Publication No.

FHWA-RD-00-067

FHWA Project Manager:

Joe Bared

Joe.Bared@fhwa.dot.gov

202-493-3314

http://www.tfhrc.gov

Roundabouts

An Informational

Guide

Kittelson & Associates, Inc.

Queensland University of Technology

Ruhr-University Bochum

University of Florida

University of Idaho

Pennsylvania State University

Hurst-Rosche Engineers

Eppell Olsen & Partners

Buckhurst Fish & Jacquemart

U.S. Department

of Transportation 

Federal Highway 

Administration
Foreword

Roundabouts are a form of intersection control in common use throughout the world. Until recently, many

transportation professionals and agencies in the United States have been hesitant to recommend and

install roundabouts, however, due to a lack of objective nationwide guidelines on planning, performance,

and design of roundabouts. Prior to the development of this guide, transportation professionals who were

interested in roundabouts had to rely on foreign roundabout design guides, consultants with roundabout

experience, or in some States, statewide roundabout design guides. To facilitate safe, optimal operation

and designs that are both consistent at a national level and consequential for driver expectation and

safety, the Federal Highway Administration (FHWA) developed this informational guide on roundabouts.

The information supplied in this document, Roundabouts: An Informational Guide, is based on established

international and U.S. practices and is supplemented by recent research. The guide is comprehensive in

recognition of the diverse needs of transportation professionals and the public for introductory material

through design detail, as well as the wide range of potential applications of roundabout intersections.

Roundabout operation and safety performance are particularly sensitive to geometric design elements.

Uncertainty regarding evaluation procedures can result in over-design and less safety. The “design problem” is essentially one of determining a design that will accommodate the traffic demand while minimizing

some combination of delay, crashes, and cost to all users, including motor vehicles, pedestrians, and

bicyclists. Evaluation procedures are suggested, or information is provided, to quantify and cost how well

a design achieves each of these aims.

Since there is no absolutely optimum design, this guide is not intended as an inflexible “rule book,” but

rather attempts to explain some principles of good design and indicate potential tradeoffs. In this respect,

the “design space” consists of performance evaluation models and design principles such as those provided in this guide, combined with the expert heuristic knowledge of a designer. Adherence to these

principles still does not ensure good design, which remains the responsibility of the designer.

Michael F. Trentacoste

Director, Office of Safety Research and Development

NOTICE

This publication is disseminated under the sponsorship of the Department of Transportation in the interest

of information exchange. The publication does not constitute a standard, specification, or regulation. Any

trade or manufacturers’ names that appear herein are included solely because they are considered essential to the object of the publication.
Roundabouts: An Informational Guide V

List of Exhibits viii

Photo Credits xiv

Chapter 1 - Introduction 1

1.1 Scope of Guide 2

1.2 Organization of Guide 3

1.3 Defining Physical Features 5

1.4 Key Dimensions 5

1.5 Distinguishing Roundabouts from Other Circular Intersections 8

1.6 Roundabout Categories 12

1.7 References 20

Chapter 2 - Policy Considerations 21

2.1 Characteristics 23

2.2 Multimodal Considerations 32

2.3 Costs Associated with Roundabouts 36

2.4 Legal Considerations 37

2.5 Public Involvement 40

2.6 Education 43

2.7 References 48

Chapter 3 - Planning 49

3.1 Planning Steps 51

3.2 Considerations of Context 53

3.3 Number of Entry Lanes 55

3.4 Selection Categories 58

3.5 Comparing Operational Performance of Alternative

Intersection Types 64

3.6 Space Requirements 69

Table of Contents
VI Federal Highway Administration

3.7 Economic Evaluation 70

3.8 References 76

Chapter 4 - Operation 79

4.1 Traffic Operation at Roundabouts 82

4.2 Data Requirements 83

4.3 Capacity 86

4.4 Performance Analysis 91

4.5 Computer Software for Roundabouts 96

4.6 References 98

Chapter 5 - Safety 101

5.1 Introduction 103

5.2 Conflicts 104

5.3 Crash Statistics 111

5.4 Crash Prediction Models 122

5.5 References 125

Chapter 6 - Geometric Design 127

6.1 Introduction 130

6.2 General Design Principles 132

6.3 Geometric Elements 145

6.4 Double-Lane Roundabouts 172

6.5 Rural Roundabouts 176

6.6 Mini-Roundabouts 179

6.7 References 181
Roundabouts: An Informational Guide VII

Chapter 7 - Traffic Design and Landscaping 183

7.1 Signing 185

7.2 Pavement Markings 197

7.3 Illumination 202

7.4 Work Zone Traffic Control 205

7.5 Landscaping 207

7.6 References 209

Chapter 8 - System Considerations 211

8.1 Traffic Signals at Roundabouts 213

8.2 At-Grade Rail Crossings 215

8.3 Closely Spaced Roundabouts 217

8.4 Roundabout Interchanges 219

8.5 Roundabouts in an Arterial Network 223

8.6 Microscopic Simulation 227

8.7 References 229

Glossary 231

Bibliography 240

Appendix A: Operations Analysis Formulas 251

Appendix B: Example Roundabout Designs 257

Appendix C: MUTCD Recommendations 265
VIII Federal Highway Administration

List of Exhibits

Chapter 1 - Introduction

Exhibit 1-1. Drawing of key roundabout features. 6

Exhibit 1-2. Description of key roundabout features. 6

Exhibit 1-3. Drawing of key roundabout dimensions. 7

Exhibit 1-4. Description of key roundabout dimensions. 7

Exhibit 1-5. Comparison of roundabouts with traffic circles. 8

Exhibit 1-6. Common design elements at roundabouts. 10

Exhibit 1-7. Basic design characteristics for each of the six

roundabout categories. 13

Exhibit 1-8. Typical mini-roundabout. 14

Exhibit 1-9. Typical urban compact. 15

Exhibit 1-10. Typical urban single-lane roundabout. 16

Exhibit 1-11. Typical urban double-lane roundabout. 17

Exhibit 1-12. Typical rural single-lane roundabout. 18

Exhibit 1-13. Typical rural double-lane roundabout 19

Chapter 2 - Policy Considerations

Exhibit 2-1. Average annual crash frequencies at 11 U.S.

intersections converted to roundabouts. 23

Exhibit 2-2. Pedestrian’s chances of death if hit by a motor vehicle. 25

Exhibit 2-3. Comparisons of vehicle-vehicle conflict points for

intersections with four single-lane approaches. 26

Exhibit 2-4. Fastest vehicle path through a double-lane roundabout. 27

Exhibit 2-5. Examples of aesthetic treatments. 31

Exhibit 2-6. Examples of informational brochures. 42

Exhibit 2-7. Driving straight through a roundabout. 45

Exhibit 2-8. Turning left at a roundabout. 46

Chapter 3 - Planning

Exhibit 3-1. Maximum daily service volumes for a

four-leg roundabout. 57

Exhibit 3-2. Planning-level maximum daily service volumes for

mini-roundabouts. 57
Roundabouts: An Informational Guide IX

Exhibit 3-3. Example of community enhancement roundabout. 59

Exhibit 3-4. Example of traffic calming roundabouts. 60

Exhibit 3-5. Comparison of predicted rural roundabout injury crashes

 with rural TWSC intersections. 61

Exhibit 3-6. Comparisons of predicted injury crashes for single-lane

and double-lane roundabouts with rural or urban signalized

intersections. 61

Exhibit 3-7. Average delay per vehicle at the MUTCD peak hour

signal warrant threshold. 63

Exhibit 3-8. Comparison of TWSC and single-lane roundabout capacity. 65

Exhibit 3-9. Sample hourly distribution of traffic. 66

Exhibit 3-10. Annual savings in delay of single-lane roundabout versus

AWSC, 50 percent of volume on the major street. 67

Exhibit 3-11. Annual savings in delay of single-lane roundabout versus

AWSC, 65 percent of volume on the major street. 67

Exhibit 3-12. Delay savings for roundabouts vs. signal, 50 percent

volume on major street. 69

Exhibit 3-13. Delay savings for roundabouts vs. signal, 65 percent

volume on major street. 69

Exhibit 3-14. Assumptions for spatial comparison of roundabouts

 and comparable conventional intersections. 70

Exhibit 3-15. Area comparison: Urban compact roundabout vs.

comparable signalized intersection. 71

Exhibit 3-16. Area comparison: Urban single-lane roundabout vs.

comparable signalized intersection. 71

Exhibit 3-17. Area comparison: Urban double-lane roundabout vs.

comparable signalized intersection. 72

Exhibit 3-18. Area comparison: Urban flared roundabouts vs.

comparable signalized intersection. 72

Exhibit 3-19. Estimated costs for crashes of varying levels

of severity. 74

Chapter 4 - Operation

Exhibit 4-1. Conversion factors for passenger car equivalents (pce). 84

Exhibit 4-2. Traffic flow parameters. 85

Exhibit 4-3. Approach capacity of a single-lane roundabout. 87

Exhibit 4-4. Approach capacity of a double-lane roundabout. 88
X Federal Highway Administration

Exhibit 4-5. Capacity reduction factors for short lanes. 89

Exhibit 4-6. Capacity comparison of single-lane and

double-lane roundabouts. 89

Exhibit 4-7. Capacity reduction factor M for a single-lane

roundabout assuming pedestrian priority. 90

Exhibit 4-8. Capacity reduction factor M for a double-lane

roundabout assuming pedestrian priority. 91

Exhibit 4-9. Control delay as a function of capacity and

circulating flow. 93

Exhibit 4-10. 95th-percentile queue length estimation. 95

Exhibit 4-11. Summary of roundabout software products for

operational analysis. 97

Chapter 5 - Safety

Exhibit 5-1. Vehicle conflict points for “T” Intersections with

single-lane approaches. 105

Exhibit 5-2. Vehicle conflict point comparison for intersections

with single-lane approaches. 106

Exhibit 5-3. Improper lane-use conflicts in double-lane roundabouts. 107

Exhibit 5-4. Improper turn conflicts in double-lane roundabouts. 108

Exhibit 5-5. Pedestrian-vehicle conflicts at signalized intersections. 109

Exhibit 5-6. Pedestrian-vehicle conflicts at single-lane roundabouts. 109

Exhibit 5-7. Bicycle conflicts at conventional intersections. 110

Exhibit 5-8. Bicycle conflicts at roundabouts. 111

Exhibit 5-9. Average annual crash frequencies at 11 U.S.

intersections converted to roundabouts. 112

Exhibit 5-10. Mean crash reductions in various countries. 112

Exhibit 5-11. Reported proportions of major crash types at

roundabouts. 113

Exhibit 5-12. Comparison of collision types at roundabouts. 114

Exhibit 5-13. Graphical depiction of collision types at roundabouts. 115

Exhibit 5-14. Accident percentage per type of user urban

roundabouts in 15 towns in western France. 116

Exhibit 5-15. British crash rates for pedestrians at roundabouts and

signalized intersections. 117

Exhibit 5-16. Percentage reduction in the number of accidents by

mode at 181 converted Dutch roundabouts. 117
Roundabouts: An Informational Guide XI

