

Table 20. Average hourly events by event type and site type.

Site Type

Conflicts: NearCrashesConflicts: 

Incidents 

Potential 

Conflicts Violations Total 

Control 0.33 2.17 4.33 208.50 215.33 

Problem 1 3.80 8.60 210.50 223.90 

Table 21 shows the average hourly events by violation type for each location. Lane-change 

violations accounted for most events, particularly lane-straddling and exit lane violations. Two 

control sites, Carmel 106th and Oshkosh, had some of the highest numbers of these violations. 

The Melbourne roundabout was the only location that had a higher number of turn violations 

than lane-change violations. 
48 

Table 21. Average hourly events by violation type and roundabout location. 

Violation Type

Carmel 106th (C)

De Pere (C)

Oshkosh (C)

Carmel 116th (P) 

Hilliard (P) 

Malta (P) 

Melbourne (P)

Slingerlands(P) 

Wales(P) 

Average 

for All 

Sites

a. Yield violation—failure to 

yield to vehicle in inner lane 4 0 2 7 4 1 2 18 2 4 

b. Yield violation—failure to 

yield to vehicle in outer lane 2 0 1 1 2 2 1 10 1 2 

c. Yield violation—failure to 

yield to vehicles in both 

lanes

0 0 0 1 1 1 0 3 0 1 

d. Lane change violation—

entrance lane 9 4 1 8 3 10 14 13 2 7 

e. Lane-change violation—

exit lane 103 23 38 109 15 123 59 31 43 60

f. Lane-change violation—

occupying or straddling both 

lanes

166 68 179 35 16 267 40 104 80 106

g. Lane-change violation—

cutting lanes when going 

straight through

13 11 7 4 1 7 15 18 16 10

h. Turn violation—turning 

right from the inner lane 2 0 1 2 2 0 1 0 0 1 

i. Turn violation—turning 

left from the outer lane 5 3 1 33 6 25 200 2 8 31

j. Turn violation—turning 

more than 270 degrees from 

the outer lane

1 0 0 4 1 2 2 1 1 1 

m. Stop violation—vehicle 

in circle stopping to yield to 

entering vehicle

0 0 0 0 0 0 0 0 0 0 

n. Stop violation—general 

unjustified stopping 1 0 0 0 0 0 0 1 0 0 

o. Other 3 0 0 2 1 0 0 0 0 1

C = control site; P = problem site.

Table 22 shows the average hourly events by violation type and site type. Control sites had a 

higher rate of lane-straddling and lane-cutting violations but lower rates of yield violations and 

turn violations.
49 

Table 22. Average hourly events by violation type and site type. 

Violation Type Control Problem

a. Yield violation—failure to yield to vehicle in inner lane 2.08 4.05

b. Yield violation—failure to yield to vehicle in outer lane 0.92 1.70

c. Yield violation—failure to yield to vehicles in both lanes 0 0.70

d. Lane change violation—entrance lane 4.58 7.35

e. Lane change violation—exit lane 54.67 65.15

f. Lane change violation—occupying or straddling both lanes 137.42 79.40

g. Lane change violation—cutting lanes when going straight 

through 10.17 8.95 

h. Turn violation—turning right from the inner lane 0.92 1

i. Turn violation—turning left from the outer lane 2.75 53.20

j. Turn violation—turning more than 270 degrees from the outer 

lane 0.50 1.75 

m. Stop violation—vehicle in circle stopping to yield to entering 

vehicle 0.08 0 

n. Stop violation—general unjustified stopping 0.25 0.10

o. Other 1 0.55

Table 23 shows average hourly conflicts by violation type and roundabout location. When 

looking only at events that resulted in a conflict (incident or near-crash), yield violations were 

the most common cause. Failure to yield to a vehicle in the inner lane was the most common 

cause of conflicts (42 events) followed by failure to yield to a vehicle in the outer lane (21 

events). The next most common cause of conflicts was turning left in the outer lane (18 events), 

for which the Melbourne roundabout was the most problematic. 
50 

Table 23. Average hourly conflicts by violation type and roundabout location. 

Violation Type

Carmel 106th (C)

De Pere (C)

Oshkosh (C)

Carmel 116th (P) 

Hilliard (P) 

Malta (P) 

Melbourne (P) 

Slingerlands (P) 

Wales (P) 

Average 

for All 

Sites

a. Yield violation—failure to 

yield to vehicle in inner lane 0.8 0 1 3.3 2 0.7 1.8 3 0.5 1.4

b. Yield violation—failure to 

yield to vehicle in outer lane 0.5 0 0.8 0.5 1.3 1 0.8 2 0.3 0.8

c. Yield violation—failure to 

yield to vehicles in both lanes 0 0 0 0.8 0.5 0.7 0 2 0.3 0.5

d. Lane-change violation—

entrance lane 0.3 0 0 0 0.3 0 0 0 0 0.1

e. Lane-change violation—

exit lane 1.8 0 0 0.8 0 0.7 0.3 2 0.3 0.6

f. Lane-change violation—

occupying or straddling both 

lanes

0 0 0 0 0 0.3 0 0 0.5 0.1

g. Lane-change violation—

cutting lanes when going 

straight through

0 0.3 0 0 0 0 0.5 0 0 0.1

h. Turn violation—turning 

right from the inner lane 0.5 0 0 0.3 0.5 0 0 0 0 0.1

i. Turn violation—turning left 

from the outer lane 0.8 0.3 0.3 0.5 0.3 1 1.8 0 0 0.5

j. Turn violation—turning 

more than 270 degrees from 

the outer lane

0 0 0.3 0.8 0.3 0 0 0 0 0.1

n. Stop violation—general 

unjustified stopping 0.3 0 0 0 0 0 0 0 0 0 

o. Other 0 0 0 0 0 0 0 0 0 0

C = control site; P = problem site.

Table 24 shows the average hourly conflicts by violation type and site type. Problem sites had a 

much higher rate of yield violations that resulted in a conflict. Problem sites also had slightly 

higher rates of turn violations resulting in conflicts. The rates of all other violation types that 

resulted in a conflict were similar between problem and control sites.
51 

Table 24. Rate of conflicts per hour by violation type and site type.

Violation Type Control Problem

Yield violation—failure to yield to vehicle in inner lane 0.58 1.75 

Yield violation—failure to yield to vehicle in outer lane 0.42 0.80 

Yield violation—failure to yield to vehicles in both lanes 0 0.50 

Lane-change violation—entrance lane 0.08 0.05 

Lane-change violation—exit lane 0.58 0.45 

Lane-change violation—occupying or straddling both lanes 0 0.15 

Lane-change violation—cutting lanes when going straight 

through 0.08 0.10 

Turn violation—turning right from the inner lane 0.17 0.15 

Turn violation—turning left from the outer lane 0.42 0.65 

Turn violation—turning more than 270 degrees from the outer 

lane 0.08 0.20 

Stop violation—general unjustified stopping 0.08 0 

Other 0 0 

EVENTS BY VOLUME

Figure 34 shows the average hourly number of events by the average hourly entering traffic 

volume for control and problem sites. For both control and problem sites, there is a slight trend 

in which the number of events decreases as the volume increases. Because lane-change 

violations caused most events, it may be that an increase in volume makes these types of 

violations less likely to occur because there is less room to maneuver within the roundabout. 

Source: FHWA.

Figure 34. Graph. Average hourly events by entering volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
52 

Figure 35 shows the average hourly conflicts by the average hourly entering traffic. For the 

control sites, the number of conflicts decrease as the volume increases. However, at the problem 

sites, the number of conflicts increase as volume increases. Conflicts were more likely to occur 

as the result of failure-to-yield violations, which were more common for problem sites. As 

volume increases, these types of violations may be more likely to occur, as entering drivers have 

fewer opportunities to enter and may take more risks. 

Source: FHWA.

Figure 35. Graph. Average hourly conflicts by entering volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
53 

CHAPTER 5. DESIGN INFLUENCES

This section summarizes observations related to potential design influences on observed crash 

and conflict patterns. Design influences encompass both geometric design and traffic control 

devices. This section focuses on design influences at the roundabout itself; network influences 

away from the roundabout are discussed in Chapter 6. 

The research team identified the following 10 general categories of geometric design and traffic 

control device influences at the roundabout: 

1. Large conflict areas: Large conflict areas appear to be associated with failure-to-yield 

crashes and may contribute to other crashes and violations related to lane changes 

through the entry or exit. At study sites, large conflict areas resulted from use of large

ICDs, skewed angles between legs, and other factors. 

2. Mandatory turn lanes: Approach lanes that are designated at the roundabout as mandatory 

left- or right-turn lanes appear to influence turning movements from incorrect lanes and 

lane changes within the roundabout. 

3. Alignment of entry lane additions and exit merging: The taper design used to add entry 

lanes appears to affect driver lane selection and subsequently influence turning 

movements from incorrect lanes. 

4. Reverse curves through exits: Reverse curves without a tangent section between curves 

appear to increase lane changes through the exit area. 

5. Small diameters: Sites with small ICDs had increased violations related to maintaining 

lanes through the roundabout. 

6. Entry alignment: The alignment of entering lanes with corresponding circulating lanes 

affects entry lane changes and the likelihood that drivers straddle lanes.

7. Circulatory roadway striping: For roundabouts where painted spirals are used, drivers 

often follow the central island curb and then change lanes abruptly to exit. 

8. Yield signs and markings: The roundabouts in this study had a variety of signing and 

pavement marking treatments in the yielding area. However, no clear pattern emerged to 

distinguish failure-to-yield crash patterns as a function of yield signing and markings. 

9. Advance lane-use signs and pavement arrows: The roundabouts in this study had a 

variety of signing and pavement-marking treatments to convey lane use in advance of the 

roundabout. Overhead lane control signs were anecdotally found to be more visible and 

simpler to read at a glance. However, the effect of signing and marking differences on 

driver lane selection could not be isolated in this analysis.

10. Circulating lane-use pavement arrows: The roundabouts in this study had a variety of 

signing and pavement-marking treatments to convey lane use within the circulatory 

roadway of the roundabout. No clear pattern emerged to distinguish crash patterns as a 

function of pavement arrow style or placement configurations.
54 

Table 25 through table 28 summarize information related to the 10 generalized categories of 

design influences for each of the control sites and problem sites, respectively. Design influences 

were identified at each of the study sites. The relative magnitude of design impact on vehicle 

conflicts and historical crashes varies by site with other contributing factors, including overall 

volumes and turning movement patterns during peak hours. 

Table 25. Geometric design influences for control sites. 

Intersection

Large 

Conflict 

Areas or 

Large ICD

Mandatory 

Turn Lanes 

Lane 

Additions or 

Lane Merge 

Exit 

Reverse 

Curves

Small 

ICD

Entry 

Misalignment

Carmel 106th Northeast 

quadrant WB left EB entry — — EB and WB entries

Oshkosh — — EB entry — 135 ft ICD Minor;

all entries

De Pere NB and SB 

entries — 

EB and 

WB entries; 

EB and 

WB exits

— — Minor;

all entries

—No data.

Table 26. Summary of design influences for problem sites. 

Intersection

Large 

Conflict 

Areas or 

Large ICD

Mandatory 

Turn Lanes 

Lane 

Additions or 

Lane Drops 

Exit 

Reverse 

Curves

Small 

ICD

Entry 

Misalignment

Carmel 116th All quadrants — — Yes — All entries

Hilliard — — 

SB and WB 

entry; EB and 

NB exit

— — — 

Melbourne All entries. 

200 ft ICD. WB right

Added 

right-turn 

bypass lanes 

on SB and NB 

entry

All exits — All entries

Slingerlands

198 ft ICD.

EB and WB 

entries.

EB and 

WB left

EB and 

WB entry — — — 

Malta EB and WB 

entries

EB and 

WB left WB entry — — Varies by entry

Wales — WB left WB entry SB exit — Minor;

all entries

—No data.
55 

Table 27. Traffic control influences for control sites. 

Intersection

Circulatory 

Roadway Striping

Yield Signs and 

Markings

Advance Lane 

Control Signs and 

Pavement Arrows

Circulating 

Lane-Use 

Arrow Type 

and Position

Carmel 106th

Combination of solid 

and dotted lane line Dual yield signs; yield 

lines

One roadside sign; 

1–2 sets of fishhook 

arrows

Standard;

centered

Oshkosh

Dotted lane line Dual yield signs with 

plaques; YIELD word 

markings

Overhead signs;

two sets of standard 

arrows

Standard;

centered

De Pere

Dotted lane line Dual yield signs with 

plaques; YIELD word 

markings

One roadside sign; 

one set of standard 

arrows

Standard; 

centered

Table 28. Traffic control influences for problem sites. 

Intersection

Circulatory 

Roadway Striping

Yield Signs and 

Markings

Advance Lane 

Control Signs and 

Pavement Arrows

Circulating 

Lane-Use Arrow 

Type and 

Position

Carmel 116th

Dotted lane line; 

partially concentric 

markings in NE and 

SW quadrants

Dual yield signs; yield 

lines

One roadside sign; 

1–2 sets of fishhook 

arrows

Fishhook; 

centered

Hilliard

Dotted lane line Dual yield signs with 

plaques; yield lines

One roadside sign; 

1–2 sets of fishhook 

arrows

Standard; start of 

lane line

Melbourne

Combination of solid 

and dotted lane line Dual yield signs with 

plaques; yield lines

One set of roadside 

signs, one set of 

fishhook arrows

Standard; 

centered

Slingerlands

Combination of solid 

and dotted lane line

Dual yield signs with 

plaques; YIELD word 

markings and yield 

lines

One roadside sign; 

1–2 sets of fishhook 

arrows

Standard with 

“ONLY”; start of 

lane line

Malta

Combination of solid 

and dotted lane line

Dual yield signs with 

plaques; YIELD word 

markings and yield 

lines

One roadside sign; 

two sets of 

fish-hook arrows

Standard with 

“ONLY”; varies

Wales

Dotted lane line Dual yield signs with 

plaques; YIELD word 

markings

Overhead plus

roadside signs;

2–3 sets of standard 

arrows

Standard; 

start of raised 

splitter

LARGE CONFLICT AREAS

One of the geometric elements associated with crashes involving failure of entering vehicles to 

yield to both circulating lanes is the presence of a large conflict area. A large conflict area is one 

where there is separation between an entry and the immediate downstream exit, creating an 

alignment where the entering and exiting paths become coincident over a given length. A small 
56 

conflict area, by contrast, has entering and exiting paths crossing one another without a 

coincident segment. The size of the conflict area is influenced by a combination of several 

interrelated geometric elements, described as follows:

• Angles between legs greater than 90 degrees.