Exhibit 5-17. British crash rates (crashes per million trips) for

bicyclists and motorcyclists at roundabouts and

signalized intersections. 120

Exhibit 5-18. A comparison of crashes between signalized and

roundabout intersections in 1998 in 15 French towns. 120

Chapter 6 - Geometric Design

Exhibit 6-1. Basic geometric elements of a roundabout. 131

Exhibit 6-2. Roundabout design process. 131

Exhibit 6-3. Sample theoretical speed profile (urban compact

roundabout). 133

Exhibit 6-4. Recommended maximum entry design speeds. 133

Exhibit 6-5. Fastest vehicle path through single-lane roundabout. 134

Exhibit 6-6. Fastest vehicle path through double-lane roundabout. 135

Exhibit 6-7. Example of critical right-turn movement. 135

Exhibit 6-8. Side friction factors at various speeds (metric units). 137

Exhibit 6-9. Side friction factors at various speeds (U.S.

customary units). 137

Exhibit 6-10. Speed-radius relationship (metric units). 138

Exhibit 6-11. Speed-radius relationship (U.S. customary units). 138

Exhibit 6-12. Vehicle path radii. 139

Exhibit 6.13. Approximated R4 values and corresponding R1

values (metric units). 141

Exhibit 6-14. Approximated R4 values and corresponding R1

values (U.S. customary units). 141

Exhibit 6-15. Through-movement swept path of WB-15 (WB-50)

vehicle. 143

Exhibit 6-16. Left-turn and right-turn swept paths of WB-15

(WB-50) vehicle. 143

Exhibit 6-17. Key dimensions of nonmotorized design users. 144

Exhibit 6-18. Radial alignment of entries. 145

Exhibit 6-19. Recommended inscribed circle diameter ranges. 146

Exhibit 6-20. Approach widening by adding full lane. 148

Exhibit 6-21. Approach widening by entry flaring. 148

Exhibit 6-22. Minimum circulatory lane widths for two-lane

roundabouts. 150
XII Federal Highway Administration

Exhibit 6-23. Example of central island with a traversable apron. 151

Exhibit 6.24. Single-lane roundabout entry design. 153

Exhibit 6-25. Single-lane roundabout exit design. 154

Exhibit 6-26. Minimum splitter island dimensions. 157

Exhibit 6-27. Minimum splinter island nose radii and offsets. 158

Exhibit 6-28. Design values for stopping sight distance. 159

Exhibit 6-29. Approach sight distance. 160

Exhibit 6-30. Sight distance on circulatory roadway. 160

Exhibit 6-31. Sign distance to crosswalk on exit. 161

Exhibit 6-32. Intersection sight distance. 162

Exhibit 6-33. Computed length of conflicting leg of

intersection sight triangle. 163

Exhibit 6-34. Sample plan view. 164

Exhibit 6-35. Sample approach profile. 165

Exhibit 6-36. Sample central island profile. 165

Exhibit 6-37. Typical circulatory roadway section. 166

Exhibit 6-38. Typical section with a truck apron. 166

Exhibit 6-39. Provisions for bicycles. 168

Exhibit 6-40. Sidewalk treatments. 169

Exhibit 6-41. Example of right-turn bypass lane. 170

Exhibit 6-42. Configuration of right-turn bypass lane with

acceleration lane. 171

Exhibit 6-43. Configuration of right-turn bypass lane with

yield at exit leg. 172

Exhibit 6-44. Sketched natural paths through a double-lane

 roundabout. 173

Exhibit 6-45. Path overlap at a double-lane roundabout. 174

Exhibit 6-46. One method of entry design to avoid path overlap

 at double-lane roundabouts. 175

Exhibit 6-47. Alternate method of entry design to avoid path

|overlap at double-lane roundabouts. 175

Exhibit 6-48. Extended splitter island treatment. 178

Exhibit 6-49. Use of successive curves on high-speed

approaches. 179

Exhibit 6-50. Example of mini-roundabout. 180
Roundabouts: An Informational Guide XIII

Chapter 7 - Traffic Design and Landscaping

Exhibit 7-1. YIELD sign (R1-2). 186

Exhibit 7-2. ONE WAY sign (R6-1R). 186

Exhibit 7-3. KEEP RIGHT sign (R4-7). 186

Exhibit 7-4. Lane-use control signing for roundabouts with doublelane entries. 188

Exhibit 7-5. Lane-use control signing for roundabouts with heavy

turning traffic. 188

Exhibit 7-6. Circular Intersection sign (W2-6). 189

Exhibit 7-7. Advisory speed plate (W13-1). 189

Exhibit 7-8. Roundabout Ahead Sign. 189

Exhibit 7-9. YIELD AHEAD sign (W3-2a). 189

Exhibit 7-10. Large Arrow sign (W1-6). 190

Exhibit 7-11. Chevron plate (W1-8a. 190

Exhibit 7-12. Pedestrian Crossing sign (W11-2a). 190

Exhibit 7-13. Examples of advance destination guide signs. 191

Exhibit 7-14. Exit guide sign (D1-1). 192

Exhibit 7-15. Sample signing plan for an urban roundabout. 193

Exhibit 7-16. Sample signing plan for a rural roundabout. 194

Exhibit 7-17. Examples of speed reduction treatments. 195

Exhibit 7-18. Sample signing plan for a mini-roundabout. 196

Exhibit 7-19. Examples of yield lines. 198

Exhibit 7-20. Approach pavement markings. 199

Exhibit 7-21. Sample pavement marking plan for a mini-roundabout. 201

Exhibit 7-22. Illumination of a roundabout. 202

Exhibit 7-23. Recommended street illumination levels. 204

Exhibit 7-24. Landscaping of the central island. 208
XIV Federal Highway Administration

Chapter 8 - System Considerations

Exhibit 8-1. Rail crossing treatments at roundabouts. 216

Exhibit 8-2. Methods for accommodating a rail crossing adjacent

to a roundabout. 217

Exhibit 8.3. Example of closely spaced offset T-intersections with

roundabouts. 218

Exhibit 8-4. Through bypass lanes at staggered T-intersections. 218

Exhibit 8-5. Two-bridge roundabout interchange. 219

Exhibit 8-6. Example of two-bridge roundabout interchanges. 220

Exhibit 8-7. Examples of one-bridge roundabout interchanges with

circular central islands. 221

Exhibit 8.8. One-bridge roundabout interchange with raindropshaped central islands. 222

Exhibit 8-9. Roundabouts in an arterial network. 223

Exhibit 8-10. Wide nodes and narrow roads. 226

Exhibit 8-11. Summary of simulation models for roundabout

analysis. 228

Photo Credits

Barry Crown: Exhibits 8-6, 8-7

Ken Courage: Exhibit 1-5 (g, Portland)

Lee Rodegerdts: Exhibits 1-5 (all except g, Portland), 1-6 (all except Fort Pierce), 2-

4 (all except Fort Pierce), 3-3, 3-4, 6-23, 6-42, 7-10 (all), 7-11 (all), 7-14 (all), 7-16 (all),

7-22, 8-7, 8-8, 8-9, C-3 (a, d–i, k–n)

Paul Ryus: Exhibits 1-6 (Fort Pierce), 2-4 (Fort Pierce), C-3 (b, c, j)
Roundabouts: An Informational Guide • 1: Introduction 1

Introduction

1.1 Scope of the Guide 2

1.2 Organization of the Guide 3

1.3 Defining Physical Features 5

1.4 Key Dimensions 5

1.5 Distinguishing Roundabouts from Other Circular Intersections 8

1.6 Roundabout Categories 12

1.6.1 Comparison of roundabout categories 13

1.6.2 Mini-roundabouts 14

1.6.3 Urban compact roundabouts 15

1.6.4 Urban single-lane roundabouts 16

1.6.5 Urban double-lane roundabouts 17

1.6.6 Rural single-lane roundabouts 18

1.6.7 Rural double-lane roundabouts 19

1.7 References 20

Exhibit 1-1. Drawing of key roundabout features. 6

Exhibit 1-2. Description of key roundabout features. 6

Exhibit 1-3. Drawing of key roundabout dimensions. 7

Exhibit 1-4. Description of key roundabout dimensions. 7

Exhibit 1-5. Comparison of roundabouts with traffic circles. 8

Exhibit 1-6. Common design elements at roundabouts. 10

Exhibit 1-7. Basic design characteristics for each of the six roundabout

categories. 13

Exhibit 1-8. Typical mini-roundabout. 14

Exhibit 1-9. Typical urban compact roundabout. 15

Exhibit 1-10. Typical urban single-lane roundabout. 16

Exhibit 1-11. Typical urban double-lane roundabout. 17

Exhibit 1-12. Typical rural single-lane roundabout. 18

Exhibit 1-13. Typical rural double-lane roundabout. 19

1
2 Federal Highway Administration

Traffic circles have been part of the transportation system in the United States

since 1905, when the Columbus Circle designed by William Phelps Eno opened in

New York City. Subsequently, many large circles or rotaries were built in the United

States. The prevailing designs enabled high-speed merging and weaving of vehicles. Priority was given to entering vehicles, facilitating high-speed entries. High

crash experience and congestion in the circles led to rotaries falling out of favor in

America after the mid-1950’s. Internationally, the experience with traffic circles

was equally negative, with many countries experiencing circles that locked up as

traffic volumes increased.

The modern roundabout was developed in the United Kingdom to rectify problems

associated with these traffic circles. In 1966, the United Kingdom adopted a mandatory “give-way” rule at all circular intersections, which required entering traffic

to give way, or yield, to circulating traffic. This rule prevented circular intersections

from locking up, by not allowing vehicles to enter the intersection until there were

sufficient gaps in circulating traffic. In addition, smaller circular intersections were

proposed that required adequate horizontal curvature of vehicle paths to achieve

slower entry and circulating speeds.

These changes improved the safety characteristics of the circular intersections by

reducing the number and particularly the severity of collisions. Thus, the resultant

modern roundabout is significantly different from the older style traffic circle both

in how it operates and in how it is designed. The modern roundabout represents a

substantial improvement, in terms of operations and safety, when compared with

older rotaries and traffic circles (1, 2, 3). Therefore, many countries have adopted

them as a common intersection form and some have developed extensive design

guides and methods to evaluate the operational performance of modern roundabouts.

1.1 Scope of the Guide

This guide provides information and guidance on roundabouts, resulting in designs

that are suitable for a variety of typical conditions in the United States. The scope

of this guide is to provide general information, planning techniques, evaluation procedures for assessing operational and safety performance, and design guidelines

for roundabouts.

This guide has been developed with the input from transportation practitioners and

researchers from around the world. In many cases, items from national and international practice and research indicate considerable consensus, and these items

have been included in this guide. However, other items have generated considerable differences of opinion (e.g., methods of estimating capacity), and some practices vary considerably from country to country (e.g., marking of the circulatory

roadway in multilane roundabouts). Where international consensus is not apparent, a reasoned approach is presented that the authors believe is currently most

appropriate for the United States. As more roundabouts are built, the opportunity

to conduct research to refine—or develop better—methods will enable future editions of this guide to improve.

Circular intersections

were first introduced

in the U.S. in 1905.

The modern roundabout was

developed in the United

Kingdom in the 1960’s.

Modern roundabouts

provide substantially better

operational and safety

characteristics than

older traffic circles

and rotaries.

International consensus has

not been achieved on some

aspects of roundabout design.

Chapter 1 Introduction
Roundabouts: An Informational Guide • 1: Introduction 3

Despite the comprehensive nature of this document, it cannot discuss every issue

related to roundabouts. In particular, it does not represent the following

topics:

• Nonmountable traffic calming circles. These are small traffic circles with raised

central islands. They are typically used on local streets for speed and volume

control. They are typically not designed to accommodate large vehicles, and

often left-turning traffic is required to turn left in front of the circle. Mini-roundabouts, which are presented, may be an appropriate substitute.

• Specific legal or policy requirements and language. The legal information that is

provided in this guide is intended only to make the reader aware of potential

issues. The reader is encouraged to consult with an attorney on specific legal

issues before adopting any of the recommendations contained herein. Similarly, regarding policy information, the guide refers to or encompasses applicable policies, such as those of the American Association of State Highway and

Transportation Officials (AASHTO) (4). It does not, however, establish any new

policies.

• Roundabouts with more than two entry lanes on an approach. While acknowledging the existence and potential of such large roundabouts, the guide does

not provide specific guidance on the analysis or design of such roundabouts.

However, the design principles contained in this document are also applicable

to larger roundabouts. The relative safety advantages of roundabout intersections diminish at high traffic flows, particularly with regard to pedestrians and

bicyclists. The advantages of larger roundabouts are their higher capacities that

may make them attractive alternatives at sites with high traffic volumes. More

intricate design is required to ensure adequate operational and safety performance. Therefore, expert operations and design advice should be sought and

roundabout analysis software should be utilized in such circumstances. As users and designers in the United States become more familiar with roundabouts,

this experience may then be extended to such applications.

1.2 Organization of the Guide

This guide has been structured to address the needs of a variety of readers including the general public, policy-makers, transportation planners, operations and safety

analysts, conceptual and detailed designers. This chapter distinguishes roundabouts

from other traffic circles and defines the types of roundabouts addressed in the

remainder of the guide. The remaining chapters in this guide generally increase in

the level of detail provided.

Chapter 2—Policy Considerations: This chapter provides a broad overview of the

performance characteristics of roundabouts. The costs associated with roundabouts

versus other forms of intersections, legal issues, and public involvement techniques

are discussed.

Chapter 3—Planning: This chapter discusses general guidelines for identifying

appropriate intersection control options, given daily traffic volumes, and procedures

for evaluating the feasibility of a roundabout at a given location. Chapters 2 and 3

provide sufficient detail to enable a transportation planner to decide under which

circumstances roundabouts are likely to be appropriate, and how they compare to

alternatives at a specific location.

Topics not discussed in this guide.
4 Federal Highway Administration

Chapter 4—Operational Analysis: Methods are presented for analyzing the operational performance of each category of roundabout in terms of capacity, delay,

and queuing.

Chapter 5—Safety: This chapter discusses the expected safety performance of

roundabouts.

Chapter 6—Geometric Design: Specific geometric design principles for roundabouts are presented. The chapter then discusses each design element in detail,

along with appropriate parameters to use for each type of roundabout.

Chapter 7—Traffic Design and Landscaping: This chapter discusses a number of

traffic design aspects once the basic geometric design has been established. These

include signs, pavement markings, and illumination. In addition, the chapter provides discussion on traffic maintenance during construction and landscaping.

Chapter 8—System Considerations: This chapter discusses specific issues and

treatments that may arise from the systems context of a roundabout. The material

may be of interest to transportation planners as well as operations and design

engineers. Signal control at roundabouts is discussed. The chapter then considers

the issue of rail crossings through the roundabout or in close proximity. Roundabouts in series with other roundabouts are discussed, including those at freeway

interchanges and those in signalized arterial networks. Finally, the chapter presents simulation models as supplementary operational tools capable of evaluating

roundabout performance within an overall roadway system.

Appendices: Three appendices are provided to expand upon topics in certain chapters. Appendix A provides information on the capacity models in Chapter 4. Appendix B provides design templates for each of the categories of roundabout described

in Chapter 1, assuming four perpendicular legs. Appendix C provides information

on the alternative signing and pavement marking in Chapter 7.

Several typographical devices have been used to enhance the readability of the

guide. Margin notes, such as the note next to this paragraph, highlight important

points or identify cross-references to other chapters of the guide. References have

been listed at the end of each chapter and have been indicated in the text using

numbers in parentheses, such as: (3). New terms are presented in italics and are

defined in the glossary at the end of the document.

Margin notes have been

used to highlight important

points.
Roundabouts: An Informational Guide • 1: Introduction 5

1.3 Defining Physical Features

A roundabout is a type of circular intersection, but not all circular intersections can

be classified as roundabouts. In fact, there are at least three distinct types of circular intersections:

• Rotaries are old-style circular intersections common to the United States prior

to the 1960’s. Rotaries are characterized by a large diameter, often in excess of

100 m (300 ft). This large diameter typically results in travel speeds within the

circulatory roadway that exceed 50 km/h (30 mph). They typically provide little or

no horizontal deflection of the paths of through traffic and may even operate

according to the traditional “yield-to-the-right” rule, i.e., circulating traffic yields

to entering traffic.

• Neighborhood traffic circles are typically built at the intersections of local streets

for reasons of traffic calming and/or aesthetics. The intersection approaches

may be uncontrolled or stop-controlled. They do not typically include raised

channelization to guide the approaching driver onto the circulatory roadway. At

some traffic circles, left-turning movements are allowed to occur to the left of

(clockwise around) the central island, potentially conflicting with other circulating traffic.

• Roundabouts are circular intersections with specific design and traffic control

features. These features include yield control of all entering traffic, channelized

approaches, and appropriate geometric curvature to ensure that travel speeds

on the circulatory roadway are typically less than 50 km/h (30 mph). Thus, roundabouts are a subset of a wide range of circular intersection forms.

To more clearly identify the defining characteristics of a roundabout, consistent

definitions for each of the key features, dimensions, and terms are used throughout this guide. Exhibit 1-1 is a drawing of a typical roundabout, annotated to identify the key features. Exhibit 1-2 provides a description of each of the key features.

1.4 Key Dimensions

For operational analysis and design purposes, it is useful to define a number of key

dimensions. Exhibit 1-3 shows a number of key dimensions that are described in

Exhibit 1-4. Note that these exhibits do not present all of the dimensions needed in

the detailed analysis and design of roundabouts; these will be presented and defined in later chapters as needed.

Types of circular intersections.

Key roundabout features include:

• Yield control of entering traffic

• Channelized approaches

• Appropriate geometric curvature to

slow speeds
6 Federal Highway Administration

Exhibit 1-1. Drawing of key

roundabout features.

Exhibit 1-2. Description of key

roundabout features.

Feature Description

Central island The central island is the raised area in the center of a roundabout around which

traffic circulates.

Splitter island A splitter island is a raised or painted area on an approach used to separate entering

from exiting traffic, deflect and slow entering traffic, and provide storage space for

pedestrians crossing the road in two stages.

Circulatory roadway The circulatory roadway is the curved path used by vehicles to travel in a counterclockwise fashion around the central island

Apron If required on smaller roundabouts to accommodate the wheel tracking of large

vehicles, an apron is the mountable portion of the central island adjacent to the

circulatory roadway.

Yield line A yield line is a pavement marking used to mark the point of entry from an approach into the circulatory roadway and is generally marked along the inscribed

circle. Entering vehicles must yield to any circulating traffic coming from the left

before crossing this line into the circulatory roadway.

Accessible pedestrian crossings Accessible pedestrian crossings should be provided at all roundabouts. The crossing location is set back from the yield line, and the splitter island is cut to allow

pedestrians, wheelchairs, strollers, and bicycles to pass through.

Bicycle treatments Bicycle treatments at roundabouts provide bicyclists the option of traveling through

the roundabout either as a vehicle or as a pedestrian, depending on the bicyclist’s

level of comfort.

Landscaping buffer Landscaping buffers are provided at most roundabouts to separate vehicular and

pedestrian traffic and to encourage pedestrians to cross only at the designated

crossing locations. Landscaping buffers can also significantly improve the aesthetics of the intersection.

Splitter islands have multiple

 roles. They:

• Separate entering and

exiting traffic

• Deflect and slow

entering traffic

• Provide a pedestrian

refuge
Roundabouts: An Informational Guide • 1: Introduction 7

Exhibit 1-3. Drawing of key

roundabout dimensions.

Exhibit 1-4. Description of

key roundabout dimensions.

Dimension Description

Inscribed circle diameter The inscribed circle diameter is the basic parameter used to define the size of a roundabout. It is measured between the outer edges of the circulatory roadway.

Circulatory roadway width The circulatory roadway width defines the roadway width for vehicle circulation around the

central island. It is measured as the width between the outer edge of this roadway and the

central island. It does not include the width of any mountable apron, which is defined to be

part of the central island.

Approach width The approach width is the width of the roadway used by approaching traffic upstream of

any changes in width associated with the roundabout. The approach width is typically no

more than half of the total width of the roadway.

Departure width The departure width is the width of the roadway used by departing traffic downstream of

any changes in width associated with the roundabout. The departure width is typically less

than or equal to half of the total width of the roadway.

Entry width The entry width defines the width of the entry where it meets the inscribed circle. It is

measured perpendicularly from the right edge of the entry to the intersection point of the

left edge line and the inscribed circle.

Exit width The exit width defines the width of the exit where it meets the inscribed circle. It is measured perpendicularly from the right edge of the exit to the intersection point of the left

edge line and the inscribed circle.

Entry radius The entry radius is the minimum radius of curvature of the outside curb at the entry.

Exit radius The exit radius is the minimum radius of curvature of the outside curb at the exit.
8 Federal Highway Administration

1.5 Distinguishing Roundabouts from

Other Circular Intersections

Since the purpose of this guide is to assist in the planning, design, and performance evaluation of roundabouts, not other circular intersections, it is important to

be able to distinguish between them. Since these distinctions may not always be

obvious, the negative aspects of rotaries or neighborhood traffic circles (hereafter

referred to as “traffic circles”) may be mistaken by the public for a roundabout.