• Large ICDs (especially 200 ft or larger), even for sites with 90-degree angles between 

legs.

• Approach alignments and corresponding entry angle: Overly deflected entries with small 

entry angles have the potential to result in larger conflict areas.

• Exit radius and alignment of the adjacent exit: Small exit radii and reversing curves 

through the exit can result in increased separation between an entry and an adjacent exit.

• Design of spiral curves within the circulatory roadway.

As illustrated in figure 36, the Melbourne site has large conflict areas in each of the four 

quadrants. The large conflict areas are the result of a large ICD of 200 ft, relatively small exit 

radii, and entry and exit alignments that combine to create over 50 ft of separation (measured 

along the outside curb) between each entry and the adjacent exit to the right. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 36. Photo. Melbourne—conflict areas.
57 

The distance from the yield line to the start of the circulating lane lines is another indication of 

the size of the conflict area. The longer the distance of travel through the conflict area, the more 

likely a driver may be to enter beside another vehicle, as illustrated in figure 37. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 37. Photo. Melbourne—Failure-to-yield crash patterns associated with large conflict 

areas.

Longer distances also make it more difficult for drivers to maintain alignment with the correct 

circulating lane, which increases the potential for lane changing through the entry. The distances 

from the yield line to the start of the circulating lane line range from 83 to 96 ft (figure 36). 

Failure of entering vehicles to yield to both circulating lanes resulted in 37 crashes over 3 yr 

(20 percent of the intersection total), with the crashes predominantly on the southbound and 

westbound entries. 

The large size of the Melbourne roundabout also appears to influence crashes related to lane 

changes within the roundabout, right turns from incorrect lanes, and left turns from incorrect 

lanes. In particular, the large size and exit geometry result in smooth paths for left turns and right 

turns from incorrect lanes, which may lead to some drivers navigating the roundabout incorrectly 

without realizing it. Left turns from incorrect lanes account for 40 percent of the intersection 

crashes and are discussed in more detail in other sections of chapter 5 and chapter 6. Figure 38
58 

illustrates observed locations of crashes related to right turns from incorrect lanes, accounting for 

10 percent of the intersection total.

 Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 38. Melbourne—Crash patterns for right turns from incorrect lanes associated with 

large conflict areas.

The Slingerlands site has an ICD of 198 ft with exclusive left-turn lanes and corresponding 

single-lane exits on two legs, as illustrated in figure 39. 
59 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 38′ 13.51′′ N and 73° 51′ 21.50′′ W. Elevation 217 ft. 

Eye altitude 1,205 ft.

Figure 39. Photo. Slingerlands—conflict areas.

However, despite the similar size, the relative alignments of each entry and exit reduce the 

separation between entries and adjacent exits compared to the Melbourne site. Still, Slingerlands 

has large conflict areas at two entries, which are influenced by the large ICD in combination with 

painted spiral markings that result in 72–78 ft of distance between the yield line and the start of 

the circulating lane line. These 2 entries with the large conflict areas corresponded to the 

locations with the majority of crashes related to failure of entering vehicles to yield to both 

circulating lanes (32 crashes over 3 yr). 

Malta, with an ICD of 165 ft, has a more compact size than the nearby Slingerlands site. 

However, as part of a previous retrofit, the eastbound and westbound approaches were modified 

to be exclusive left-turn lanes with single-lane exits, as illustrated in figure 40. 
60 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft. 

Eye altitude 1,415 ft.

Figure 40. Photo. Malta—conflict areas.

Painted spirals were added to the circulatory roadway to support the modified lane 

configurations. However, due to the retrofit conditions, the addition of the painted spirals 

increased the length of conflict areas adjacent to the eastbound and westbound entries to 68 ft 

and 60 ft, respectively. This increase was observed to adversely affect vehicle alignment and 

increase lane straddling.

At Carmel 106th, which has an ICD of 165 ft, the size of the conflict areas varies. Key 

dimensions are illustrated in figure 41. 
61 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 10, 2018. 39° 56′ 29.71′′ N and 86° 09′ 40.36′′ W. Elevation 821 ft. 

Eye altitude 1,752 ft.

Figure 41. Photo. Carmel 106th—conflict areas.

At this location, other factors—the angle between legs and the alignment of the entries and 

exits—appear to have a greater impact on the size of the conflict areas than the ICD. The largest 

conflict area is adjacent to the westbound entry, which has a distance of 77 ft from the yield line 

to the start of the circulating lane line. The width of this conflict area is created by a combination 

of the design of the spiral within the circulatory roadway, the skew angle between legs, and the 

alignment of the east leg being heavily offset to the left of the roundabout center. The size of the 

conflict area is likely contributing to observed violations of vehicles crossing and straddling 

lanes westbound. The two largest conflict areas correspond to the locations of the highest 

numbers of crashes related to entering drivers failing to yield to both circulating lanes, with six 

crashes at the eastbound entry and two crashes at the westbound entry. Traffic patterns and 

network influences also have a major impact on crash patterns at this location; these patterns are 

further discussed in chapter 6.

Carmel 116th has an ICD of 177 ft, which is larger than that for Carmel 106th. The conflict areas 

at Carmel 116th, illustrated in figure 42, are also larger than those at Carmel 106th, ranging from 

67 to 74 ft when measured from the entrance line to the start of the circulating lane lines.
62 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 42. Photo. Carmel 116th—conflict areas.

For the northeast and southwest corners of the roundabout, concentric lane markings are present. 

In these corners, the conflict area was measured to coincide with the edge of the splitter island. 

The exit geometry influences the size of the conflict areas at this location. Carmel 116th has a 

higher number of crashes related to failure of entering vehicles to yield to both circulating lanes, 

with 31 crashes of this type over 3 yr compared to 10 crashes over 3 yr at Carmel 106th. 

However, this crash type represented 16–20 percent of the total intersection crashes at both 

locations. At Carmel 116th, the failure-to-yield crashes were distributed between the northbound, 

southbound, and eastbound entries, with 8–12 crashes over 3 yr per entry. These crashes were 

also influenced by traffic volume patterns and network considerations, which are discussed 

further in chapter 6.

The control site in De Pere had the fewest crashes and the lowest volumes. This site has an ICD 

of 182 ft, with a slight skew angle between intersection legs that results in larger conflict areas in 

front of the southbound and northbound entries, as illustrated in figure 43. However, the size of 

the conflict areas did not correlate to the location of crashes at De Pere. Over the 3-yr period, 13 

of the 23 total crashes occurred in the southwest corner adjacent to the eastbound entry, which 

has the smallest conflict area. At this site, other factors, such as turning movement patterns and 

network influences, appear to have a greater impact; these factors are discussed in more detail in 

chapter 6. 
63 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: May 17, 2018. 44° 25′ 57.57′′ N and 88° 00′ 58.87′′ W. Elevation 598 ft. 

Eye altitude 1,485 ft.

Figure 43. Photo. De Pere—conflict areas.

Each of the remaining three sites at Hilliard (figure 44), Wales (figure 45), and Oshkosh 

(figure 46) have more compact ICDs, ranging from 135 to 159 ft. These reduced ICDs generally 

result in smaller conflict areas than the other study sites. However, crashes involving failure to 

yield were the highest at the Hilliard site, which had 21–35 crashes of this type at each entry over 

3 yr. At Wales, 11–15 crashes involving failure to yield were reported for each entry over 3 yr, 

which is comparable with the number of crashes recorded at the other problem sites where large 

conflict areas were present. In contrast, the Oshkosh site had 1–3 failure-to-yield crashes at each 

entry over 3 yr. Oshkosh had a smaller ICD than Hilliard and Wales but generally had 

comparably compact conflict areas. These findings suggest that other factors were likely to 

contribute to the observed failure-to-yield crashes at Hilliard and Wales.
64 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 48.28′′ N and 83° 09′ 40.58′′ W. Elevation 937 ft. 

Eye altitude 1,935 ft.

Figure 44. Photo. Hilliard—conflict areas.
65 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 43.21′′ N and 88° 23′ 02.71′′ W. Elevation 987 ft. 

Eye altitude 2,450 ft.

Figure 45. Photo. Wales—conflict areas.
66 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: April 13, 2015. 44° 02′ 21.52′′ N and 88° 32′ 33.17′′ W. Elevation 760 ft. 

Eye altitude 1,539 ft.

Figure 46. Photo. Oshkosh—conflict areas.

APPROACH LANES ENDING IN MANDATORY TURN LANES

Where an approach lane ends at the roundabout as a mandatory turn lane, drivers may use the 

incorrect lane or make lane changes within the roundabout if additional emphasis is not provided 

upstream of entry. 

At the westbound approach to Carmel 106th, there are two approaching lanes; however, the 

inside lane is an exclusive left-turn lane. The outside lane allows a vehicle to make a left turn, 

continue straight westbound, or turn right. The combination of the lane drop with the relatively 

short spacing (685 ft) to the upstream roundabout at the adjacent interchange may be influencing 

driver lane selection or ability to change lanes during periods of heavier volume. Over 4 h, 

194 through vehicles were noted to incorrectly use the inside lane to continue westbound 

(13-25 percent of westbound through vehicles). These data correspond to a key crash pattern, 

illustrated in figure 47, associated with vehicles using the incorrect lane to make a through 

movement and changing lanes at the westbound exit. This crash pattern generated 11 percent 

(or 5) of the intersection’s total crashes. 
67 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 10, 2018. 39° 56′ 29.71′′ N and 86° 09′ 40.36′′ W. Elevation 821 ft. 

Eye altitude 1,752 ft.

Figure 47. Photo. Carmel 106th—Westbound through from incorrect exclusive left-turn 

lane.

Figure 48 illustrates an observed near-crash on the westbound exit at Carmel 106th. A vehicle 

incorrectly making a through movement from the exclusive left-turn lane (inside lane) almost 

collided with an adjacent vehicle making the through movement from the correct outside lane. 

Source: FHWA.

Figure 48. Carmel 106th—Observed near-crash from westbound through from incorrect 

exclusive left-turn lane (image facing south).
68 

At Malta, as illustrated in figure 49, the eastbound approach has two continuous lanes, with the 

inner lane becoming a mandatory left turn only lane at the roundabout. The outer entry lane 

serves the eastbound through and right-turn movements. During the morning peak hour, high 

eastbound-to-southbound right turns were observed for vehicles traveling from I–87. Due to the 

high right-turn demand, there is a substantial lane imbalance, with 69 percent of vehicles in the 

outside lane. This imbalance resulted in 5 percent of the eastbound through vehicles 

(10 vehicles) during the morning peak using the incorrect inside lane, which is designated as 

left-turn only, to make an eastbound through movement. The westbound approach also has a 

mandatory left turn only lane; however, the second westbound entry lane is developed as a lane 

addition, as discussed in the next section.

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft. 

Eye altitude 1,415 ft.

Figure 49. Photo. Malta—Exclusive eastbound left-turn lane.

Exclusive left turns are also present at Wales (westbound entry) and Slingerlands (eastbound and 

westbound entries). However, in both cases, the exclusive lanes are developed through a lane 

addition. These cases are further discussed in the next section. 

ALIGNMENT OF ENTRY LANE ADDITIONS AND EXIT MERGING

At roundabouts, the addition of lanes on the approach and then the merging of lanes on exit is a 

relatively common strategy. According to the 2000 version of Roundabouts: An Informational 

Guide, the wide-nodes/narrow-roads strategy focuses on putting the capacity at the intersection 

where it is needed and allowing for fewer lanes along the roadway segment between 

intersections.(4)

 A common application is the use of a two-lane, undivided roadway (one lane in 

each direction) that widens to two entering lanes at the roundabout. Depending on the lane 

configuration, the downstream exit lane can be a single-lane exit or a two-lane exit that merges 

to a single lane beyond the roundabout exit. 
69 

Seven of the intersections in this study had lane additions, and 2 of these 7 intersections had lane 

merging immediately beyond the exit. The design approach to adding entry lanes and merging 

exit lanes appeared to affect vehicle lane selection and lane utilization.

At Hilliard, the westbound and southbound approaches each have lane additions. Each uses a 

standard taper to add a shared through-right lane toward the outside. As illustrated in figure 50, 

the single approach lane is aligned with the inside entry lane that is designated as a shared 

through-left lane. Each of the northbound, westbound, and southbound entries had between 8 and 

11 reported crashes related to right turns from incorrect lanes over 3 yr. These crashes may be 

attributable, in part, to drivers aligning into the inside left through lane on entry, as well as 

influences from downstream exit-lane merging in close proximity to the circulatory roadway. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 48.28′′ N and 83° 09′ 40.58′′ W. Elevation 937 ft. 

Eye altitude 1,935 ft.

Figure 50. Photo. Hilliard—Lane additions and lane merging.

During each of the 4 h, the westbound approach at Hilliard was imbalanced, with 56–78 percent 

of the westbound drivers using the inside entry lane. This result was likely further influenced by 

two factors: relatively low right-turn demand, and network influences related to a downstream 

lane drop beyond the roundabout to the west and exit-lane merging to the north. However, the 

combination of crashes related to right turns from incorrect lanes and fewer drivers using the 

correct outside lane suggests that the alignment of vehicles to favor the inside lane may be 

contributing to drivers making incorrect lane selections for right turns.
70 

On the southbound entry at Hilliard, which experienced the highest conflicting flows at the 

intersection, the alignment of the lane addition appeared to have less impact on lane utilization, 

which was more balanced, with a maximum of 54 percent in the dominant lane. In addition to 

right turns from incorrect lanes, a primary crash type for the southbound entry was crashes 

related to failure to yield to both circulating lanes. The imbalance in volume on the westbound 

entry results in higher volume in the inside circulating lane, leaving the outside lane more vacant. 