Therefore, the ability to carefully distinguish roundabouts from traffic circles is important in terms of public understanding.

How then does one distinguish a roundabout from other forms of circular intersection? Exhibit 1-5 identifies some of the major characteristics of roundabouts and

contrasts them with other traffic circles. Note that some of the traffic circles shown

have many of the features associated with roundabouts but are deficient in one or

more critical areas. Note also that these characteristics apply to yield-controlled

roundabouts; signalized roundabouts are a special case discussed in Chapter 8.

Exhibit 1-5. Comparison of

roundabouts with traffic

circles.

Roundabouts Traffic Circles

(a) Traffic control

Yield control is used on all entries. The

circulatory roadway has no control.

Santa Barbara, CA

Some traffic circles use stop control, or

no control, on one or more entries.

Hagerstown, MD

(b) Priority to circulating vehicles

Circulating vehicles have the right-ofway. Santa Barbara, CA

Some traffic circles require circulating

traffic to yield to entering traffic.

Sarasota, FL

Circular intersections that do not

conform to the characteristics of

modern roundabouts are called

“traffic circles” in this guide.

Roundabouts must have

all of the characteristics

listed in the left column.

Chapter 8 discusses signalization

at roundabouts.
Roundabouts: An Informational Guide • 1: Introduction 9

Exhibit 1-5. (continued).

Comparison of roundabouts

with traffic circles.

(c) Pedestrian access

Pedestrian access is allowed only across

the legs of the roundabout, behind the

yield line. Santa Barbara, CA

Some traffic circles allow pedestrian access to the central island. Sarasota, FL

(d) Parking

No parking is allowed within the circulatory roadway or at the entries. Avon, CO

(e) Direction of circulation

All vehicles circulate counter-clockwise

and pass to the right of the central island. Naples, FL

Some traffic circles allow parking within

the circulatory roadway. Sarasota, FL

Roundabouts Traffic Circles

Some neighborhood traffic circles allow

left-turning vehicles to pass to the left

of the central island. Portland, OR

All traffic circulates counter-clockwise

around a roundabouts central island.
10 Federal Highway Administration

In addition to the design elements identified in Exhibit 1-5, roundabouts often include one or more additional design elements intended to enhance the safety and/

or capacity of the intersection. However, their absence does not necessarily preclude an intersection from operating as a roundabout. These additional elements

are identified in Exhibit 1-6.

Characteristic Description

 (a) Adequate

speed reduction

Good roundabout design requires entering vehicles to negotiate a small enough radius to slow speeds to no greater than

50 km/h (30 mph). Once within the circulatory roadway, vehicles’ paths are further deflected by the central island. West

Boca Raton, FL

Some roundabouts allow high-speed entries for major movements. This increases the risk for more severe collisions for

vehicles, bicycles, and pedestrians. Bradenton Beach, FL

Roundabouts may have these

additional design features.

Exhibit 1-6. Common

design elements at

roundabouts.
Roundabouts: An Informational Guide • 1: Introduction 11

Good roundabout design makes accommodation for the appropriate design vehicle. For small roundabouts, this may require the use of an apron. Lothian, MD

Characteristic Description Exhibit 1-6 (continued).

Common design elements

at roundabouts.

(b) Design

vehicle

Some roundabouts are too small to accommodate large vehicles that periodically approach the intersection. Naples, FL

Flare on an entry to a roundabout is the widening of an approach to multiple lanes to provide additional capacity and

storage at the yield line. Long Beach, CA

(c) Entry flare

Aprons can be used in small

roundabouts to accommodate

the occasional large vehicle that

may use the intersection.
12 Federal Highway Administration

(d) Splitter

island

Characteristic Description

All except mini-roundabouts have raised splitter islands. These

are designed to separate traffic moving in opposite directions,

deflect entering traffic, and to provide opportunities for pedestrians to cross in two stages. Mini-roundabouts may have splitter islands defined only by pavement markings. Tavares, FL

(e) Pedestrian

crossing locations

Pedestrian crossings are located at least one vehicle length

upstream of the yield point. Fort Pierce, FL

Exhibit 1-6 (continued).

Common design elements at

roundabouts.

1.6 Roundabout Categories

For the purposes of this guide, roundabouts have been categorized according to size

and environment to facilitate discussion of specific performance or design issues.

There are six basic categories based on environment, number of lanes, and size:

• Mini-roundabouts

• Urban compact roundabouts

• Urban single-lane roundabouts

• Urban double-lane roundabouts

• Rural single-lane roundabouts

• Rural double-lane roundabouts

Multilane roundabouts with more than two approach lanes are possible, but they

are not covered explicitly by this guide, although many of the design principles contained in this guide would still apply. For example, the guide provides guidance on the

This guide uses six basic

roundabout categories.

Multilane roundabouts with

more than two approach

lanes are possible, but not

explicitly covered in this guide.
Roundabouts: An Informational Guide • 1: Introduction 13

design of flaring approaches from one to two lanes. Although not explicitly discussed,

this guidance could be extended to the design of larger roundabout entries.

Note that separate categories have not been explicitly identified for suburban environments. Suburban settings may combine higher approach speeds common in

rural areas with multimodal activity that is more similar to urban settings. Therefore, they should generally be designed as urban roundabouts, but with the highspeed approach treatments recommended for rural roundabouts.

In most cases, designers should anticipate the needs of pedestrians, bicyclists,

and large vehicles. Whenever a raised splitter island is provided, there should also

be an at-grade pedestrian refuge. In this case, the pedestrian crossing facilitates

two separate moves: curb-to-island and island-to-curb. The exit crossing will typically require more vigilance from the pedestrian and motorist than the entry crossing. Further, it is recommended that all urban crosswalks be marked. Under all

urban design categories, special attention should be given to assist pedestrian

users who are visually impaired or blind, through design elements. For example,

these users typically attempt to maintain their approach alignment to continue

across a street in the crosswalk, since the crosswalk is often a direct extension of

the sidewalk. A roundabout requires deviation from that alignment, and attention

needs to be given to providing appropriate informational cues to pedestrians regarding the location of the sidewalk and the crosswalk, even at mini-roundabouts.

For example, appropriate landscaping is one method of providing some information. Another is to align the crosswalk ramps perpendicular to the pedestrian’s line

of travel through the pedestrian refuge.

1.6.1 Comparison of roundabout categories

Exhibit 1-7 summarizes and compares some fundamental design and operational

elements for each of the six roundabout categories developed for this guide. The

following sections provide a qualitative discussion of each category.

Mini- Urban Urban Urban Rural Rural

Design Element Roundabout Compact Single-Lane Double-Lane Single-Lane Double-Lane

Recommended 25 km/h 25 km/h 35 km/h 40 km/h 40 km/h 50 km/h

maximum entry (15 mph) (15 mph) (20 mph) (25 mph) (25 mph) (30 mph)

design speed

Maximum number 111212

of entering lanes

per approach

Typical inscribed 13 m to 25 m 25 to 30 m 30 to 40 m 45 to 55 m 35 to 40 m 55 to 60 m

circle diameter1 (45 ft to 80 ft) (80 to 100 ft) (100 to 130 ft) (150 to 180 ft) (115 to 130 ft) (180 to 200 ft)

Splitter island Raised if Raised, with Raised, with Raised, with Raised and Raised and

treatment possible, crosswalk cut crosswalk cut crosswalk cut extended, with extended, with

crosswalk crosswalk cut crosswalk cut

cut if raised

Typical daily service 10,000 15,000 20,000 Refer to 20,000 Refer to

volumes on 4-leg Chapter 4 Chapter 4

roundabout (veh/day) procedures procedures

1. Assumes 90-degree entries and no more than four legs.

Suburban roundabouts incorporate

elements of both urban and rural

roundabouts.

Roundabout design should generally

accommodate pedestrian, bicycle,

and large vehicle use.

Exhibit 1-7. Basic design

characteristics for each of the six

roundabout categories.
14 Federal Highway Administration

1.6.2 Mini-roundabouts

Mini-roundabouts are small roundabouts used in low-speed urban environments,

with average operating speeds of 60km/h (35mph) or less. Exhibit 1-8 provides an

example of a typical mini-roundabout. They can be useful in low-speed urban environments in cases where conventional roundabout design is precluded by right-ofway constraints. In retrofit applications, mini-roundabouts are relatively inexpensive because they typically require minimal additional pavement at the intersecting

roads-for example, minor widening at the corner curbs. They are mostly recommended when there is insufficient right-of-way for an urban compact roundabout.

Because they are small, mini-roundabouts are perceived as pedestrian-friendly with

short crossing distances and very low vehicle speeds on approaches and exits. The

mini-roundabout is designed to accommodate passenger cars without requiring

them to drive over the central island. To maintain its perceived compactness and

low speed characteristics, the yield lines are positioned just outside of the swept

path of the largest expected vehicle. However, the central island is mountable, and

larger vehicles may cross over the central island, but not to the left of it. Speed

control around the mountable central island should be provided in the design by

requiring horizontal deflection. Capacity for this type of roundabout is expected to

be similar to that of the compact urban roundabout. The recommended design of

these roundabouts is based on the German method, with some influence from the

United Kingdom.

Exhibit 1-8. Typical

mini-roundabout.

Mini-roundabouts can be useful

in low-speed urban

environments with right-of-way

constraints.
Roundabouts: An Informational Guide • 1: Introduction 15

1.6.3 Urban compact roundabouts

Like mini-roundabouts, urban compact roundabouts are intended to be pedestrianand bicyclist-friendly because their perpendicular approach legs require very low

vehicle speeds to make a distinct right turn into and out of the circulatory roadway.

All legs have single-lane entries. However, the urban compact treatment meets all

the design requirements of effective roundabouts. The principal objective of this

design is to enable pedestrians to have safe and effective use of the intersection.

Capacity should not be a critical issue for this type of roundabout to be considered.

The geometric design includes raised splitter islands that incorporate at-grade pedestrian storage areas, and a nonmountable central island. There is usually an apron

surrounding the nonmountable part of the compact central island to accommodate

large vehicles. The recommended design of these roundabouts is similar to those

in Germany and other northern European countries. Exhibit 1-9 provides an example of a typical urban compact roundabout.

Exhibit 1-9. Typical urban

compact roundabout.

Urban compact roundabouts are

intended to be pedestrian-friendly;

capacity should not be a critical issue

when considering this type.
16 Federal Highway Administration

1.6.4 Urban single-lane roundabouts

This type of roundabout is characterized as having a single lane entry at all legs and

one circulatory lane. Exhibit 1-10 provides an example of a typical urban single-lane

roundabout. They are distinguished from urban compact roundabouts by their larger

inscribed circle diameters and more tangential entries and exits, resulting in higher

capacities. Their design allows slightly higher speeds at the entry, on the circulatory roadway, and at the exit. Notwithstanding the larger inscribed circle diameters

than compact roundabouts, the speed ranges recommended in this guide are somewhat lower than those used in other countries, in order to enhance safety for bicycles and pedestrians. The roundabout design is focused on achieving consistent

entering and circulating vehicle speeds. The geometric design includes raised splitter islands, a nonmountable central island, and preferably, no apron. The design of

these roundabouts is similar to those in Australia, France, and the United Kingdom.