This imbalance may be creating conditions favorable to failure-to-yield crashes on the 

southbound entry, where a driver may attempt to enter beside another circulating vehicle. 

As illustrated in figure 50, the northbound and eastbound exit lanes at Hilliard merge into a 

single lane immediately beyond the crosswalks. This factor may also be influencing crashes 

involving right turns from incorrect lanes on the northbound and westbound entries. Some 

drivers may be using the incorrect lane to avoid the downstream lane drop immediately after 

their turn. The lane merging northbound and eastbound may also be influencing lane changing 

through the exits, which was a top source for violations at the Hilliard site.

At Wales, the westbound approach widens from a single approach lane to two entry lanes with 

the addition of an exclusive left-turn lane. As illustrated in figure 51, the approaching vehicles 

are aligned into the outside lane, which is designated as a shared through-right lane.

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 43.20′′ N and 88° 23′ 00.62′′ W. Elevation 987 ft. 

Eye altitude 1,628 ft.

Figure 51. Photo. Wales—Westbound lane addition (plan view).

As illustrated in figure 52, an abrupt alignment shift on the westbound approach upstream of the 

roundabout further directs vehicles into the outside lane and may be contributing to 
71 

54 left-turning vehicles over 4 h observed to use the incorrect outside lane from the westbound 

entry. However, this alignment shift is not translating to a crash pattern related to the inside lane 

being designated left turn only. While the signing and markings do not allow a left turn from the 

outside lane, the overall intersection lane configurations would support left turns from both 

westbound entry lanes. Higher numbers of crashes may be expected with the same westbound 

entry geometry if the inside lane is designated as a shared through-left with two westbound exit 

lanes.

Source: FHWA.

Figure 52. Photo. Wales—Westbound lane addition (ground-level view).

Figure 53 shows Malta, where the westbound approach widens from a single lane to two entry 

lanes approximately 270-ft upstream of the yield line.
72 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft. 

Eye altitude 1,415 ft.

Figure 53. Photo. Malta—Westbound lane addition.

At Malta, the second entry lane is developed by widening toward the outside with a 120-ft taper. 

Widening toward the outside aligns approaching drivers into the inside entry lane, which is 

designated as left turn only. Drivers must use the outside entry lane to continue straight toward a 

downstream interchange, the primary movement on the westbound entry. Compared to the 

shorter taper used at Hilliard, the longer taper at Malta appears to better support drivers selecting 

the correct lane for the through movement. During each of the 3 h evaluated at Malta, between 

69 and 78 percent of drivers used the outside lane on the westbound entry. During each hour, 

between 7 and 14 drivers (or 2–6 percent of the westbound through volume) were observed to 

use the incorrect lane. These instances of incorrect lane use appeared to be influenced by volume 

imbalances and drivers aligning into the inside lane. 

Figure 54 shows Slingerlands, where two legs have one travel lane in each direction with a 

two-way center turn lane. The two-way left-turn lanes transition into exclusive left-turn lanes at 

the roundabout entry. 
73 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 38′ 13.51′′ N and 73° 51′ 21.50′′ W. Elevation 217 ft. 

Eye altitude 1,205 ft.

Figure 54. Photo. Slingerlands—Entry lane additions.

Drivers approaching the entry are aligned into the outside entry lane, which has a shared 

left-through-right lane-use designation. Therefore, drivers intending to use the inside entry lane 

must make a lane change, similar to moving into a left-turn lane at an intersection with other 

control types. This required lane change results in heavy lane imbalances but lower instances of 

through vehicles using the incorrect lane. For the 1 h of data evaluated at Slingerlands, the 

westbound and eastbound approaches had 65 percent and 85 percent of vehicles in the outside 

lane, respectively. The team observed 3‒4 through vehicles per approach using the incorrect 

lane, likely in an attempt to bypass vehicle queues due to the heavy lane imbalances. Over a 3-yr 

period, only two crashes were reported related to through vehicles using the incorrect lane. 

Figure 55 shows De Pere, where the eastbound and westbound legs widen from a single 

approach lane to two entry lanes. This widening is accomplished through a uniform flare to the 
74 

left and right to reduce priority being given to one entry lane. Drivers on the westbound approach 

favored the outside lane (56–65 percent of vehicles during each hour). Meanwhile, drivers on the 

eastbound approach favored the inside lane (72–75 percent of vehicles during each hour). On the 

westbound entry, six drivers were observed to use the incorrect lane for either a left-turn or rightturn movement over the 4 h analyzed. Nine drivers used the incorrect lane for the left turn or 

right turn on the eastbound approach, with most using the incorrect outside lane for the left turns 

to bypass queuing in the inside lane. At this site, the turning movement patterns appeared to have 

a greater influence on the lane selection due to the geometry of the entry lane additions. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: May 17, 2018. 44° 25′ 57.57′′ N and 88° 00′ 58.87′′ W. Elevation 598 ft. 

Eye altitude 1,485 ft.

Figure 55. Photo. De Pere—Flared lane additions and lane merging.

At De Pere, the eastbound and westbound exit lanes also merge from two lanes to one lane 

within 140–180 ft beyond the roundabout exit. The exit lanes are posted with a sign indicating 

that the right lane ends to direct drivers in the outside exit lane to merge left. On the westbound 

entry, 64–80 percent of drivers making a through movement during each hour use the inside 

lane. However, this use is also influenced by the lane imbalance on the westbound entry favoring 

the outside lane due to high westbound right-turn volume. On the eastbound entry, 64–75 percent 

of drivers during each hour also use the inside lane to make a through movement, despite the 

heavy lane imbalance favoring the inside lane. This trend suggests that the downstream lane drop 

on the westbound exit may be influencing drivers’ lane choices. 
75 

REVERSE CURVES THROUGH EXITS

Lane changing through the roundabout exits is a common violation identified in the conflict 

analysis across the study sites. The use of reverse curves through the exits with no tangent 

provided between the circulating lane line and exit curve appears to be increasing the potential 

for vehicles to change lanes through the exit. 

Melbourne is one example site where this lane changing occurs, with each exit featuring 

relatively small exit radii and back-to-back reversing curves coming out of the circulatory 

roadway, as illustrated in figure 56. Over a 4-h period, 237 vehicles were observed to change 

lanes through the exits when making a left turn, and 59 vehicles were observed to cut across 

lanes when going straight. Traffic patterns influence the actual number of lane-change violations 

at each exit; however, the southbound exit was observed to have the highest number of vehicles 

changing from the inside to the outside lane through the exit. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 56. Photo. Melbourne—Lane changing through exit reverse curves (plan view).

Figure 57 provides a ground-level view of a vehicle approaching the exit and the path that the 

vehicle traveled as it crossed lanes through the exit.
76 

Source: FHWA.

Figure 57. Photo. Melbourne—Lane changing through exit reverse curves (ground view).

At Melbourne, drivers become accustomed to the curvature around the central island due to the 

large 200-ft ICD and often made smooth and natural—but incorrect—left-turn movements using 

the outside lane. An example is illustrated in figure 58 for southbound left-turning vehicles, 

where 685 vehicles over a 4-h period were observed to make a left turn from the incorrect lane. 

Network influences, discussed in chapter 6, contribute to incorrect lane use; however, the smooth 

left-turn path from the outside lane created by the reverse exit curvature also appeared to 

influence driver perception of allowable movements at Melbourne. 

Source: FHWA.

Figure 58. Photo. Melbourne—Exit reverse curves influencing left turns from the incorrect 

lane.
77 

At Carmel 116th, a larger exit radius is used compared to Melbourne. However, the short reverse 

curves along the lane line leading into the eastbound and westbound exits appear to be 

influencing higher observed lane-change violations through the exit. On the northbound and 

southbound exits, concentric lane markings do not provide guidance to drivers to lead them to 

the correct exit lane. Also, the physical alignment of the exit in relation to the circulatory 

roadway aligns drivers using the inside circulating lane into the outside exit lane on the 

northbound and southbound exits, as shown with the solid lines in figure 59. Lane changes 

through the exits accounted for approximately 55 percent of the total violations recorded at 

Carmel 116th, with 424 violations recorded over a 4-h period at all exits. The highest instances 

were on the southbound and eastbound exits, which are also influenced by network 

considerations that are discussed in more detail in chapter 6. 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 59. Photo. Carmel 116th—Lane changing through exit reverse curves.

SMALL DIAMETERS

Figure 60 shows Oshkosh, which has the smallest ICD of any study site of 135 ft; all the other 

study sites have ICDs ranging from 150–200 ft. While Oshkosh was a control site that 

experienced among the lowest total annual crashes (13 per year), it generated unique patterns of 

crashes compared to the other sites. A higher proportion (15 percent) of the crashes at Oshkosh 

were related to sideswiping and lane changing within the circulatory roadway. An additional 

10 percent of the crashes at Oshkosh were related to rear-end crashes at the north, west, and east 

exits.
78 

Source: FHWA.

Figure 60. Photo. Oshkosh—Photo from drone facing southeast.

At Oshkosh, violations related to lane straddling or occupying both lanes accounted for 

78 percent of all events. Over a 4-h period, 743 events were recorded related to drivers straddling 

the lane line in the circulatory roadway or cutting across lanes when going straight. This 

represents 9.2 percent of the total entering volume. Balancing competing objectives of vehicle 

alignment and speed control becomes more challenging with very small ICDs. At Oshkosh, the 

outside entry lane is aligned toward the circulating lane line. This overall design composition is a 

likely contributing factor to the observed lane straddling. However, despite the high violations, 

there were no incidents or near-crashes from lane straddling during the analysis period.

While independent of ICD, the location of the pedestrian crossings at Oshkosh may also be 

contributing to unexpected driver behavior. Pedestrian crossings at this site are less than 20 ft

from the circulatory roadway. The proximity of these crossings may be influencing rear-end 

crashes at exits where drivers are not expecting the driver in front of them to stop for pedestrians 

while still partially in the circulatory roadway. The compact size of the roundabout and the close 

spacing of the pedestrian crossings result in a compressed space for driver reactions. This finding

was consistent with field observations where drivers were noted to be somewhat more aggressive 

in accelerating from the yield line when entering the roundabout, possibly due to the shorter time 

and distance to make decisions.

ENTRY ALIGNMENT

The alignment of entering lanes into the correct receiving lanes affects observed violation types, 

including lane changes through the entry (sometimes referred to as vehicle path overlap) and lane 

straddling through the circulatory roadway. While these violation types and associated crashes 

were not found to be a primary crash type near roundabout exits, they contribute to drivers’ 

perceptions of safety and the total PDO crashes at the intersections.
79 

Among the study sites, Melbourne had the most severe misalignment. This misalignment 

resulted from the combination of several geometric elements: a large ICD; a small entry radius; a 

lack of large radius or tangent near the yield line; and the distance between the entry and 

downstream exit, which created a long distance between the yield line and the start of the 

circulating lane line. Figure 61 illustrates the misalignment of the outside entry lane on each 

approach with the inside circulating lane. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 61. Photo. Melbourne—Entry lane alignment.

On a similar note, at Carmel 106th the eastbound entering lanes are not well-aligned with the 

corresponding circulating lanes. As a result, vehicles were observed to straddle the circulating 

lane line when making an eastbound through movement. This trend is consistent with the tire 

marks visible in figure 62. The large shift to the left for the westbound entry creates a skewed 

angle of intersection to the adjacent northbound exit. This angle was observed to affect vehicle 

alignment, particularly when there were not two vehicles side-by-side at the yield line. Vehicles 

traveling westbound across the northern portion of the circulatory roadway were frequently 

observed to cross the lane line and encroach into the inside circulating lane.
80 

Source: FHWA.

Figure 62. Photo. Carmel 106th—Eastbound lane straddling within circulatory roadway 

due to entry lane alignment.

Figure 63 shows how the entry lane alignment for the eastbound and westbound entries at 

Carmel 106th guides drivers entering from the outside entry lane into a position that straddles the 

circulating lane line. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 10, 2018. 39° 56′ 29.71′′ N and 86° 09′ 40.36′′ W. Elevation 821 ft. 

Eye altitude 1,752 ft.

Figure 63. Photo. Carmel 106th—Entry lane alignment.
81 

Like the Carmel 106th eastbound entry, all the approaches at Carmel 116th have entry lane 

misalignment. The northbound and southbound entries at Carmel 116th have a longer distance 

between the yield line and the start of the circulating lane line, as illustrated in figure 64, which 

may influence the potential for poor path alignment. Vehicles were observed to straddle both 

circulating lanes, particularly in the northbound and southbound directions. The use of a wider 

inside circulating lane (16 ft) relative to the outside lane may also have contributed to the 

potential for lane straddling and poor path alignment. Conversely, the eastbound and westbound 

entries had a shorter distance between the yield line and the start of the circulating lane lines due 

to the partially concentric striping pattern in the circulatory roadway. This shorter distance and 

striping pattern appeared to help eastbound and westbound drivers maintain the correct lanes as 

they entered. 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 64. Photo. Carmel 116th—Entry lane alignment.

At Malta, the southbound, eastbound, and westbound entries each have varying degrees of 

misalignment with the corresponding circulating lanes. As illustrated in figure 65, the 

southbound entry has the strongest misalignment, with vehicles in the outside entry lane 

generally aligned with the circulating lane line. The team identified 63 southbound through 

vehicles to be straddling both lanes over the 3-h period analyzed. The eastbound and westbound 

approaches generally had reasonable alignment; however, the shape of the spiral markings within 

the circulatory roadway resulted in a larger conflict area, which was observed to affect how well 

drivers maintained their alignment. Over a 3-h period, 144 eastbound and 157 westbound 

through vehicles were observed to straddle lanes within the circulatory roadway. 
82 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft. 

Eye altitude 1,415 ft.

Figure 65. Photo. Malta—Entry lane alignment.