Exhibit 1-10. Typical urban

single-lane roundabout.

Urban single-lane roundabouts have

slightly higher speeds and capacities

than urban compact roundabouts.

The design focuses on consistent

entering and exiting speeds.
Roundabouts: An Informational Guide • 1: Introduction 17

1.6.5 Urban double-lane roundabouts

Urban double-lane roundabouts include all roundabouts in urban areas that have at

least one entry with two lanes. They include roundabouts with entries on one or

more approaches that flare from one to two lanes. These require wider circulatory

roadways to accommodate more than one vehicle traveling side by side. Exhibit 1-

11 provides an example of a typical urban multilane roundabout. The speeds at the

entry, on the circulatory roadway, and at the exit are similar to those for the urban

single-lane roundabouts. Again, it is important that the vehicular speeds be consistent throughout the roundabout. The geometric design will include raised splitter

islands, no truck apron, a nonmountable central island, and appropriate horizontal

deflection.

Alternate routes may be provided for bicyclists who choose to bypass the roundabout. Bicycle and pedestrian pathways must be clearly delineated with sidewalk

construction and landscaping to direct users to the appropriate crossing locations

and alignment. Urban double-lane roundabouts located in areas with high pedestrian or bicycle volumes may have special design recommendations such as those

provided in Chapters 6 and 7. The design of these roundabouts is based on the

methods used in the United Kingdom, with influences from Australia and France.

Exhibit 1-11. Typical urban

double-lane roundabout.

The urban double-lane roundabout

category includes roundabouts with

one or more entries that flare from

one to two lanes.

See Chapters 6 and 7 for special

design considerations for

pedestrians and bicycles.
18 Federal Highway Administration

1.6.6 Rural single-lane roundabouts

Rural single-lane roundabouts generally have high average approach speeds in the

range of 80 to 100 km/h (50 to 60 mph). They require supplementary geometric and

traffic control device treatments on approaches to encourage drivers to slow to an

appropriate speed before entering the roundabout. Rural roundabouts may have

larger diameters than urban roundabouts to allow slightly higher speeds at the

entries, on the circulatory roadway, and at the exits. This is possible if few pedestrians are expected at these intersections, currently and in future. There is preferably

no apron because their larger diameters should accommodate larger vehicles.

Supplemental geometric design elements include extended and raised splitter islands, a nonmountable central island, and adequate horizontal deflection. The design of these roundabouts is based primarily on the methods used by Australia,

France, and the United Kingdom. Exhibit 1-12 provides an example of a typical rural

single-lane roundabout.

Rural roundabouts that may one day become part of an urbanized area should be

designed as urban roundabouts, with slower speeds and pedestrian treatments.

However, in the interim, they should be designed with supplementary approach

and entry features to achieve safe speed reduction.

Exhibit 1-12. Typical rural

single-lane roundabout.

Because of their higher

approach speeds, rural

single-lane roundabouts

require supplementary geometric

and traffic control device

treatments on the approaches.

Rural roundabouts that may

become part of an urbanized

area should include urban

roundabout design features.
Roundabouts: An Informational Guide • 1: Introduction 19

1.6.7 Rural double-lane roundabouts

Rural double-lane roundabouts have speed characteristics similar to rural singlelane roundabouts with average approach speeds in the range of 80 to 100 km/h (50

to 60 mph). They differ in having two entry lanes, or entries flared from one to two

lanes, on one or more approaches. Consequently, many of the characteristics and

design features of rural double-lane roundabouts mirror those of their urban counterparts. The main design differences are designs with higher entry speeds and

larger diameters, and recommended supplementary approach treatments. The

design of these roundabouts is based on the methods used by the United Kingdom, Australia, and France. Exhibit 1-13 provides an example of a typical rural doublelane roundabout. Rural roundabouts that may one day become part of an urbanized

area should be designed for slower speeds, with design details that fully accommodate pedestrians and bicyclists. However, in the interim they should be designed with approach and entry features to achieve safe speed reduction.

Exhibit 1-13. Typical rural

double-lane roundabout.

Rural double-lane roundabouts

have higher entry speeds and

larger diameters than their

urban counterparts.
20 Federal Highway Administration

1.7 References

1. Brown, M. TRL State of the Art Review—The Design of Roundabouts. London:

HMSO, 1995.

2. Todd, K. “A history of roundabouts in Britain.” Transportation Quarterly, Vol. 45,

No. 1, January 1991.

3. Jacquemart, G. Synthesis of Highway Practice 264: Modern Roundabout Practice in the United States. National Cooperative Highway Research Program. Washington, D.C: National Academy Press, 1998.

4. American Association of State Highway and Transportation Officials (AASHTO).

A Policy on Geometric Design of Highways and Streets. Washington, D.C.:

AASHTO, 1994.
Roundabouts: An Informational Guide • 2: Policy Considerations 21

Policy Considerations

2.1 Characteristics 23

2.1.1 Safety 23

2.1.2 Vehicle delay and queue storage 28

2.1.3 Delay of major movements 28

2.1.4 Signal progression 29

2.1.5 Environmental factors 29

2.1.6 Spatial requirements 29

2.1.7 Operation and maintenance costs 30

2.1.8 Traffic calming 30

2.1.9 Aesthetics 30

2.1.10 Design for older drivers 31

2.2 Multimodal Considerations 32

2.2.1 Pedestrians 32

2.2.2 Bicycles 34

2.2.3 Large vehicles 34

2.2.4 Transit 35

2.2.5 Emergency vehicles 35

2.2.6 Rail crossings 35

2.3 Costs Associated with Roundabouts 36

2.4 Legal Considerations 37

2.4.1 Definition of “intersection” 37

2.4.2 Right-of-way between vehicles 38

2.4.3 Required lane position at intersections 38

2.4.4 Priority within the circulatory roadway 38

2.4.5 Pedestrian accessibility 39

2.4.6 Parking 40

2
22 Federal Highway Administration

2.5 Public Involvement 40

2.5.1 Public meetings 41

2.5.2 Informational brochures 41

2.5.3 Informational videos 43

2.5.4 Media announcements 43

2.6 Education 43

2.6.1 Driver education 43

2.6.2 Bicyclist education 47

2.6.3 Pedestrian education 47

2.7 References 48

Exhibit 2-1. Average annual crash frequencies at 11 23

U.S. intersections converted to roundabouts.

Exhibit 2-2. Pedestrian’s chances of death if hit by a motor vehicle. 25

Exhibit 2-3. Comparisons of vehicle-vehicle conflict points for 26

intersections with four single-lane approaches.

Exhibit 2-4. Fastest vehicle path through a double-lane roundabout. 27

Exhibit 2-5. Examples of aesthetic treatments. 31

Exhibit 2-6. Examples of informational brochures. 42

Exhibit 2-7. Driving straight through a roundabout. 45

Exhibit 2-8. Turning left at a roundabout. 46
Roundabouts: An Informational Guide • 2: Policy Considerations 23

Roundabouts have unique characteristics that warrant consideration by developers

and managers of the road system. This chapter provides a general overview of the

characteristics of roundabouts and policy considerations pertaining to them. The

information may be useful to policy makers and the general public. The reader is

encouraged to refer to later chapters on the specifics associated with planning,

operation, safety, and design of roundabouts.

2.1 Characteristics

The previous chapter described the physical features of a roundabout. This section

describes performance characteristics that need to be considered, either at a policy

level when introducing roundabouts into a region or at specific locations where a

roundabout is one of the alternatives being considered.

2.1.1 Safety

This section provides an overview of the safety performance of roundabouts and

then discusses the general characteristics that lead to this performance. It does

not attempt to discuss all of the issues related to safety; the reader is encouraged

to refer to Chapter 5 for a more detailed discussion.

Roundabouts are generally safer than other forms of intersection in terms of aggregate crash statistics for low and medium traffic capacity conditions (1). Injury

crash rates for motor vehicle occupants are generally lower, although the proportion of single-vehicle crashes is typically higher. However, bicyclists and pedestrians are involved in a relatively higher proportion of injury accidents than they are at

other intersections (2).

Exhibit 2-1 presents comparisons of before and after aggregate crash frequencies

(average annual crashes per roundabout) involving users of eleven roundabouts

constructed in the United States (3). The decrease in severe injury crashes is noteworthy. However, the “before” situation at these intersections required mitigation

for safety. Therefore, some other feasible alternatives may also be expected to

have resulted in a reduction in the crash frequencies. This study yielded insufficient

data to draw conclusions regarding the safety of bicyclists and pedestrians.

Type of Before roundabout Roundabout Percent change

roundabout Sites Total Inj.3 PDO4 Total Inj. PDO Total Inj. PDO

Single-Lane1 8 4.8 2.0 2.4 2.4 0.5 1.6 -51% -73% -32%

Multilane2 3 21.5 5.8 15.7 15.3 4.0 11.3 -29% -31% -10%

Total 11 9.3 3.0 6.0 5.9 1.5 4.2 -37% -51% -29%

Notes:

1. Mostly single-lane roundabouts with an inscribed circle diameter of 30 to 35 m (100 to 115 ft).

2. Multilane roundabouts with an inscribed circle diameter greater than 50 m (165 ft).

3. Inj. = Injury crashes.

4. PDO = Property Damage Only crashes.

Source: (3)

Roundabouts have been

demonstrated to be generally safer

for motor vehicles and pedestrians

than other forms of at-grade

intersections.

Exhibit 2-1. Average annual

crash frequencies at 11 U.S.

intersections converted to

roundabouts.

Chapter 2 Policy Considerations
24 Federal Highway Administration

Good roundabout design places a high priority on speed reduction and speed consistency. Such designs require that vehicles negotiate the roundabout through a

series of turning maneuvers at low speeds, generally less than 30 km/h (20 mph).

Speed consistency refers to the design objective of slowing vehicles in stages

down to the desired negotiating speed to be consistent with the expectations of

drivers. Speed control is provided by geometric features, not only by traffic control

devices or by the impedance of other traffic. Because of this, speed reduction can

be achieved at all times of day. If achieved by good design, then in principle, lower

vehicle speeds should provide the following safety benefits:

• Reduce crash severity for pedestrians and bicyclists, including older pedestrians, children, and impaired persons;

• Provide more time for entering drivers to judge, adjust speed for, and enter a

gap in circulating traffic;

• Allow safer merges into circulating traffic;

• Provide more time for all users to detect and correct for their mistakes or mistakes of others;

• Make collisions less frequent and less severe; and

• Make the intersection safer for novice users.