Each of the Wisconsin sites at Wales, Oshkosh, and De Pere has similar entry lane alignment. 

The entry geometry at these sites varies slightly, depending on accommodations for trucks, with 

Wales incorporating gore striping between entry lanes. However, at each site, drivers in the 

outside entry lane are generally aligned toward the lane line within the circulatory roadway 

instead of toward the center of the outside circulating lane. As a result, the team observed higher 

instances of lane straddling. Figure 66 illustrates an example of a near-crash associated with lane 

straddling for southbound entering vehicles at Wales. 
83 

Source: FHWA.

Figure 66. Photo. Wales—Observed lane-straddling conflict.

As illustrated in figure 67, the alignment of each entry at Wales generally guides drivers from the 

outside entry lane toward the correct outside receiving lane. However, at the entrance line, the 

vehicle path is aligned more toward the circulating lane line, resulting in increased lane 

straddling. The lane straddling also results in degradation of the lane markings within the 

circulatory roadway, as illustrated in figure 66, which the team observed from drone video 

footage. The degradation of the lane markings further complicates the ability of drivers to 

identify and maintain lanes. Sideswipes and lane changes account for 18 percent of the total 

intersection crashes (19 crashes over 3 yr), with most of these crashes associated with the 

northbound, southbound, and eastbound movements. Occupying or straddling both lanes was the 

leading cause for violations, conflicts, and near-crashes based on conflict analysis. Over a 4-h 

period, the team noted 309 lane-straddling violations, 10 lane-straddling conflicts or 

near-crashes, and 62 lane-change violations through the circulatory roadway. Entry alignments 

may have factored into these events. 
84 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 42.79′′ N and 88° 23′ 02.10′′ W. Elevation 988 ft. Eye altitude 1,656 ft.

Figure 67. Photo. Wales—Entry lane alignment.

The two Wisconsin sites show similar effects of entry alignment on lane straddling. At the De 

Pere site, lane straddling was the top violation type, with 258 violations and 13 potential 

conflicts over a 4-h period. Lane straddling was the top violation type at the Oshkosh site, as 

discussed previously in the section titled “Small Diameters.”

CIRCULATORY ROADWAY STRIPING

Circulatory roadway striping was found to contribute to crashes and conflicts in two ways, as 

follows:

• Concentric striping that is inconsistent with approach lane-use assignments.

• Painted spiral striping to shift circulating vehicles to the outside lane.

Striping in the northeast and southwest corners of Carmel 116th may contribute to left turns from 

incorrect lanes. Figure 68 shows the Carmel 116th roundabout and the variation in striping 

between the northeast and southwest corners and the southeast and northwest corners. 
85 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 68. Photo. Carmel 116th—Concentric lane-line markings.

In the southeast and northwest corner, the circulatory roadway striping leads drivers in the 

outside circulatory lane toward the east and west exits and provides drivers in the inside lane the 

option of continuing to circulate or exiting the roundabout. In the northeast and southwest 

corners, concentric lane-line markings require drivers who want to exit the roundabout from the 

inside lane to cross the dotted circulatory lane lines. These concentric lane-line markings are 

inconsistent with the entry lane-use assignments on the northbound and southbound entries, 

which allow through movements from either entry lane. However, these two different lane-line

marking treatments yielded similar numbers of crashes in the northwest and southwest corners. 

The northwest corner had 48 crashes over 3 yr involving left turns from incorrect lanes, 

compared to 47 crashes over 3 yr of the same type in the southwest corner. These data reinforce 

that getting drivers into the correct lane before entry is critical in reducing the potential for these 

crash types.

At Carmel 116th, the southbound, eastbound, and northbound entries each had high numbers of 

crashes related to failure to yield to vehicles in the inside lane. In each case, the outside 

circulating lane was less likely to be occupied due to the patterns of left turns from the 

southbound and westbound entries. 
86 

At Slingerlands, the spiral striping in the northeast and southwest quadrants is developed with 

only pavement markings and is very abrupt, as shown in figure 69. As a result, many left-turning 

drivers followed the circular central island curb instead of following the spiral markings, which 

led them into the inside circulating lane. Once in the inside circulating lane, drivers were 

required to make a lane change to exit eastbound or westbound. The curvature of the circulatory 

roadway before the exit may give some drivers the false impression that they may continue to 

circulate (and make improper left turns) from the outside lane.

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 38′ 13.51′′ N and 73° 51′ 21.50′′ W. Elevation 217 ft. 

Eye altitude 1,205 ft.

Figure 69. Photo. Slingerlands—Spiral pavement markings.

Similar to Slingerlands, the Malta site has abrupt painted spirals, as shown in figure 70. Many 

northbound left-turning drivers were observed to continue to follow the circular central island 

curb into the inside circulating lane instead of following the spiral markings to the intended 

outside circulating lane. Furthermore, high volumes of traffic make the 

northbound-to-westbound left-turn movement toward the I–87 interchange throughout the day. 

The team observed drivers making this movement from the incorrect outside lane during each 

peak hour. 
87 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: July 15, 2015. 42° 58′ 16.76′′ N and 73° 47′ 33.94′′ W. Elevation 338 ft. 

Eye altitude 1,415 ft.

Figure 70. Photo. Malta—Spiral pavement markings.

Figure 71 shows Wales, where the design explicitly incorporates a geometric spiral into the 

shape of the central island, giving the central island a noncircular shape. The spiral begins along 

the east side of the central island to start drivers along the spiral path while they are adjacent to 

the central island curb. Dotted lane markings continue the spiraled path to guide drivers from the 

inside lane on the east side of the circulatory roadway to the outside lane on the north side of the 

circulatory roadway. This design was observed to result in fewer left-turning vehicles failing to 

follow the intended spiral path. However, over a 4-h period, the team observed 56 northbound 

left-turning vehicles staying in the inside lane along the north side of the circulatory roadway 

instead of spiraling to the outside lane. This trend resulted in late lane changes near the 

westbound exit. 
88 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 42.79′′ N and 88° 23′ 02.10′′ W. Elevation 988 ft. Eye altitude 1,656 ft.

Figure 71. Photo. Wales—Geometric spiral and spiral markings.

As illustrated in figure 72, the condition of the circulating lane markings was poor, which is 

likely contributing to these observations. Figure 72 shows a sequence of photographs where a 

northbound left-turning white van follows the geometric portion of the spiral toward the correct 

outside circulating lane, cuts back toward the inside lane, and eventually changes lanes to exit 

westbound. It is unclear whether more visible markings would have altered this observed 

behavior. 

Source: FHWA.

Figure 72. Photo. Wales—Vehicles navigating the northbound left-turn spiral.
89 

YIELD SIGNS AND PAVEMENT MARKINGS

The team observed various treatments across the study sites with respect to yield signs and 

pavement markings. For signing, the following combinations were seen in the field across the 

study sites: 

• Yield signs only on each side of the entrance, used in Carmel 106th and Carmel 116th. 

Figure 73 shows an example from Carmel 116th. 

• Yield signs on each side of the entrance with a one-way sign over the left yield sign, as 

shown in figure 74 in Slingerlands. 

• Yield signs on each side of the entrance with a one-way sign over the left yield sign and a 

“TO ALL LANES IN CIRCLE” supplemental plaque underneath each yield sign, used in 

Slingerlands and Malta. Figure 75 shows an example from Slingerlands. 

• Yield signs on each side of the entrance with a one-way sign under the left yield sign and 

a “TO TRAFFIC FROM LEFT” supplemental plaque under the right yield sign, used in 

De Pere, Wales, and Oshkosh. Figure 76 shows an example from De Pere.

• Yield signs on each side of the entrance with a “TO BOTH LANES IN 

ROUNDABOUT” supplemental plaque under each yield sign, as shown in figure 77 in 

Hilliard.

Source: FHWA.

Figure 73. Photo. Carmel 116th yield signs only on each side of entrance.
90 

Source: FHWA.

Figure 74. Photo. Slingerlands —Yield signs with a one-way sign.

Source: FHWA.

Figure 75. Photo. Slingerlands—Yield signs with a one-way sign and ‘TO ALL LANES IN 

CIRCLE’ supplemental plaques.

Source: FHWA.

Figure 76. Photo. De Pere—Yield signs with a one-way sign and a ‘TO TRAFFIC FROM 

LEFT’ supplemental plaque.
91 

Source: FHWA.

Figure 77. Photo. Hilliard yield signs with ‘TO BOTH LANES IN ROUNDABOUT’ 

supplemental plaques.

Despite the variety of yield signing configurations, no clear pattern emerged to distinguish crash 

patterns as a function of these configurations. 

For markings, the following combinations were seen in the field across the eight study sites:

• Entrance line only (circulatory roadway edge-line extension). 

• Entrance line with “YIELD” word legend marking used in De Pere and Wales. Figure 78

shows an example from De Pere.

• Entrance line with staggered yield lines, used in Carmel 106th, Carmel 116th, Hilliard, 

Malta, and Slingerlands. Figure 79 shows an example from Carmel 106th. 

• Entrance line with yield line running approximately parallel to the entrance line, as 

shown for Melbourne in figure 80. This use of the yield line is inconsistent with MUTCD, 

which requires the yield line to be perpendicular to the direction of traffic.(6)
92 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 42.79′′ N and 88° 23′ 02.10′′ W. Elevation 988 ft. 

Eye altitude 1,656 ft.

Figure 78. Photo. Wales entrance line and ‘YIELD’ word markings. 

Source: FHWA.

Figure 79. Photo. Carmel 106th entrance line and yield lines.
93 

Source: FHWA.

Figure 80. Photo. Melbourne entrance line and parallel yield line.

Similar to signing, no clear pattern emerged to distinguish crash patterns as a function of 

marking configurations.

ADVANCE LANE-CONTROL SIGNS AND PAVEMENT MARKING ARROWS

Lane-control signs and pavement marking arrows support drivers’ lane selections prior to 

reaching the roundabout entry. Various treatments were observed across the study sites for 

advance lane-control signs and pavement marking arrows. A wide range of combinations was 

seen in the field across the eight study sites, including the following: 

• A single, side-mounted lane-control sign and one set of fishhook pavement marking 

arrows were used in Melbourne, as shown in figure 81. 

• A single, side-mounted lane-control sign and two sets of fishhook pavement marking 

arrows were used in Hilliard, Carmel 106th, and Carmel 116th. Figure 82 shows an 

example from Hilliard.

• Overhead lane-control signs with two sets of standard pavement marking arrows were

used in Oshkosh and Wales. Figure 83 shows an example from Oshkosh. 

Source: FHWA.

Figure 81. Photo. Melbourne side-mounted lane-use sign and single set of fishhook 

markings.
94 

Source: FHWA.

Figure 82. Photo. Hilliard side-mounted lane-use sign and two sets of fishhook markings.

Source: FHWA.

Figure 83. Photo. Oshkosh overhead lane-use signs and two sets of standard arrow 

markings.

The team anecdotally found the overhead lane-control signs with standard arrows to be more 

visible and simpler to read at a glance than lane-control signs with fishhook arrows. However, 

the effect of signing and marking differences on drivers’ lane selections could not be isolated in 

the analysis.

CIRCULATORY LANE-USE PAVEMENT MARKING ARROWS

Circulatory lane-use pavement marking arrows reinforce the allowable movements from each 

circulating lane based on the posted lane configurations at each entry. The study sites varied in 

terms of arrow style and placement. No clear pattern emerged to distinguish crash patterns as a 

function of pavement arrow style or placement configurations.
95 

While specific patterns of crashes could not be tied to arrow placement as part of this study, 

arrow placement may serve two purposes for influencing how drivers use the arrows for 

navigation: first, reinforcing entry alignment and second, reinforcing the presence of two 

circulating lanes. For the first purpose, arrows such as those seen at Hilliard that are placed near 

the start of the circulating lane line (i.e., near the exit side of the splitter island) may be primarily 

visible to entering drivers traveling in the same direction as the arrow, thus reinforcing the entry 

lane configuration and supporting vehicle alignment. For the second, circulatory lane-use 

pavement marking arrows placed closer to the entry side of the raised splitter island are more 

likely to reinforce the presence of two conflicting circulating lanes and the associated conflicting 

lane configurations to entering drivers. None of the study sites had arrows placed near the entry 

sides of the splitter islands during the observations. However, arrows have since been relocated 

to this position at Hilliard as part of recent modifications. Arrows placed adjacently to the center 

of the splitter island may serve both purposes for smaller diameter roundabouts such as Oshkosh. 

As described in NCHRP Report 672 and MUTCD, standard arrows are typically used in the 

circulatory roadway for legibility.(7,8)

 However, as illustrated in figure 84, fishhook arrows were 

used within the circulatory roadway at Carmel 116th. While the use of fishhook arrows may 

affect legibility, the team could not isolate their impact on actual crash patterns from other 

possible factors.

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.24′′ N and 86° 09′ 44.43′′ W. Elevation 854 ft. Eye altitude 1,723 ft.

Figure 84. Photo. Carmel 116th—Fishhook arrows within the circulatory roadway. 

The remaining sites all used standard pavement arrows; however, the placement of the pavement 

arrows varied. Two examples of sites using standard pavement arrows included Melbourne 
96 

(figure 85) and Oshkosh (figure 86). The Melbourne site has the pavement arrows placed toward 

the start of the circulating lane line (i.e., the exit side of the splitter island); the Oshkosh site has 

the pavement arrows placed near the middle of the circulating lane line (i.e., centered in front of 

the splitter island). 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 47.22′′ N and 80° 43′ 32.82′′ W. Elevation 32 ft. 

Eye altitude 1,013 ft.

Figure 85. Photo. Melbourne—Circulatory lane-use pavement arrows. 
97 

Original map: © 2015 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: April 13, 2015. 44° 02′ 21.52′′ N and 88° 32′ 33.17′′ W. Elevation 760 ft. 

Eye altitude 1,539 ft.

Figure 86. Photo. Oshkosh—Circulatory lane-use pavement arrows.

As illustrated in figure 87, Hilliard also used standard arrows; however, the arrow position was 

initially located at the start of the circulating lane line.