For example, Exhibit 2-2 shows that a pedestrian is about three times more likely

to die when struck at 50 km/h (30 mph) than at 32 km/h (20 mph), across a range of

only 18 km/h (10 mph) difference in speed (4). Typical commuter bicyclist speeds

are in the range of 20 to 25 km/h (12 to 15 mph). Therefore, the difference in design

speed is critical to all users who are not within the protective body of a motorized

vehicle. The minor additional delay or inconvenience to drivers of lower-speed roundabout designs (as compared to higher-speed roundabout designs) is a tradeoff for

the substantial safety benefit to pedestrians and bicyclists. Older drivers may benefit from the additional time to perceive, think, react, and correct for errors (as may

all users). It should be clarified that there has been no specific research performed

on older drivers, older pedestrians, and older bicyclists at roundabouts. It should

also be noted that visually impaired pedestrians are not provided the audible cues

from vehicle streams that are available at a signal controlled intersection. For example, at roundabout exits, it may be difficult to discern the sound of vehicles

which will continue to circulate from those exiting the roundabout. Therefore, information needs to be provided to these users through various appropriate design

features to assist them in safely locating and navigating the crossings at roundabouts.

Furthermore, the operational efficiency (capacity) of roundabouts is probably greater

at lower circulating speed, because of these two phenomena:

• The faster the circulating traffic, the larger the gaps that entering traffic will

comfortably accept. This translates to fewer acceptable gaps and therefore more

instances of entering vehicles stopping at the yield line.

• Entering traffic, which is first stopped at the yield line, requires even larger gaps

in the circulating traffic in order to accelerate and merge with the circulating

traffic. The faster the circulating traffic, the larger this gap must be. This translates into even fewer acceptable gaps and therefore longer delays for entering

traffic.

Good roundabout designs

encourage speed reduction

and speed consistency.

Potential safety benefits of low

vehicle speeds.

Visually impaired pedestrians

are not provided with audible

cues from vehicle streams.

Lower circulating speeds can

provide greater capacity.
Roundabouts: An Informational Guide • 2: Policy Considerations 25

2.1.1.1 Single-lane roundabouts

The safety characteristics of single-lane and multilane roundabouts are somewhat

different and are discussed separately. Single-lane roundabouts are the simplest

form of roundabout and thus are a good starting point for discussing the safety

characteristics of roundabouts relative to other forms of intersections.

The frequency of crashes at an intersection is related to the number of conflict

points at an intersection, as well as the magnitude of conflicting flows at each

conflict point. A conflict point is a location where the paths of two vehicles, or a

vehicle and a bicycle or pedestrian diverge, merge, or cross each other. For example, Exhibit 2-3 presents a diagram of vehicle-vehicle conflict points for a traditional four-leg intersection and a four-leg roundabout intersection of two-lane roads.

The number of vehicle-vehicle conflict points for four-leg intersections drops from

thirty-two to eight with roundabouts, a 75 percent decrease. Fewer conflict points

means fewer opportunities for collisions. These are not the only conflict points at

roundabouts or traditional intersections, but are illustrative of the differences between intersection types. Chapter 5 contains a more detailed comparison of conflicts at more complex intersections and for pedestrians and bicyclists.

The severity of a collision is determined largely by the speed of impact and the

angle of impact. The higher the speed, the more severe the collision. The higher the

angle of impact, the more severe the collision. Roundabouts reduce in severity or

eliminate many severe conflicts that are present in traditional intersections.

Exhibit 2-2. Pedestrian’s

chances of death if hit by a

motor vehicle.

Source: United Kingdom (4)

Roundabouts bring the simplicity of

a “T” intersection to intersections

with more than three legs.

A four-leg intersection has 75 percent

fewer conflicts between vehicles and

pedestrians and other vehicles,

compared to a conventional four-leg

intersection.

See Chapter 5 for a comparison of

intersection conflicts.
26 Federal Highway Administration

As Exhibit 2-3 shows, a roundabout eliminates vehicle-vehicle crossing conflicts by

converting all movements to right turns. Separate turn lanes and traffic control

(stop signs or signalization) can often reduce but not eliminate the number of crossing

conflicts at a traditional intersection by separating conflicts in space and/or time.

However, the most severe crashes at signalized intersections occur when there is

a violation of the traffic control device designed to separate conflicts by time (e.g.,

a right-angle collision due to a motorist running a red light, or vehicle-pedestrian

collisions). The ability of roundabouts to reduce conflicts through physical, geometric features has been demonstrated to be more effective than the reliance on driver

obedience to traffic control devices. At intersections with more than four legs, a

roundabout or pair of roundabouts may sometimes be the most practical alternative to minimize the number of conflicts.

Drivers approaching a single-lane roundabout have five basic decisions regarding

other users. First, drivers must be mindful of any bicyclists merging into motor

vehicle traffic from the right side of the road or a bicycle lane or shoulder. Then they

must yield to any pedestrians crossing at the entry. Third, they must choose an

acceptable gap in which to enter the roundabout. Then they must choose the correct exit, and finally, they must yield to any pedestrians crossing the exit lane.

By contrast, a driver making a left turn from the minor leg of a two-way stopcontrolled intersection has to yield to pedestrians and bicyclists, and judge gaps in

both of the major street through movements from both directions, as well as the

major street left and right turns and opposing minor through and right turns.

Signalized intersections have simplified the decision-making process for drivers,

especially at locations where protected left-turn phasing is provided, by separating

conflicts in time and space. However, the rules and driver decisions for negotiating

signalized intersections are still quite complex when all the possible signal phasing

schemes are accounted for. For signals with permitted left-turn phasing, the driver

Exhibit 2-3. Comparisons of

vehicle-vehicle conflict points for

intersections with four single-lane

approaches.

g r

r

r

Roundabouts eliminate crossing

conflicts by converting all movements

to right turns.

Types of intersection

conflicts.
Roundabouts: An Informational Guide • 2: Policy Considerations 27

must be cognizant of the opposing traffic including pedestrians, and the signal

indication (to ensure a legal maneuver). At roundabouts, once at the yield line, the

entering driver can focus attention entirely on the circulating traffic stream approaching from the left. A driver behind the entering driver can focus entirely on crossing

pedestrians.

2.1.1.2 Double-lane roundabouts

As discussed in Chapter 1, double-lane roundabouts are those with at least one

entry that has two lanes. In general, double-lane roundabouts have some of the

same safety characteristics for vehicle occupants as their less complicated singlelane counterparts. However, due to the presence of multiple entry lanes and the

accompanying need to provide wider circulatory and exit roadways, double-lane

roundabouts have complications that result in poorer safety characteristics, particularly for bicyclists and pedestrians, than single-lane roundabouts serving similar traffic demands. This makes it important to use the minimum number of entry,

circulating, and exit lanes, subject to capacity considerations.

Due to their typically larger size compared to single-lane roundabouts, double-lane

roundabouts often cannot achieve the same levels of speed reduction as their singlelane counterparts. Wider entering, circulating, and exiting roadways enable a vehicle to select a path that crosses multiple lanes, as shown in Exhibit 2-4. Because

of the higher-speed geometry, single-vehicle accidents can be more severe. However, design of double-lane roundabouts according to the procedures in Chapter 6,

especially the approach and entry, can substantially reduce the speeds of entering

vehicles and consequently reduce the severity of conflicts. Even so, speed control

cannot occur to the extent possible with single-lane roundabouts.

Pedestrians crossing double-lane roundabouts are exposed for a longer time and

to faster vehicles. They can also be obscured from, or not see, approaching vehicles in adjacent lanes if vehicles in the nearest lane yield to them. Children, wheelExhibit 2-4. Fastest vehicle

path through a double-lane

roundabout.

Increasing the number of lanes

increases the number of conflict

points.

The design of double-lane

roundabouts to control the

speed of the fastest vehicle path

is covered in Chapter 6.
28 Federal Highway Administration

chair users, and visually impaired pedestrians face particular risks. Bicycles

are also more exposed to severe conflicts when choosing to circulate with

motor vehicles.

Driver decisions are more complex at double-lane roundabouts. The requirement

to yield to pedestrians still applies. The primary additional decisions are the choices

of the proper lane for entering, lateral position for circulating, and proper lane for

exiting the roundabout. Lane choice on approaching a double-lane roundabout is no

different from approaching a signalized intersection: to turn left, stay left; to turn

right, stay right. However, the decisions for circulating within and especially exiting

a double-lane roundabout are unique.

Double-lane roundabouts with legs aligned at approximately 90-degree angles allow motorists to determine the appropriate lane choice for their path through the

roundabout in a relatively easy manner. Double-lane roundabouts with more than

four legs and/or with legs aligned at angles significantly different from 90 degrees

make driver decisions more complicated. This occurs because it can be difficult on

some legs to determine which movements are left, through, and right. For this reason, it is desirable that multilane roundabouts be limited to a maximum of four legs,

with legs aligned at approximately 90-degree angles. If this is not possible, special

advance guide signs showing appropriate lane choice should be considered.

When double-lane roundabouts are first introduced to an area, there is a need for

adequate user education. Recommendations for user education material specifically related to this issue are presented later in this chapter.

2.1.2 Vehicle delay and queue storage

When operating within their capacity, roundabout intersections typically operate

with lower vehicle delays than other intersection forms and control types. With a

roundabout, it is unnecessary for traffic to come to a complete stop when no conflicts present themselves, or else deceleration will avoid a conflict. When there are

queues on one or more approaches, traffic within the queues usually continues to

move, and this is typically more tolerable to drivers than a stopped or standing

queue. The performance of roundabouts during off-peak periods is particularly good

in contrast to other intersection forms, typically with very low average delays.

2.1.3 Delay of major movements

Roundabouts tend to treat all movements at an intersection equally. Each approach

is required to yield to circulating traffic, regardless of whether the approach is a

local street or major arterial. In other words, all movements are given equal priority.

This may result in more delay to the major movements than might otherwise be

desired. This problem is most acute at the intersection of high-volume major streets

with low- to medium-volume minor streets (e.g., major arterial streets with minor

collectors or local streets). Therefore, the overall street classification system and

hierarchy should be considered before selecting a roundabout (or stop-controlled)

intersection. This limitation should be specifically considered on emergency response routes in comparison with other intersection types and control. The delays

depend on the volume of turning movements and should be analyzed individually

for each approach, according to the procedures in Chapter 4.

Double-lane roundabouts can be

confusing without proper engineering and user education.

Consider guide signs for roundabouts

with skewed approaches or

more than four legs.

Sections 2.5 and 2.6 cover user

education topics.

Techniques for estimating delay

are given in Chapter 4.