As part of modifications implemented after field evaluations were conducted for this TPF study, 

circulatory lane-use pavement marking arrow positions were shifted closer to the entry side of 

the splitter island at Hilliard. The purpose of this adjustment was to make the arrows more 

visible to entering drivers to reinforce the presence of two conflicting circulating lanes. A second 

through arrow was also added immediately prior to the exit in the outside lane to reinforce the 

requirement to exit. Figure 88 illustrates the recently implemented modified arrow position at 

Hilliard. 
98 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 48.28′′ N and 83° 09′ 40.58′′ W. Elevation 937 ft. 

Eye altitude 1,935 ft.

Figure 87. Photo. Hilliard—Observed circulatory lane-use pavement arrows. 

Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 40° 01′ 47.68′′ N and 83° 09′ 40.14′′ W. Elevation 937 ft. Eye altitude 1,410 ft.

Figure 88. Photo. Hilliard—Current circulatory lane-use pavement arrows.
99 

The Wales site applies standard arrows with the positioning near the start of the raised splitter 

island, as illustrated in figure 89. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 16, 2018. 43° 00′ 42.79′′ N and 88° 23′ 02.10′′ W. Elevation 988 ft. 

Eye altitude 1,656 ft.

Figure 89. Photo. Wales—Circulatory lane-use pavement arrows.

Some sites also applied supplemental “ONLY” markings for exclusive left-turn lanes or to

supplement the outside through lane. As illustrated in figure 90 and figure 91, at Malta and 

Slingerlands standard through arrows are supplemented with “ONLY” messages for outside 

circulating lanes to emphasize the requirement that vehicles in the outside lane must exit. 

“ONLY” word markings are also used in conjunction with a standard left-turn arrow for 

receiving exclusive left-turn lanes. Arrow placement varies at Malta due to a previous 

intersection retrofit. Arrows are placed near the start of the raised splitter islands on each leg. 
100 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 58′ 16.43′′ N and 73° 47′ 34.33′′ W. Elevation 339 ft. 

Eye altitude 1,045 ft.

Figure 90. Photo. Malta—Circulatory lane-use pavement arrows and ‘ONLY’ markings.
101 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 38′ 13.51′′ N and 73° 51′ 21.50′′ W. Elevation 217 ft. 

Eye altitude 1,205 ft.

Figure 91. Photo. Slingerlands—Circulatory lane-use pavement arrows and ‘ONLY’ 

markings.

103 

CHAPTER 6. NETWORK INFLUENCES

In addition to details of the design at each roundabout, as covered throughout chapter 5, the 

research team captured field observations and other information regarding the site context and 

surrounding roadway network. This chapter summarizes observations related to potential 

network influences on observed crash and conflict patterns. For each of the study sites, the team 

noted some level of network influence that directly related to observed patterns of conflicts and 

historical crashes.

The observations at these sample sites reinforce that considering the broader site context and 

roadway network in the planning and design of multi-lane roundabouts is crucial to achieving 

safety and operational performance. The team noted network influences that affected selection of 

incorrect lanes for navigating the roundabout, lane imbalances at entry, circulating lane 

imbalances, and approach platooning. 

The research team identified five general categories of network influences, as follows:

1. Interchange or other major network connection proximity: This category includes having 

a downstream interchange or other major network connection that may influence driver 

lane selection or create heavy turning movement patterns.

2. Adjacent land uses and access: This category includes the context of the area and types of 

uses, which may impact driver familiarity, as well as the intensity of uses and locations of 

access points. Trip origins may influence a driver’s starting lane position, and lane 

selection for roundabout navigation may be further influenced by the downstream 

destination. Both conditions could result in incorrect lane selection or lane changing 

within the roundabout. 

3. Close intersection spacing near the roundabout: For very close intersection spacing, 

roundabouts can provide desirable operational benefits over other intersection control 

forms. However, they may also create challenges for drivers to change lanes between 

intersections if vehicles are traveling in platoons or if queues are present that further limit 

the space available for lane changing.

4. Adjacent traffic signal influences: Upstream signalized intersections have the potential to 

create platooned arrivals that limit drivers’ abilities to interpret lane-use information and 

change to the correct lane on approach. Upstream signals can also result in fluctuating 

volume levels at the roundabout, with alternating periods of relatively intense volume and 

lower volume. During the more intense volume periods within the signal cycle, drivers 

have an increased potential for vehicle conflicts and a greater navigation task load than 

during lower volume periods within the signal cycle. 

5. Downstream lane drop: This category could include the merging of two lanes on an exit 

or one of the exit lanes dropping as right turn only or left turn only at a downstream 

intersection or driveway. These conditions can impact upstream lane utilization for 

movements with more than one lane. 
104 

Table 29 summarizes information related to the five generalized categories of network influences 

for each of the control sites. Network influences were identified at each of the study sites. The 

relative magnitude of impact on vehicle conflicts and historical crashes varies by site, with other 

contributing factors including overall volume conditions and peak-hour turning movement 

patterns.

Table 29. Summary of network influences for control sites.

Intersection

Proximity to 

Interchange or 

Major Network 

Connection 

Adjacent 

Land Use 

Influences

and Access

Close 

Intersection 

Spacing 

(<600 ft) 

Adjacent 

Signal 

Platooning 

Influences

Downstream 

Lane Drop

Carmel 106th

Interchange ramp 

gore 580 ft from 

exit; start of 

entrance ramp 

360 ft from 

roundabout exit 

Hospital 

685 ft to 

upstream 

roundabout 

No No

Oshkosh 

Interchanges 2.5 

and 4 mi away 

influence traffic 

patterns.

Business 

access points 

near entry 

and exits 

240 ft to 

upstream 

signal; 550 ft

to another 

upstream 

signal 

Yes 170 ft from 

WB exit

De Pere

Interchange 2 mi 

away influences 

traffic patterns. 

Limited

influences

2,700 ft to 

nearest 

upstream 

controlled 

intersection

Yes No

Table 30 summarizes information related to the five generalized categories of network influences 

for each of the problem sites.
105 

Table 30. Summary of the network influences for problem sites.

Intersection

Proximity to 

Interchange or 

Major Network 

Connection 

Adjacent Land 

Use Influences 

and Access

Close Intersection 

Spacing 

(<600 ft)

Adjacent 

Signal 

Platooning 

Influences

Downstream 

Lane Drop

Carmel 116th

Interchange ramp 

gore 700 ft from 

exit; start of 

entrance ramp 

500 ft from 

roundabout exit

Hospital shopping 

center

450 ft to upstream 

roundabout No No

Hilliard Interchanges 

2–4 mi away

Elementary 

school; nearby 

business access 

points

225 ft to upstream 

roundabout; 225 ft 

to signal on 

separate leg

Yes 650 ft from 

WB exit

Melbourne Interchange 

0.35 mi away

Hospital; places 

of worship

1,000 ft or greater 

to upstream signals 

on all legs

Yes No

Slingerlands

Network 

connections induce 

heavy left-turn and 

right-turn 

movements

Limited 

influences

1,370 ft or greater 

to upstream signals 

on all legs

Possible. 

Signals 0.5 to 

1 mi from 

entry on 3 

legs

No

Malta Interchange 

0.35 mi away

High U-turns due 

to corridor access 

management

700 ft to upstream 

signal Yes No

Wales Interchange 2.8 mi 

away High school 480 ft to upstream roundabout Yes 0.2 mi from EB exit

INTERCHANGE OR OTHER MAJOR NETWORK CONNECTION PROXIMITY

Four of the study sites (one control site and three problem sites) are located in close proximity to 

an interchange. Each of the remaining study sites has interchanges 2–4 mi away or other major 

network connections in the broader site vicinity that directly influence vehicle turning movement 

patterns and directional traffic flows throughout the day.

For each of the four sites in close proximity to interchanges, the study team noted increased 

instances of left turns and right turns from incorrect lanes for movements traveling to the 

interchange area. Some drivers appear to be pre-positioning for their desired downstream 

movement onto the entrance ramp by using the incorrect outside entry lane rather than using the 

designated inside entering and circulating lanes for their intended left-turn maneuver and then 

changing lanes between the roundabout and the interchange. Exits with nearby downstream 

ramps or network connections also tended to have higher instances of lane-change violations and 

conflicts as vehicles exited the circulatory roadway. 

Carmel 106th (control site) and Carmel 116th (problem site) are approximately 1 mi apart, with 

each located 580–700 ft west of an interchange ramp, as illustrated in figure 92 and figure 93, 

respectively.
106 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 10, 2018. 39° 56′ 30.38′′ N and 86° 09′ 33.81′′ W. Elevation 836 ft. 

Eye altitude 2,523 ft.

Note: SB vehicles destined for the downstream freeway entrance ramp (line with short dashes) were frequently 

observed to either make a left turn using the incorrect lane (solid line) or change lanes through the roundabout exit 

(line with long dashes).

Figure 92. Photo. Carmel 106th—Influences of adjacent interchange ramp.

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: October 22, 2018. 39° 57′ 22.30′′ N and 86° 09′ 37.02′′ W. Elevation 856 ft. 

Eye altitude 2,523 ft.

Note: SB vehicles destined for the downstream freeway entrance ramp were frequently observed to make a left turn 

using the incorrect lane (line with short dashes) or change lanes through the eastbound exit. Vehicles destined for the 

adjacent shopping center were frequently observed to make a WB left turn using the incorrect lane (line with long 

dashes) or change lanes through the southbound exit.

Figure 93. Photo. Carmel 116th—Examples of possible system influences
107 

For both sites, key crash and violation patterns included use of the incorrect lane for southbound 

left turns and northbound right turns as vehicles traveled toward the interchange: 

• For Carmel 106th, 11 percent of southbound left-turning vehicles during the afternoon 

and evening peak hours entered the roundabout in the incorrect outside lane. This pattern 

appears to be influenced by the driver’s desire to access the freeway entrance ramp, 

which begins 360 ft from the roundabout exit. Crashes associated with southbound left 

turns from incorrect lanes comprised 14 percent (6 crashes over 3 yr) of the intersection

total. Drivers using the correct inside lane on the southbound entry were also frequently 

observed changing lanes through the eastbound exit to position for the freeway entrance 

ramp. This use of the incorrect lane may also be influenced by lane imbalances on the 

southbound approach, where 61–67 percent of the entering vehicles use the inside lane.

• For Carmel 116th, southbound left turns from incorrect lanes accounted for 24 percent (or 

47 crashes over 3 yr) of the intersection total. During the noon and afternoon peak hours, 

26–40 vehicles per hour (12–14 percent of left-turning vehicles) entered the roundabout 

using the incorrect outside lane on the southbound approach. Similar to Carmel 106th, 

these data may be influenced by drivers positioning to be in the outside lane on the 

downstream eastbound exit to access the southbound freeway entrance ramp. Drivers

using the correct inside lane on the southbound entry were also frequently observed 

making lane changes as they exited eastbound. Increases in volume on the southbound 

approach, and corresponding increases in left-turn violations, may also be influenced by a 

shift change at the adjacent hospital during the 3:45–4:45 p.m. period. 

For both Carmel 106th and Carmel 116th, one of the key crash types involved northbound right 

turns from incorrect lanes. During peak hours, approximately 1–2.5 percent of northbound right 

turns used an incorrect inside lane. However, over a 3-yr period, these data translated to 5 

crashes (11 percent of the intersection total) at Carmel 106th and 11 crashes (6 percent of the 

intersection total) at Carmel 116th. Pre-positioning for the downstream interchange may be a 

factor. Entry lane imbalances may also contribute to drivers selecting the incorrect lane to bypass 

vehicle queues. At Carmel 106th, approximately 74–83 percent of the northbound entry volume 

used the outside lane during each hour analyzed. At Carmel 116th, approximately 60–70 percent 

of the northbound entering volume used the outside lane during each hour analyzed. 

At Carmel 106th and Carmel 116th, lane imbalances associated with turning movements to and 

from the interchange also resulted in volume imbalances where drivers favored the inside 

circulating lane. This imbalance may be contributing to crash types involving drivers in the 

outside entry lane failing to yield to drivers in the inside circulating lane, resulting in a crash near 

the adjacent exit. For the eastbound entry at Carmel 106th, 78–80 percent of the conflicting 

circulating vehicles during each hour are positioned in the inside circulating lane, likely due to 

origin-destination influences from the adjacent interchange and the hospital in the southeast 

corner of the intersection. Crashes associated with the outside lane on the eastbound entry failing 

to yield to the inside circulating lane comprised 14 percent (6 crashes over 3 yr) of the 

intersection total. At Carmel 116th, the patterns of left turns on the southbound and westbound 

entries also result in more frequent instances where a conflicting circulating vehicle is only 

present in the inside lane on the southbound, eastbound, and northbound entries. These patterns 
108 

correspond to the locations for all 30 crashes involving failure to yield as reported over the 3-yr 

period. 

The Melbourne roundabout and surrounding roadway network are illustrated in figure 94. At the 

Melbourne site, 685 vehicles (representing 33.5–39.5 percent of the southbound left-turn volume 

during each hour analyzed) were observed over a 4-h period to use the incorrect outside lane to 

make the southbound left turn when traveling toward the interchange. These left turns using 

incorrect lanes likely resulted in 16 crashes over 3 yr near the southbound exit. Due to a heavy 

southbound left-turn pattern traveling toward the freeway during the noon, afternoon, and 

evening peak hours, the inside lane on the southbound entry operates as a de facto left turn only 

lane. Fewer than six through vehicles per hour were observed to use the inside lane for the 

southbound through movement. This lane use pattern likely helped to limit higher instances of 

conflicts and crashes near the southbound exit than might be expected with a higher through 

volume. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 51.56′′ N and 80° 43′ 23.71′′ W. Elevation 30 ft. Eye altitude 3,396 ft.

Note: Adjacent traffic signal locations are identified with a dashed circle.

Figure 94. Photo. Melbourne—Roadway network and adjacent signalized intersections.