Since all intersection

 movements at a roundabout

have equal priority, major street

movements may be delayed

more than desired.
Roundabouts: An Informational Guide • 2: Policy Considerations 29

2.1.4 Signal progression

It is common practice to coordinate traffic signals on arterial roads to minimize

stops and delay to through traffic on the major road. By requiring coordinated platoons to yield to traffic in the circulatory roadway, the introduction of a roundabout

into a coordinated signal system may disperse and rearrange platoons of traffic if

other conflicting flows are significant, thereby reducing progressive movement. To

minimize overall system delay, it may be beneficial to divide the signal system into

subsystems separated by the roundabout, assigning each subsystem its own cycle.

The traffic performance of the combination roundabout-signal system should be

tested in advance with signal systems and roundabout analysis tools. In some

cases, total delay, stops, and queues will be reduced by the roundabout. The number of available gaps for midblock unsignalized intersections and driveways may

also be reduced by the introduction of roundabouts, although this may be offset by

the reduced speeds near roundabouts. In addition, roundabouts can enable safe

and quick U-turns that can substitute for more difficult midblock left turns, especially where there is no left turn lane.

2.1.5 Environmental factors

Roundabouts may provide environmental benefits if they reduce vehicle delay and

the number and duration of stops compared with an alternative. Even when there

are heavy volumes, vehicles continue to advance slowly in moving queues rather

than coming to a complete stop. This may reduce noise and air quality impacts and

fuel consumption significantly by reducing the number of acceleration/deceleration cycles and the time spent idling.

In general, if stop or yield control is insufficient, traffic through roundabouts generates less pollution and consumes less fuel than traffic at fixed-time signalized intersections. However, vehicle-actuated signals typically cause less delay, less fuel

consumption, and less emissions than roundabouts as long as traffic volumes are

low. During busy hours, vehicle-actuated signals tend to operate like fixed-time

signals, and the percentage of cars that must stop becomes high (5).

2.1.6 Spatial requirements

Roundabouts usually require more space for the circular roadway and central island than the rectangular space inside traditional intersections. Therefore, roundabouts often have a significant right-of-way impact on the corner properties at the

intersection, especially when compared with other forms of unsignalized intersection. The dimensions of a traditional intersection are typically comparable to the

envelope formed by the approaching roadways. However, to the extent that a comparable roundabout would outperform a signal in terms of reduced delay and thus

shorter queues, it will require less queue storage space on the approach legs. If a

signalized intersection requires long or multiple turn lanes to provide sufficient

capacity or storage, a roundabout with similar capacity may require less space on

the approaches. As a result, roundabouts may reduce the need for additional rightof-way on the links between intersections, at the expense of additional right-ofway requirements at the intersections themselves (refer to Chapters 3 and 8). The

right-of-way savings between intersections may make it feasible to accommodate

parking, wider sidewalks, planter strips, wider outside lanes, and/or bicycle lanes

in order to better accommodate pedestrians and/or bicyclists. Another space-saving strategy is the use of flared approach lanes to provide additional capacity at the
30 Federal Highway Administration

intersection while maintaining the benefit of reduced spatial requirements upstream

and downstream of an intersection.

At interchange ramp terminals, paired roundabouts have been used to reduce the

number of lanes in freeway over- and underpasses. In compact urban areas, there

are typically signalized intersections at both ends of overpass bridges, necessitating two additional overpass lanes to provide capacity and storage at the signalized

intersections.

2.1.7 Operation and maintenance costs

Compared to signalized intersections, a roundabout does not have signal equipment that requires constant power, periodic light bulb and detection maintenance,

and regular signal timing updates. Roundabouts, however, can have higher landscape maintenance costs, depending on the degree of landscaping provided on

the central island, splitter islands, and perimeter. Illumination costs for roundabouts

and signalized intersections are similar. Drivers sometimes face a confusing situation when they approach a signalized intersection during a power failure, but such

failures have minimal temporary effect on roundabouts or any other unsignalized

intersections, other than the possible loss of illumination. The service life of a roundabout is significantly longer, approximately 25 years, compared with 10 years for a

typical signal (6).

2.1.8 Traffic calming

Series of roundabouts can have secondary, traffic calming effects on streets by

reducing vehicle speeds. As discussed previously, speed reduction at roundabouts

is caused by geometry rather than by traffic control devices or traffic volume. Consequently, speed reduction can be realized at all times of day and on streets of any

traffic volume. It is difficult to speed through an appropriately designed roundabout

with raised channelization that forces vehicles to physically change direction. In

this way, roundabouts can complement other traffic calming measures.

Roundabouts have also been used successfully at the interface between rural and

urban areas where speed limits change. In these applications, the traffic calming

effects of roundabouts force drivers to slow and reinforce the notion of a significant change in the driving environment.

2.1.9 Aesthetics

Roundabouts offer the opportunity to provide attractive entries or centerpieces to

communities. However, hard objects in the central island directly facing the entries

are a safety hazard. The portions of the central island and, to a lesser degree, the

splitter islands that are not subject to sight-distance requirements offer opportunities for aesthetic landscaping. Pavement textures can be varied on the aprons as

well. Exhibit 2-5 presents examples of the aesthetic treatments that have been

applied to roundabouts. They can also be used in tourist or shopping areas to facilitate safe U-turns and to demarcate commercial uses from residential areas. They

have been justified as a spur to economic development, conveying to developers

that the area is favorable for investment in redevelopment. Some are exhibited as

a “signature” feature on community postcards, advertisements, and travelogues.

By reducing speeds,

roundabouts complement

other traffic calming

measures.

Landscaping issues are

discussed in detail in

Chapter 7.
Roundabouts: An Informational Guide • 2: Policy Considerations 31

2.1.10 Design for older drivers

In the United States, there is a trend toward an aging population, as well as individuals, continuing to drive until an older age. This trend has implications for all

roadway design, including roundabout design, ranging from operations through

geometric and sign design. In this regard, designers should consult available documents such as the Federal Highway Administration (FHWA) Older Driver Highway

Design Handbook (7):

• The single greatest concern in accommodating older road users, both drivers

and pedestrians, is the ability of these persons to safely maneuver through

intersections.

• Driving situations involving complex speed-distance judgments under time constraints are more problematic for older drivers and pedestrians than for their

younger counterparts.

• Older drivers are much more likely to be involved in crashes where the drivers

were driving too fast for the curve or, more significantly, were surprised by the

curved alignment.

• Many studies have shown that loss-of-control crashes result from an inability to

maintain lateral position through the curve because of excessive speed, with

inadequate deceleration in the approach zone. These problems in turn stem

from a combination of factors, including poor anticipation of vehicle control requirements, induced by the driver’s prior speed, and inadequate perception of

the demands of the curve.

• Older drivers have difficulties in allocating attention to the most relevant aspects of novel driving situations.

• Older drivers generally need more time than average drivers to react to events.

Exhibit 2-5. Examples

of aesthetic treatments.

(a) West Boca Raton, FL (b) Santa Barbara, CA

(c) Fort Pierce, FL (d) Vail, CO
32 Federal Highway Administration

While the Handbook is not specific to roundabouts, and since no age-related research has been conducted with U.S. roundabouts to date, these findings may

apply to older persons encountering roundabouts, as well. The excerpts above all

imply that lower, more conservative design speeds are appropriate. Roundabouts

designed for low, consistent speeds cater to the preferences of older drivers: slower

speeds; time to make decisions, act, and react; uncomplicated situations to interpret; simple decision-making; a reduced need to look over one’s shoulder; a reduced need to judge closing speeds of fast traffic accurately; and a reduced need

to judge gaps in fast traffic accurately. For example, two-way stop-controlled intersections may be appropriate for replacement with a roundabout when a crash

analysis indicates that age-related collisions are prevalent.

2.2 Multimodal Considerations

As with any intersection design, each transportation mode present requires careful consideration. This section presents some of the general issues associated

with each mode; additional detail on mode-specific safety and design issues is

provided in subsequent chapters.

2.2.1 Pedestrians

Pedestrians are accommodated by crossings around the perimeter of the roundabout. By providing space to pause on the splitter island, pedestrians can consider

one direction of conflicting traffic at a time, which simplifies the task of crossing

the street. The roundabout should be designed to discourage pedestrians from

crossing to the central island, e.g., with landscape buffers on the corners. Pedestrian crossings are set back from the yield line by one or more vehicle lengths to:

• Shorten the crossing distance compared to locations adjacent to the inscribed

circle;

• Separate vehicle-vehicle and vehicle-pedestrian conflict points; and

• Allow the second entering driver to devote full attention to crossing pedestrians

while waiting for the driver ahead to enter the circulatory roadway.

If sidewalks on the intersecting roads are adjacent to the curbs, this setback may

require the sidewalks to deviate from a straight path. This is not the case if sidewalks are separated from the curbs by a generous landscape buffer.

Most intersections are two-way stop-controlled, or uncontrolled. Compared to twoway stop-controlled intersections, roundabouts may make it easier and safer for

pedestrians to cross the major street. At both roundabouts and two-way stopcontrolled intersections, pedestrians have to judge gaps in the major (uncontrolled)

stream of traffic. By reducing stopping distance, the low vehicular speeds through

a roundabout generally reduce the frequency and severity of incidents involving

pedestrians. In addition, when crossing an exit lane on the minor road, the sight

angle is smaller than when watching for left-turning vehicles at a conventional intersection.

The comparison between roundabouts and all-way stop-controlled intersections is

less clear. All-way stop control is virtually nonexistent in foreign countries that have

Pedestrian crossings should be set

back from the yield line by one or

more vehicle lengths.
Roundabouts: An Informational Guide • 2: Policy Considerations 33

roundabouts, and so there is little international experience with which to compare.

All-way stop-controlled intersections may be preferred by pedestrians with visual

impairment because vehicles are required to stop before they enter the intersection. However, crossing the exit leg of an all-way stop-controlled intersection can

be intimidating for a pedestrian since traffic may be turning onto the exit from

multiple directions. Roundabouts, on the other hand, allow pedestrians to cross

one direction of traffic at a time; however, traffic may be moving (albeit at a slow

speed), thus making it more challenging to judge gaps, especially for visually impaired users, children, and the elderly.

The biggest difference may be that all-way stop-controlled intersections, like twoway stops, do not provide positive geometric features to slow vehicles and instead

rely entirely on the authority of the traffic control device. The roundabout geometry

physically slows and deflects vehicles, reducing the likelihood of a high-speed collision due to a traffic control device violation.

Signalized intersections offer positive guidance to pedestrians by providing visual

and occasionally audible pedestrian signal indications. In this respect, the decision

process for pedestrians requires less judgment at signalized intersections than at

roundabouts, particularly for visually impaired and elderly pedestrians. However,

pedestrians are still vulnerable at signalized intersections to right-turn and left-turn

movements unprotected by a green arrow. In addition, high-speed collisions are

still possible if a vehicle runs through a red indication. In this respect, the roundabout provides a speed-constrained environment for through traffic. At two-way

and all-way stop intersections, right-turning motorists often look only to the left in

order to check for vehicular conflicts, endangering or inconveniencing pedestrians

crossing from the right or on the right. This situation is exacerbated by the fact that

many of these drivers do not come to a complete stop if they do not perceive any

conflicts. With crosswalks located back from the circulatory roadway, roundabouts

place pedestrians in a more visible location.