The team observed several compounding network influences at Melbourne that may be 

contributing to the high instances of southbound left turns from incorrect lanes. The predominant 

southbound left-turn volume creates lane imbalances favoring the inside lane at the southbound 

entry, which may result in some drivers selecting the incorrect outside lane for the southbound 

left turn to avoid queues and reduce delay. An upstream signal also creates platoons of arriving

vehicles that may make it more difficult for drivers to identify the correct lane for the left-turn 

maneuver and then change lanes if needed. Drivers destined for I–95 southbound may also be 
109 

choosing to use the incorrect outside lane to minimize the number of lane changes required to 

access the freeway entrance ramp. The combination of queuing from the signalized intersection 

between the roundabout and interchange and weaving from the northbound roundabout free-flow 

bypass lane makes it more difficult to change lanes during peak periods of the afternoon. 

At Melbourne, the westbound approach has three primary lanes, with the outside lane being 

dropped at the roundabout as a right turn only, free-flow bypass lane. An upstream signal is 

located approximately 1,050 ft from the roundabout entry. An interchange exit ramp is located 

650 ft further upstream of the signal. Westbound vehicles arriving from the interchange in the 

outside lane must change lanes within platoons created by the signal to continue straight or turn 

left at the roundabout. Vehicles desiring to make a left turn would need to make two lane 

changes to get to the inside entry lane. The density of vehicle platoons from the signal makes 

lane changing more difficult and may affect lane selection.

Figure 95 illustrates the roadway network and land uses within the vicinity of the Malta site, 

which is located 0.35 mi from an interchange with I–87. The Malta site is also a major network 

connection with US 9, which parallels I–87. This network configuration results in regional travel 

patterns to and from I–87 that influence volume levels for various intersection turning

movements throughout the day and may influence potential violations and conflicts. Entry lane 

imbalances due to increases in turning volumes traveling to and from the interchange may 

contribute to increased instances of left, right, and through movements from incorrect lanes 

during individual hours on each leg. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 58′ 15.68′′ N and 73° 47′ 58.80′′ W. Elevation 338 ft. Eye altitude 4,333 ft. 

Note: Study intersection is identified with a dashed circle.

Figure 95. Photo. Malta—Roadway network and adjacent land uses. 
110 

ADJACENT LAND USES AND ACCESS

Surrounding land uses and area contexts may impact driver familiarity. As previously noted, four 

of the study sites are located immediately adjacent to freeway interchanges. Each of these sites 

also has hospitals, retail shopping center uses, or recreational areas nearby that create the 

potential for increased numbers of unfamiliar drivers. However, driver familiarity alone is not 

sufficient for predicting the potential for vehicle crashes. For the study site in Hilliard, which is 

located in an urbanized city-center context, the city previously reviewed at-fault driver addresses 

and determined that crashes were predominantly caused by drivers that are local to the city of 

Hilliard and surrounding counties and would thus be expected to be familiar with roundabout 

navigation.(11)

Based on site observations, the intensity of uses and locations of property access points appear to 

influence driver navigation and possible frequency of various violations or conflicts. Trip origins 

and destinations also appear to influence incorrect lane selection, with some drivers prepositioning for the downstream driveway destination instead of using the designated lanes for 

their maneuver. Some drivers were also observed to change lanes within the roundabout or 

through the exit to access driveways located immediately beyond the exit. 

At the Carmel 116th site, the team observed a strong pattern of drivers making the westbound 

left turn through the roundabout destined to the shopping center in the southwest quadrant of the 

study intersection. As previously illustrated by the blue line in figure 42, the team observed 

drivers incorrectly using the outside circulating lane when making the left turn to position for the 

right turn into the shopping center immediately after exiting the roundabout to the south. 

Vehicles using the correct inside circulating lane to make the westbound-to-southbound left turn 

frequently crossed over to the outside lane through the exit to position for turning into the 

shopping center driveway. Westbound vehicles turning left from the incorrect lane represented 

25 percent (48 crashes over 3 yr) of the intersection total.

At the Hilliard site, an elementary school is adjacent to the northwest quadrant of the 

roundabout. The school influences traffic patterns, particularly during the morning peak hour 

(heavier northbound left-turn and westbound through volumes) and during the early afternoon 

period during school dismissal. During the morning peak, heavy northbound-to-westbound leftturn flows create queuing in the inside lane on the northbound approach. To bypass the queue, 

multiple northbound left-turning drivers were observed to intentionally use the incorrect outside 

lane to make a right turn into the school after exiting the roundabout to the west. Crashes 

associated with northbound left turns from incorrect lanes reflected approximately 4 percent (10 

crashes over 3 yr) of the intersection total. This crash pattern is illustrated in figure 96. 
111 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 48.28′′ N and 83° 09′ 40.58′′ W. Elevation 937 ft. Eye altitude 1,935 ft.

Figure 96. Photo. Hilliard—Left turn from the incorrect lane for trips to school. 

At the Melbourne site, the hospital adjacent to the southwest corner, annotated in figure 94, has 

access points 1,000 ft to the west of the roundabout and approximately 400 ft to the south. Some 

drivers traveling to the hospital were noted to make an improper westbound-to-southbound left 

turn from the outside lane to be able to make a right turn into the hospital immediately after 

exiting the roundabout southbound. Crashes associated with westbound left turns from incorrect 

lanes accounted for 23 percent (42 crashes over 3 yr) of the intersection total. Approximately 10–

18 percent of the hourly westbound vehicles turning left were observed to use the incorrect lane. 

Drivers using the correct inside circulating lane for the westbound left turn were observed to 

change lanes through the exit to cross to the outside exit lane. For vehicles accessing the hospital 

using Wickham Road to the west, some westbound through drivers were observed to enter the 

roundabout in the outside lane and then change to the inside lane through the roundabout exit to 

position themselves for the downstream left turn into the hospital.

Also at the Melbourne site, two places of worship are adjacent to the northwest corner of the 

roundabout. Vehicles making a right turn using the westbound-to-northbound right-turn bypass 

lane are required to make a rapid lane change across three lanes to make a northbound left turn 

into these properties. Instead, drivers appear to be using the incorrect inside lane on the 

westbound entry to make a right turn based on their downstream destination, as illustrated in 

figure 97. This trend may have contributed to up to 15 westbound right-turn crashes that were 

reported from the incorrect inside lane. Note that activity for these places of worship is likely to 
112 

be more heavily weighted toward weekends and was not captured as part of the drone data 

collection. 

Original map: © 2020 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: February 2, 2020. 28° 13′ 50.05′′ N and 80° 43′ 31.21′′ W. Elevation 30 ft. 

Eye altitude 1,789 ft.

Figure 97. Photo. Melbourne—Improper right turns to access places of worship.

Driveways in close proximity to an entry were also observed to create the potential for drivers to 

use the incorrect lane for making a left-turn maneuver. One example is at the Carmel 116th site, 

where a right-in/right-out driveway for the adjacent shopping center is located approximately
113 

200 ft west of the roundabout entry. As illustrated in figure 98, some drivers leaving the 

shopping center were observed to have difficulty during peak periods in getting into the left lane 

on 116th Street eastbound prior to the roundabout entry, which could be contributing to vehicles 

making left turns from the incorrect lanes. A total of 19 crashes over 3 yr (10 percent of the 

intersection total) were related to improper eastbound left turns from incorrect lanes. Between 

2.2 percent and 5.8 percent of all eastbound left-turning vehicles were observed to use the 

incorrect lane.

 Original map: © 2022 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: 39° 57′ 22.14′′ N and 86° 09′ 47.30′′ W. Elevation 854 ft. Eye altitude 2,049 ft.

Figure 98. Photo. Carmel 116th—Left turn from incorrect lane after departing adjacent 

driveway. 

At the Oshkosh site, a retail store is located north of the roundabout with a right-in/right-out 

driveway approximately 95 ft upstream of the roundabout entry. Drivers have a short distance to 

move into the inside lane if they desire to make a left-turn or U-turn at the roundabout. During 

congested periods with queues present, drivers may be more likely to have difficulty getting to 

the correct inside lane prior to entry. One near-crash was observed due to a driver making a Uturn from the incorrect lane after turning right from this driveway, as illustrated in figure 99. 
114 

Source: FHWA.

Figure 99. Photo. Oshkosh—U-turn from incorrect lane after departing an adjacent 

driveway. 

At the Malta site, access management between the roundabouts along the SR 67 corridor resulted 

in a relatively high number of eastbound-to-westbound U-turn movements. Some drivers were 

observed to follow the central island curb instead of the spiral markings, resulting in the 

U-turning vehicles changing lanes to continue westbound using the single-lane westbound exit, 

as illustrated in figure 100. For driveways near the entries, such as on the eastbound and 

westbound approaches, the short distance also created challenges for some vehicles to get into 

the correct inside lane for a left-turn movement during congested periods. 

 Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: June 25, 2018. 42° 58′ 16.43′′ N and 73° 47′ 34.33′′ W. Elevation 339 ft. Eye altitude 1,045 ft.

Figure 100. Photo. Malta—Observed U-turn-related lane changing.
115 

CLOSE INTERSECTION SPACING

For very close intersection spacing, roundabouts can provide desirable operational benefits due 

to shorter queues and more efficient use of lanes than comparable signalized intersections. 

However, they may also create challenges for drivers who need to change lanes between 

intersections, especially if vehicles are traveling in platoons or if queues between roundabouts 

are present that further limit the space available for lane changing. In these cases, drivers would 

benefit from having appropriate information (such as destinations assigned to each lane) to 

minimize the need for lane changing between closely spaced intersections. 

In Hilliard, the study site is located approximately 225 ft from an adjacent multi-lane roundabout 

to the south, as illustrated in figure 101. During brief periods during peak hours of the day, 

queuing on the northbound approach would make it more difficult for drivers to change lanes 

between roundabouts, which may contribute to use of the incorrect lane for left-turn or right-turn 

movements. Approximately 3 percent of vehicles used the incorrect lane to make a northbound 

left turn during the congested morning and evening peak hours, compared to approximately 1 

percent of vehicles during the uncongested midday hours. Figure 102 illustrates queuing on the 

northbound approach during the morning peak hour, with heavy northbound left-turn movements 

associated with school arrivals. A traffic signal is also located nearby, approximately 225 ft to 

the west along Cemetery Road, which similarly reduces the space for lane changing between 

intersections. 

Original map: © 2018 Google® EarthTM. Annotations by FHWA (see Acknowledgements section).

Data: Imagery date: March 17, 2018. 40° 01′ 47.07′′ N and 83° 09′ 40.75′′ W. Elevation 940 ft. Eye altitude 2,649 ft.

Figure 101. Photo. Hilliard roadway network and surrounding development patterns.
116 

Source: FHWA.

Figure 102. Photo. Hilliard—Northbound vehicle queues during peak a.m. hours.

Similar spacing considerations to adjacent roundabouts are present at the Carmel 106th site, 

illustrated in figure 92, as well as at the Carmel 116th site, illustrated in figure 98. For both sites, 

the upstream roundabout influenced lane choice, and periodic peak-hour queues between 

intersections may have made it more difficult for drivers to identify and get into the correct lane 

for their intended turning movement. The Wales site also has a roundabout located 

approximately 480 ft to the west; however, the spacing did not appear to substantially affect

drivers’ lane selections, based on the observed volume levels and queuing. 

Several sites also have traffic signals nearby; both Oshkosh and Hilliard have traffic signals 

within 240 ft. Other sites had signals farther away: Malta has a signal 700 ft from the 

roundabout, and Melbourne has a signal 1,000 ft away. Signal operational impacts are discussed 

in the next section. However, close spacing between intersections may further compound the 

ability of drivers to get into the correct lane. Queue spillback from the adjacent signal into the 

study roundabout was only observed at the Melbourne site. 

ADJACENT TRAFFIC SIGNAL INFLUENCES

Upstream signalized intersections have the potential to create platooned arrivals that can affect

drivers’ abilities to interpret lane-use information and, if needed, change to the correct lane prior 

to entry. Due to their cyclical operation, upstream signals can create periods of relatively intense 

flow followed by lower flow periods. During the more intense flow periods within signal cycles, 

drivers at the roundabout may experience an increased potential for vehicle conflicts and a 

greater navigational task load than what might be expected during lower flow periods within the 

cycle. These cyclic variations may not be detected by considering only average flow conditions 

over the peak hour or day. As summarized in table 29 and table 30, multiple roundabout sites had 

upstream signals at varying distances from the roundabout. 
117 

For the Melbourne site, the adjacent signals illustrated in figure 94 to the east and north had the 

most significant observed impact on operations among the study sites. As illustrated in 

figure 103, the signal to the east would consolidate vehicles arriving from the I–95 interchange 

and then release heavy platoons toward the roundabout. The short intersection spacing and 

platoon density may be affecting drivers in their ability (or willingness) to identify the correct 

lane and make the corresponding lane change before reaching the roundabout. The arriving 

westbound platoons also temporarily cause delay and queuing on the adjacent southbound entry. 

Given the volume imbalances on the southbound entry favoring left turns, queuing tended to be 

higher in the inside lane, which could be causing drivers to use the incorrect outside lane for 

southbound left turns to bypass the standing entry queues. Platooned arrivals on the southbound 

approach from the upstream signal may also be impacting drivers’ abilities to change lanes prior 

to entry. 

Source: FHWA.

Figure 103. Photo. Melbourne—Platooned westbound arrivals (top) resulting in 

southbound queueing (left).

During the evening peak hours, queues from the adjacent signal to the east were observed to 

temporarily extend into the roundabout on multiple occasions, as illustrated in figure 104. Also, 

note in figure 104 the lack of vehicles on the westbound entry (top) and southbound entry (left) 

due to the previous platoons already being processed through the roundabout and the next 

platoons not yet arriving from the adjacent signals.
118 

Source: FHWA.

Figure 104. Photo. Melbourne—Queues from the adjacent signal extending into the 

roundabout.

For the site in De Pere, a traffic signal 2 mi to the north was observed to influence the 

roundabout operations with vehicles arriving in platoons, as illustrated in figure 105. The crash 

pattern illustrated in figure 105 reflects 48 percent (or 11 crashes) out of the intersection’s 3-yr 

total. This trend appears to be influenced by the platooned arrival patterns where there was more 

likely to be another vehicle in the adjacent lane if a driver did not use the correct lane to make 

their left-turn movement. The platooned arrivals may also have affected drivers’ ability to 

identify the correct lane for their movement and their ability to change lanes prior to entry, 

despite the De Pere site having the lowest volumes of the study intersections. 

Source: FHWA.

Figure 105. Photo. De Pere—Platooned vehicle arrivals at southbound entry and crash 

patterns caused by left turn from incorrect lane. 
119 

Similar platooned arrival patterns from adjacent upstream signals were observed at Oshkosh, 

Hilliard, Malta, Slingerlands, and Wales. Potential impacts on crash patterns and observed 

violations and conflicts vary by intersection, depending on the turning-movement patterns 

associated with each entry.

DOWNSTREAM LANE DROPS

Downstream lane drops reflect one of the exit lanes dropping as right turn only or left turn only 

at a downstream intersection or driveway. These lane drops can impact lane utilization at the 

upstream roundabout entries. (Merging of the two exit lanes is covered separately in Chapter 5.) 

On the two-lane westbound exit at the Oshkosh site, the inside exit lane becomes a left turn only 

lane approximately 170 ft from the roundabout exit. This transition may be influencing driver 

lane selection and lane imbalances noted on the upstream westbound entry. Between 63.8 and 

68.9 percent of drivers during each hour analyzed used the outside lane on the westbound 

approach, possibly to avoid the lane drop on the westbound exit. 

As illustrated in figure 101, at the Hilliard site the outside lane on the westbound exit is dropped 

as right turn only into the school downstream of the roundabout. This dropped lane may be 

affecting the lane distribution on the westbound entry, where 56–78 percent of vehicles were 

observed in the inside entry lane during each period.

121 

CHAPTER 7. SUMMARY OF FINDINGS

Based on field evaluations and subsequent analyses documented in this report, the research team 

identified several key findings and areas for further investigation. This chapter summarizes those 

key findings and offers menus of potential countermeasures for consideration based on the 

specific issues and conditions at individual sites.

SUMMARY OF KEY FINDINGS

The research team identified the following eight key findings related to crash and conflict 

patterns occurring near multi-lane roundabout exits: 

1. The following key patterns of interest comprised most crashes and conflicts at the study 

intersections:

• Drivers in the outside entering lane failing to yield to drivers in the inside circulating 

lane. 

• Drivers making left-turn movements from the incorrect outside lane. 

• Drivers making right-turn movements from the incorrect inside lane. 

• Drivers straddling lanes or changing lanes when entering, circulating, or exiting. 

2. Key crash and conflict patterns varied across both the control and problem sites based on 

volume conditions (e.g., turning-movement patterns), roundabout geometry, and network 

considerations. The magnitudes and types of crashes and conflicts also varied based on 

the approaches at each individual site. 

3. Control sites and problem sites generated comparable average hourly violations; 

however, problem sites generated nearly twice the number of conflicts and potential 

conflicts as control sites. Specifically, the following was observed regarding these data: 

• The magnitude and types of observed violations and conflicts varied across sites 

based on volume conditions, geometric factors, and network factors. In particular, the 

presence or absence of exclusive left-turn lanes was a key factor influencing the 

number of violations. 

• The primary observed violation types across all study sites included lane straddling, 

lane changing through exits, and left turns from incorrect lanes; however, these 

violation types did not necessarily result in observed conflicts. By comparison, the 

primary violation types resulting in observed conflicts included failure to yield at 

entry, exit lane changes, and left turns from incorrect lanes. These violation types 

accounted for 91 percent of observed conflicts at problem sites and 87 percent at 

control sites. Failure to yield at entry resulted in the highest average observed 

conflicts, with approximately 3 times as many conflicts per hour at problem sites as at 

control sites.
122 

4. Volume magnitude, uniformity, and imbalances across lanes were potential contributors 

to number of failure-to-yield crashes observed. Specifically, the following was observed 

regarding these data:

• The average sums of entering and conflicting volumes at each of the entries for the 

control sites, which ranged from 900 to 1,200 vehicles per hour, were more uniform 

and consistent across entries than at problem sites, which ranged from 900 to 2,100 

vehicles per hour. 

• The 8 approaches with the most failure-to-yield crashes (more than 15 crashes over 

3 yr per approach) each had 2 conflicting circulating lanes that were both allowed to 

exit. They also generally had a large imbalance in lane use, with either a high 

percentage of entering vehicles using the right/outside lane or a high percentage of 

circulating vehicles using the left/inside lane. Network influences and turningmovement patterns played a key role in lane imbalances; however, geometric factors 

also influenced some locations, as follows: 

o Heavy right-turn volumes made the outside circulating lane at the downstream 

entry more frequently unoccupied. 

o Heavy left-turn volumes caused lane imbalances that resulted in the outside 

circulating lane being frequently unoccupied at a downstream entry. 

o Approach lane addition and lane drop design also potentially influenced volume 

imbalances across lanes. 

o Sites with large conflict areas due to large ICDs, skewed angles between legs, or 

other factors experienced more failure-to-yield crashes. However, crashes of this 

type were also identified at sites with more compact geometry, such as Hilliard. 

• Experiences at problem sites varied widely. For individual approaches, failure-toyield crashes ranged from 1 crash over 3 yr to 35 crashes over 3 yr. Volume alone 

does not explain the variation in observed crashes at the problem sites. No apparent 

strong relationship was identified between crash experiences and entering volume, 

circulating volume, or sum of entering and circulating volumes within individual 

conflict areas.

• The team identified no apparent relationships between entry signing and markings 

and failure-to-yield crashes. Varied signing and marking treatments were present at 

both the control and problem sites, including variations on the use of yield word 

markings or yield lines and a range of plaques to supplement the yield signing. 

Similar plaques were present at approaches with lower and higher numbers of 

crashes.

5. It is imperative to get drivers into the correct lane prior to the yield line to address crashes 

associated with left turns from incorrect lanes. However, the ability to for drivers to 
123 

achieve this is largely dependent on the location of the roundabout relative to nearby 

origins and destinations. Specifically, the following was observed regarding these data: 

• The volume of left-turning vehicles was not strongly correlated to the potential for 

crashes associated with left turns from incorrect lanes.

• Network influences for multiple sites, such as adjacent interchange ramps or business 

access points, appeared to directly influence drivers’ desire to select the incorrect 

outside lane to position themselves correctly for their intended downstream turning 

movement. 

• Modifications to circulatory roadway lane lines or arrow pavement markings alone 

are not anticipated to fully address crashes associated with left turns from incorrect 

lanes. Implementation of raised lane dividers (consistent with turbo roundabout 

design practices) or spirals within the circulatory roadway to guide drivers to the 

outside lane on exit may help to reduce incorrect lane use. However, in retrofit 

situations, existing geometry may still allow drivers to navigate through gaps in 

raised lane dividers to continue to make left turns from the outside lane. Similarly, the 

presence of lane dividers may shift the lane changing into a more compressed area in 

advance of or beyond the roundabout. The most substantial benefits appear to be 

gained from an emphasis on the following: 

o Providing appropriate approach lane configurations to promote balanced lane use. 

o Providing signing and markings to get drivers into the correct lane prior to entry. 

o Designing geometric elements to allow drivers to change lanes on approach and 

support balanced lane use. 

6. Design of spirals within the circulatory roadway directly influences driver navigation and 

lane changing within the roundabout. Painted spirals are strongly associated with drivers 

straddling lanes.

7. Exclusive left-turn lanes are strongly associated with incorrect lane selection and lane 

changing within the roundabout. 

8. Multiple geometric factors influence lane straddling and lane changing. This finding 

includes entry geometry/alignment, exit geometry/alignment, size of conflict areas, lane 

widths, and diameters at the edges of the typical ranges (very small or very large). 

However, network considerations also play a role in lane changing through exits.

ANALYSIS OF KEY FINDINGS

From the key findings identified in the previous section, the research team has identified a few 

key themes that appear to hold true across the studied sites, as follows:

• The study findings reinforce longstanding guidance to keep the roundabout configuration 

to the fewest lanes practical. Where 2x2 configurations are expected to be needed to 
124 

serve forecasted future growth, phased project implementation that minimizes the number 

of initial lanes, such as use of a single-lane or 2x1 configuration, should be considered. 

• Turning movement patterns and roundabout locations, relative to nearby origins and 

destinations, substantially affect PDO crashes. Drivers who have limited ability to change 

lanes prior to entering the roundabout or after exiting the roundabout due to their origin 

or destination may choose to navigate it using incorrect lanes because they perceive no 

other safe option. The research team believes that geometric changes or changes in traffic 

control device layout within the roundabout alone would be insufficient to correct these 

network-related challenges. Lane reductions (e.g., changing to single-lane movements) or 

changes to access placement away from the roundabout will be needed to reduce or 

eliminate these patterns.

• Heavy right-turn movements in shared through-right lanes appear particularly linked to 

safety-related challenges at downstream 2x2 conflict areas. If the outer lane at the entry 

upstream of the subject 2x2 conflict area is designated as shared through-right but has a 

heavy right-turning flow, the outside circulating lane can be starved relative to the inside 

circulating lane. In these cases, drivers in the right entry lane at the subject 2x2 conflict 

area may not perceive a conflict and thus may enter without yielding. It may be better to 

use right turn only lanes or channelized right-turn bypass lanes and tolerate increased 

congestion during times when two through lanes may be desired. If right turn only lanes 

are used, the design needs to be carefully arranged to discourage illegal use of the right 

turn only lane as a through lane. 

• Painted spirals within the circulatory roadway are especially prone to lane straddling and 

lane changes in the exit-circulating areas. Drivers routinely drive over painted spirals, 

following instead the edge of the truck apron. Physical extension of the truck apron to 

cover the intended area of the spiral is likely to mitigate this problem. Existing sites may 

benefit from retrofitted truck apron extensions if the cross slopes of the circulatory 

roadway and truck apron are compatible. It may not be feasible to retrofit existing sites 

with crowned circulatory roadways due to conflicts with drainage along the inside edge 

of the circulatory roadway and grade breaks within the truck apron. 

• The overall composition of the roundabout plays a greater role in safety performance than 

individual geometric, signing, or marking elements. Adjustments to individual design 

elements may support incremental reductions in specific types of violations and conflicts. 

However, the team anticipates a combination of improvements will be needed at most 

sites to address the ranges of crash types present.

MENUS OF COUNTERMEASURES

The research team observed a wide range of geometric, volume, and network conditions across 

the study sites. Each unique set of circumstances is expected to require a customized set of 

countermeasures to address the observed crash patterns. While education, enforcement, and 

potential benefits associated with connected and automated vehicles may also be beneficial, 

these areas are outside the scope of this study.
125 

Potential countermeasures are organized into six categories. For each potential countermeasure, 

the potential benefits and effectiveness are assessed based on the findings of this project and the 

research team’s judgment. The categories are as follows:

1. Lane configuration. 

2. Network modifications. 

3. Approach signing and markings. 

4. Entry signing and markings. 

5. Circulatory roadway and exit countermeasures.

6. Geometric modifications. 

Lane Configurations 

The goals of improvements to lane configurations include the following: 

• Reduced vehicle-vehicle conflicts of all types.

• Reduced vehicular speeds. 

• Improved safety and accessibility environments for pedestrians and bicyclists. 

Possible treatments are listed in table 31, along with their potential benefits. 
126 

Table 31. Menu of potential lane configuration changes. 

Treatment Potential Benefits

Potential for 

Effectiveness

Reduce entering and 

circulating lanes 

where feasible.

The overall simplification of the roundabout by reducing 

through lanes, ideally to a 2x1 configuration (or a single-lane 

roundabout for sites that have been over-designed), has the 

potential to address many crash patterns identified in this study 

and provide other safety improvements as well. Lane reduction 

may result in peak-period congestion for motor vehicles but 

may have better safety performance for all modes over the 

course of the day and year. A lifecycle cost analysis may help 

to understand the tradeoff between safety and capacity.

High.

Convert shared 

through-right entry 

functioning as a de 

facto right turn only 

lane to a permanent 

right turn only lane.

This strategy provides some additional capacity for right-turn 

movements while allowing for removal of one lane within a 

portion of the circulatory roadway. Reduction in the number of 

circulating lanes addresses many of the observed crash patterns. 

However, dropping one of the primary lanes on a four-lane 

roadway could result in increased lane changing for through 

vehicles upstream of the roundabout.

Medium.

Add a 

yield-controlled 

right-turn bypass 

lane for approaches 

with heavy 

right-turn 

movements that are 

creating lane 

imbalances for 

through movements.

This strategy is targeted at providing more balanced 

distribution of circulating vehicles to reduce the potential for 

failure-to-yield crashes at the downstream entry.

Adding a bypass lane may affect pedestrian navigation, add 

vehicle conflict points, and increase intersection footprints. 

These and other potential tradeoffs need to be considered based 

on specific site conditions.

Medium.

Effectiveness 

may be offset 

by the 

addition of 

new conflicts 

associated 

with a bypass 

lane.

Convert shared left 

through entry lane 

functioning as a de 

facto left turn only 

lane to a permanent 

left turn only lane.

This strategy provides some additional capacity for left-turn 

movements (with potential reduction to through-movement 

capacity) while allowing for the downstream exit to be reduced 

from two lanes to one lane. The expected benefit is a potential 

reduction in crashes associated with left turns from improper 

lanes and failure-to-yield crashes for vehicles entering in the 

outside lane adjacent to the exit being modified.

This treatment requires additional modifications to the 

circulatory roadway to guide left-turning vehicles from the 

inside lane to the outside lane for the new single-lane exit. 

Spiral pavement markings alone were found to be ineffective. 

Modification to the central island and truck apron may be 

needed to achieve the desired spiral vehicle paths.

Medium.

Effectiveness 

may be offset 

by the 

addition of 

new conflicts 

associated 

with spiral 

transitions 

within the 

circulatory 

roadway.
127 

Network Modifications

Improvements that address network influences aim to improve driver ability to change lanes 

between intersections. These improvements are anticipated to reduce the likelihood of turns from 

incorrect lanes.

Possible treatments are listed in table 32, along with their potential benefits. Additional signing 

and marking treatments that address network considerations are provided in the following 

section.

Table 32. Menu of potential network modifications.

Treatment Potential Benefits

Potential for 

Effectiveness

Shift access point 

further away from 

roundabout. 

This treatment may provide more distance for lane 

changing, which may reduce the likelihood of turns from 

incorrect lanes or lane changing through the roundabout 

entry and exit.

Medium.

Increase spacing 

between upstream 

or downstream 

signalized 

intersection and 

roundabout.

This treatment may provide more distance for lane 

changing, which may reduce the likelihood of turns from 

incorrect lanes at the roundabout.

Medium.

Replace upstream 

signalized 

intersection with 

roundabout. 

This treatment may reduce the intensity of platoons of 

vehicles passing from the upstream signalized 

intersection to the roundabout, making lane changes in 

advance of the roundabout more feasible and reduce the 

likelihood of turns from incorrect lanes..

Medium.

For closely 

spaced 

intersections, 

modify the lane 

configuration of 

the intersection 

upstream of the 

roundabout.

This treatment reduces lane changing and increases the 

likelihood for a vehicle to use the correct lane based on 

predominant origin and destination patterns. 

Medium.

Modify timing of 

upstream signals, 

such as by 

reducing cycle 

lengths or by 

providing 

coordination.

This treatment has the potential to influence arrival 

patterns of vehicles by reducing the intensity of vehicles 

arriving at the same time at the roundabout.

Medium.
128 

Approach Signing and Markings

Goals of making improvements to approach signing and markings include the following: 

• Appropriate lane selection prior to entry.

• Lane selection guidance for major destinations. 

Possible treatments are listed in table 33, along with their potential benefits. 

Table 33. Menu of potential signing and marking treatments.

Treatment Potential Benefits 

Potential for 

Effectiveness

Overhead 

lane-control 

signs 

This treatment is likely to significantly improve visibility of

lane-control signs and separate them from roadside visual 

clutter. Overhead signs may not be as effective on east-west 

alignments during times of the day when the sun is within 

the driver’s cone of vision. In addition, overhead signs must 

comply with illumination guidelines for effectiveness at 

night. 

High. 

Larger 

side-mounted 

lane-control 

signs 

This treatment may improve the visibility of lane-control 

signs, especially in roadway environments with visual clutter 

along the roadway.

Medium. 

Lane-control 

signs on both 

sides of 

approach 

This treatment may improve the visibility of lane-control 

signs in the left approaching lane.

Medium. 

Multiple sets 

of lane-control 

signs and 

arrow 

markings

This treatment may improve the visibility of lane-control 

signs and markings by providing multiple opportunities for 

drivers to see them as they approach the roundabout. 

Medium. 

Overhead 

destination 

guidance 

This treatment may improve driver selection of the correct 

lane prior to entry by linking lane-use selection to intended 

destination.

Medium. 

Standard arrow 

signs and 

markings (with 

optional dot in 

inside lane)

Standard arrows for both signs and markings may improve 

driver comprehension more than fishhook-style arrows. An

optional dot in the inside lane may reduce the already-low 

likelihood of a driver turning left in front of the central 

island. 

Medium. 

Add route 

shield 

markings

This treatment may improve driver selection of the correct 

lane prior to entry by linking lane-use selection to intended 

destination.

Medium. 
129 

Entry Improvement Countermeasures

Improvements to roundabout entries aim to do the following: 

• Enhance driver yielding to both circulating lanes at entry. 

• Enhance vehicle alignment between entering and circulating lanes. 

Possible treatments are listed in table 34, along with their potential benefits. 

Table 34. Menu of potential entry treatments.

Treatment Potential Benefits 

Potential for 

Effectiveness

Raise crosswalks 

to enhance 

yielding at entry 

and provide 

additional speed 

control. 

Lower speeds may increase the likelihood of drivers 

yielding at the entry. The raised crosswalk may also have 

a significant benefit to pedestrians in terms of improving 

the likelihood of drivers yielding on both the entry and 

exit sides. The raised crosswalk can be part of a package 

of improvements to make the crossings accessible to 

people who are blind or have low vision. 

High. 

Replace yield line 

with yield word 

markings. 

The use of words instead of symbols may have better 

driver recognition.

Medium. 

Implement yield 

signs on both 

sides of entry. 

This treatment is already recommended in MUTCD.

(6) Medium. 

Adjust angle or 

position of 

entrance line. 

Due to the staggered nature of two-lane entries, driver 

view angles in the outside lane are generally smaller than 

those in the inside lane. Where driver view angles at the 

entrance line are too severe, adjusting the angle or 

position of the entrance line may support improved view 

angles and corresponding improvements in driver 

yielding. 

Medium to 

low. 

Add supplemental 

signing under 

yield sign (e.g., 

“TO BOTH 

LANES”). 

These signs may have a small but unquantifiable benefit 

in improving driver yielding for unfamiliar drivers. The 

signs may lose effectiveness over time with familiar 

drivers. 

Low. 

Add supplemental 

signing in central 

island (e.g.,

custom, subject to 

approval for 

experimentation). 

These signs may have a small but unquantifiable benefit 

in improving driver yielding for unfamiliar drivers. The 

signs may lose effectiveness over time with familiar 

drivers. 

Low. 
130 

Circulatory Roadway and Exit Countermeasures 

Improvements within circulatory roadways and exits aim to do the following:

• Encourage lane discipline for posted lane configurations. 

• Reduce lane encroachment and lane changes. 

Possible treatments that seem to have medium potential for effectiveness are listed in table 35, 

along with their potential benefits.

Table 35. Menu of circulatory roadway and exit treatments with medium effectiveness 

potential.

Treatment Potential Benefits

Potential for 

Effectiveness

Add supplemental 

treatments such as 

lane line rumble 

strips, raised 

pavement markers, 

or raised lane 

dividers to 

promote lane 

discipline (e.g., 

turbo-style 

treatments). 

Rumble strips and raised pavement markers provide 

a deterrent for drivers who may cross lane lines 

within a circulatory roadway. Mountable raised 

lane dividers provide increased physical 

channelization. Because a gap in these treatments is 

required adjacent to each entry to allow entering 

drivers to cross the lane line, the size of a conflict 

area and overall geometry of a roundabout continue 

to play critical roles in potential for effectiveness. 

For retrofit situations, these treatments may provide 

limited benefit at some sites in discouraging left 

turns from incorrect lanes or lane changing through 

the entry or exit due to the size of the conflict areas.

Medium. 

Effectiveness 

expected to be 

higher for new 

construction where 

the supplemental 

treatments are 

integral to the 

overall design. 

Maintenance is a 

factor.

Adjust circulating 

lane line position 

(narrower inside 

lane) to enhance 

entry alignment 

and allow tangent 

to be added 

between 

circulating and 

exit curves.

Adjustment to the position of the lane line is 

intended to reduce potential for lane changing 

through entry and exit. A wider outside circulating 

lane may help to encourage drivers in the outside 

lane to stay in the outside circulating lane where the 

roundabout geometry results in slight misalignment 

of entering and circulating lanes. Narrowing the 

inside circulating lane also allows for a tangent to 

be added along the lane line between the circulatory 

roadway and exit curves. State-level guidance 

encourages the use of a tangent at least 40-ft long 

along the exit lane line.(10)

Medium. 

Possible treatments that seem to have low potential for effectiveness are listed in table 36, along 

with their potential benefits.
131 

Table 36. Menu of circulatory roadway and exit treatments with low effectiveness potential.

Treatment Potential Benefits

Potential for 

Effectiveness

Adjust shape of 

spiral markings. 

The shape of spiral pavement markings has the 

potential to influence the size of conflict areas and the 

corresponding alignment for entering vehicles. 

However, painted spirals are prone to lane straddling or 

lane changing where drivers were frequently observed 

to follow the central island curb instead of the spiral 

lane markings. Adjustment to the shape of the spiral 

may provide a marginal benefit for guiding left-turning 

drivers. 

Low.

Adjust lane line 

type (combination 

of solid and dotted 

lines versus

consistent “strong 

dash,” double 

lines). 

These markings may have a small but unquantifiable 

benefit in minimizing late lane changes. These 

markings may lose effectiveness over time with 

familiar drivers. No relationship was identified 

between the lane line type and observed crash patterns 

across the study sites. 

Low.

Adjust lane arrow 

positioning or 

additional set of 

lane arrows for 

larger ICDs. 

Arrows positioned closer to the exit may provide 

benefits by reinforcing the number of conflicting lanes 

and the movements associated with those lanes (i.e., 

that both circulating lanes might exit straight). 

Additionally, they may encourage entering drivers to 

yield to both lanes and reinforce guidance to drivers 

that the outside lane must exit. For larger ICDs, 

multiple sets of lane arrows may be considered to 

support driver navigation. 

Low.

Add through arrow 

to outside exit 

lane. 

The intent of this treatment is to reinforce the 

requirement for drivers in the outside circulating lane 

to exit. This arrow supplements other arrow pavement 

markings in the circulating roadway. This treatment 

presents tradeoffs at locations where pedestrian 

crossings are present due to the potential for drivers to 

interpret the arrow as an indication of priority.

Low.
132 

Geometric Countermeasures

Geometric countermeasures involve more substantive changes that may include adjustments to 

the curbs of a design. Depending on the magnitude of the changes, geometric countermeasures 

could lead to cascading design considerations, such as the relocation of drainage inlets. The goal 

of geometric countermeasures is to improve driver lane selection, driver yielding, and lane 

discipline through physical changes to the roadway curbs.

Possible geometric treatments that seem to have high potential for effectiveness are listed in table 

37, along with their potential benefits. 

Table 37. Menu of geometric treatments with high effectiveness potential.

Treatment Potential Benefits 

Potential for 

Effectiveness 

Adjust alignment 

of a leg to correct 

a skew. 

Depending on the site context, skewed entries may 

impact the size of conflict areas, vehicle speed control, 

entering vehicle lane alignment, and driver view angles. 

Adjusting the alignment of an entry or an adjacent exit to 

reduce the skew has the potential to improve driver 

yielding at entry and reduce lane changing and lane 

straddling. 

High. 

Modify the ICD

of the 

roundabout. 

For excessively large roundabouts, reducing the size of 

the roundabout may be necessary to remove 

exit-circulating conflicts inherent to separation between 

entries and the immediate downstream exit. This strategy 

requires substantial intersection reconstruction; thus, 

extensive cost-benefit analyses and exploration of 

alternative strategies to determine the viability of lane 

reductions are recommended. 

High. 

Replace painted 

spiral markings 

with truck apron 

extension. 

Physical extension of the truck apron enhances guidance 

to drivers for navigating the intended path to single-lane 

exits where exclusive lanes are used. This enhancement 

will reduce lane straddling and lane-changing conflicts 

within the circulatory roadway and exit area. 

High. 

Possible geometric countermeasure treatments that seem to have medium-to-high potential for 

effectiveness are listed in table 38, along with their potential benefits.
133 

Table 38. Menu of geometric treatments with medium-to-high effectiveness potential.

Treatment Potential Benefits

Potential for 

Effectiveness

Modify entry geometry, 

including entry radius, entry 

width, or entry alignment.

Adjusted entry geometry may be desirable for a 

variety of reasons, depending on specific site 

conditions. Possible benefits include improved entry 

speed control to enhance driver yielding, improved 

lane alignment, and adjusted driver view angles. 

Medium to 

high.

Modify exit geometry, 

including exit radius, exit 

width, or exit alignment.

Adjustment to exit geometry may be desirable for a 

variety of reasons, depending on specific site 

conditions. Possible benefits include improving exit 

alignment to reduce exit lane changing and reducing 

the size of conflict areas.

Medium to 

high.

Geometric modification: 

Modify the transition from 

one approach lane to two 

entry lanes at the roundabout.

At some roundabouts, an entry lane is added to 

either the left or right using a standard taper. This 

configuration prioritizes approaching vehicles into 

one entry lane. Depending on network origindestination travel patterns, the lane being given 

priority could result in drivers selecting the incorrect 

lane for their movement or create imbalanced lane 

use. Adjusting which entry lane is being prioritized 

or making modifications to use a flared approach 

that removes lane priority are possible strategies for 

achieving more balanced lane use and improving 

lane selection. 

Medium to

high.

Geometric modification: 

Modify the transition from 

two exiting lanes to one 

downstream lane through use 

of a zipper-style merge 

(custom; subject to approval 

for experimentation).

Compared to the common strategy to end the outside 

lane and merge vehicles to the left, a zipper-style 

merge avoids priority being given to one exit lane, 

which has the potential to influence lane imbalances 

on upstream entries as familiar drivers take positions 

to avoid needing to change lanes along their travel 

paths. The zipper-style merge has the potential to 

enhance operation of the upstream entry and reduce 

possible failure-to-yield crashes associated with an 

absence of vehicles in the outside circulating lane. 

Right turns from incorrect lanes were also 

potentially influenced at some sites by the outside 

lane on the adjacent exit ending near the roundabout, 

which may be improved with use of a zipper-style 

merge.

Medium.

135 

ACKNOWLEDGEMENTS

For the following figures, the original map is the copyright property of Google® MapsTM and 

can be accessed from https://www.google.com/maps; all north arrows, road names, labels, lines, 

dimensions, and/or other annotations were added to the maps by FHWA:

• Figure 1 and figure 2. 

• Figure 6 through figure 14. 

• Figure 36 through figure 47. 

• Figure 49 through figure 51. 

• Figure 53 through figure 56. 

• Figure 59. 

• Figure 61. 

• Figure 63 through figure 65. 

• Figure 67 through figure 71. 

• Figure 78. 

• Figure 84 through figure 98. 

• Figure 100 and figure 101. 

137 

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Roundabout Exits: Transportation Pooled Fund Study Final Report 

(Washington, DC: 2023) https://doi.org/10.21949/1521966