The two populations at opposite ends of the age continuum—children and the

elderly—and people with disabilities are particularly at risk at intersections. Children (owing to their lack of traffic experience, impulsiveness, and small size) and

the elderly (owing to their age-related physical limitations) present challenges to

the designer. In recognition of pedestrians with disabilities, intersections must comply with Americans with Disabilities Act (ADA) mandated accessibility standards

discussed in Section 2.4.5 and Chapter 5.

Elderly pedestrians, children, and the disabled find it more difficult to cross unprotected road crossings. These types of pedestrians generally prefer larger gaps in

the traffic stream, and walk at slower speeds than other pedestrians. Multilane

roadways entering and exiting double-lane roundabouts require additional skills to

cross, since pedestrians need assurance that they have been seen by drivers in

each lane they are crossing.

When crossing a roundabout, there are several areas of difficulty for the blind and

or visually impaired pedestrian. It is expected that a visually impaired pedestrian

with good travel skills must be able to arrive at an unfamiliar intersection and cross

it with pre-existing skills and without special, intersection-specific training. Roundabouts pose problems at several points of the crossing experience, from the perspective of information access.

When crossing a roundabout, there

are several areas of difficulty for the

blind and or visually impaired

pedestrian.
34 Federal Highway Administration

Unless these issues are addressed by a design, the intersection is “inaccessible”

and may not be permissible under the ADA. Chapters 5, 6, and 7 provide specific

suggestions to assist in providing the above information. However, more research

is required to develop the information jurisdictions need to determine where roundabouts may be appropriate and what design features may be appropriate for the

disabled, such as audible signalized crossings. Until specific standards are adopted,

engineers and jurisdictions must rely on existing related research and professional

judgment to design pedestrian features so that they are usable by pedestrians with

disabilities.

2.2.2 Bicycles

Roundabouts may not provide safety benefits to bicyclists (1). Nevertheless, the

recommended roundabout designs discourage erratic or undesirable driver behavior. They slow drivers to speeds more compatible with bicycle speeds, while reducing high-speed conflicts and simplifying turn movements for bicyclists. Typical commuter bicyclist speeds are around 25 km/h (15 mph), so entering a roundabout

designed for circulating traffic to flow at similar speeds should be safer compared

with larger and faster roundabout designs. Bicyclists require particular attention in

two-lane roundabout design, especially in areas with moderate to heavy bicycle

traffic.

As with pedestrians, one of the difficulties in accommodating bicyclists is their

wide range of skills and comfort levels in mixed traffic. On single-lane roundabouts,

bicyclists have the option of either mixing with traffic or using the roundabout like

a pedestrian. The former option will likely be reasonably comfortable for experienced cyclists; however, less-experienced cyclists (including children) may have

difficulty and discomfort mixing with vehicles and are more safely accommodated

as pedestrians.

The complexity of vehicle interactions within a roundabout leaves a cyclist vulnerable, and for this reason, bike lanes within the circulatory roadway should never be

used. On double-lane roundabouts, a bicycle path separate and distinct from the

circulatory roadway is preferable, such as a shared bicycle-pedestrian path of sufficient width and appropriately marked to accommodate both types of users around

the perimeter of the roundabout. While this will likely be more comfortable for the

casual cyclist, the experienced commuter cyclist will be significantly slowed down

by having to cross as a pedestrian at each approach crossing and may choose to

continue to traverse a double-lane roundabout as a vehicle. It may sometimes be

possible to provide cyclists with an alternative route along another street or path

that avoids the roundabout, which should be considered as part of overall network

planning. The provision of alternative routes should not be used to justify compromising the safety of bicycle traffic through the roundabout because experienced

cyclists and those with immediately adjacent destinations will use it.

2.2.3 Large vehicles

Roundabouts should always be designed for the largest vehicle that can be reasonably anticipated (the “design vehicle”). For single-lane roundabouts, this may require the use of a mountable apron around the perimeter of the central island to

provide the additional width needed for tracking the trailer wheels. At double-lane

roundabouts, large vehicles may track across the whole width of the circulatory

roadway to negotiate the roundabout. In some cases, roundabouts have been

Bike lanes through

roundabouts should

never be used.

Design roundabouts to

accommodate the largest

vehicle that can reasonably

be expected.
Roundabouts: An Informational Guide • 2: Policy Considerations 35

designed with aprons or gated roadways through the central island to accommodate oversized trucks, emergency vehicles, or trains.

2.2.4 Transit

Transit considerations at a roundabout are similar to those at a conventional intersection. If the roundabout has been designed using the appropriate design vehicle,

a bus should have no physical difficulty negotiating the intersection. To minimize

passenger discomfort, if the roundabout is on a bus route, it is preferable that

scheduled buses are not required to use a truck apron if present. Bus stops should

be located carefully to minimize the probability of vehicle queues spilling back into

the circulatory roadway. This typically means that bus stops located on the far side

of the intersection need to have pullouts or be further downstream than the splitter

island. Pedestrian access routes to transit should be designed for safety, comfort,

and convenience. If demand is significant, such as near a station or terminus, pedestrian crossing capacity should be accounted for.

Roundabouts may provide opportunities for giving transit (including rail) and emergency vehicles priority as can be done at signalized intersections. This may be

provided using geometry, or signals. For example, these could include an exclusive

right-turn bypass lane or signals holding entering traffic while the transit vehicle

enters its own right-of-way or mixed traffic. The roundabout can be supplemented

by signals activated by a transit, emergency, or rail vehicle. Chapters 6, 7, and 8

provide more detail on transit treatments.

2.2.5 Emergency vehicles

The passage of large emergency vehicles through a roundabout is the same as for

other large vehicles and may require use of a mountable apron. On emergency

response routes, the delay for the relevant movements at a planned roundabout

should be compared with alternative intersection types and control. Just as they

are required to do at conventional intersections, drivers should be educated not to

enter a roundabout when an emergency vehicle is approaching on another leg.

Once having entered, they should clear out of the circulatory roadway if possible,

facilitating queue clearance in front of the emergency vehicle.

Roundabouts provide emergency vehicles the benefit of lower vehicle speeds,

which may make roundabouts safer for them to negotiate than signalized crossings. Unlike at signalized intersections, emergency vehicle drivers are not faced

with through vehicles unexpectedly running the intersection and hitting them at

high speed.

2.2.6 Rail crossings

Rail crossings through or near a roundabout may involve many of the same design

challenges as at other intersections and should be avoided if better alternatives

exist. In retrofit, the rail track may be designed to pass through the central island,

or across one of the legs. Queues spilling back from a rail blockage into the roundabout can fill the circulatory roadway and temporarily prevent movement on any

approach. However, to the extent that a roundabout approach capacity exceeds

that of a signal at the same location, queues will dissipate faster. Therefore, a casespecific capacity and safety analysis is recommended. Section 8.2 addresses the

design of at-grade rail crossings.

Public transit buses should not be

forced to use a truck apron to

negotiate a roundabout.

See Section 8.2 for information on

designing roundabouts located near

at-grade rail crossings.

Chapters 6-8 provide more

detail on transit treatments.
36 Federal Highway Administration

2.3 Costs Associated with Roundabouts

Many factors influence the amount of economic investment justified for any type

of intersection. Costs associated with roundabouts include construction costs,

engineering and design fees, land acquisition, and maintenance costs. Benefits

may include reduced crash rates and severity, reduced delay, stops, fuel consumption, and emissions. Benefit-cost analysis is discussed further in Chapter 3.

When comparing costs, it is often difficult to separate the actual intersection costs

from an overall improvement project. Accordingly, the reported costs of installing

roundabouts have been shown to vary significantly from site to site. A roundabout

may cost more or less than a traffic signal, depending on the amount of new pavement area and the extent of other roadway work required. At some existing

unsignalized intersections, a traffic signal can be installed without significant modifications to the pavement area or curbs. In these instances, a roundabout is likely

to be more costly to install than a traffic signal, as the roundabout can rarely be

constructed without significant pavement and curb modifications.

However, at new sites, and at signalized intersections that require widening at one

or more approaches to provide additional turn lanes, a roundabout can be a comparable or less expensive alternative. While roundabouts typically require more pavement area at the intersection, they may require less pavement width on the upstream approaches and downstream exits if multiple turn lanes associated with a

signalized intersection can be avoided. The cost savings of reduced approach roadway widths is particularly advantageous at interchange ramp terminals and other

intersections adjacent to grade separations where wider roads may result in larger

bridge structures. In most cases, except potentially for a mini-roundabout, a roundabout is more expensive to construct than the two-way or all-way stop-controlled

intersection alternatives.

Recent roundabout projects in the United States have shown a wide range in reported construction costs. Assuming “1998 U.S. Dollars” in the following examples,

costs ranged from $10,000 for a retrofit application of an existing traffic circle to

$500,000 for a new roundabout at the junction of two State highways. National

Cooperative Highway Research Program (NCHRP) Synthesis 264 (3) reports that

the average construction cost of 14 U.S. roundabouts, none being part of an interchange, was approximately $250,000. This amount includes all construction elements, but does not include land acquisition.

Higher costs are typically incurred when a substantial amount of realignment, grading, or drainage work is required. The cost of maintaining traffic during construction

tends to be relatively high for retrofitting roundabouts. This expense is due mainly

to the measures required to maintain existing traffic flow through the intersection

while rebuilding it in stages. Other factors contributing to high roundabout costs

are large amounts of landscaping in the central and splitter islands, extensive signing and lighting, and the provision of curbs on all outside pavement edges.

Operating and maintenance costs of roundabouts are somewhat higher than for

other unsignalized intersections, but less than those for signalized intersections. In

addition, traffic signals consume electricity and require periodic service (e.g., bulb

replacement, detector replacement, and periodic signal retiming). Operating costs

for a roundabout are generally limited to the cost of illumination (similar to signalized

alternatives, but typically more than is required for other unsignalized intersections).

Roundabouts may require more

pavement area at the intersection,

compared to a traffic signal, but

less on the approaches and exits.

The cost of maintaining traffic during

construction of a roundabout retrofit

can be relatively high.
Roundabouts: An Informational Guide • 2: Policy Considerations 37

Maintenance includes regular restriping and repaving as necessary, as well as snow

removal and storage in cold climates (these costs are also incurred by conventional

intersections). Landscaping may require regular maintenance as well, including

such things as pruning, mowing, and irrigation system maintenance. To the extent

that roundabouts reduce crashes compared with conventional intersections, they

will reduce the number and severity of incidents that disrupt traffic flow and that

may require emergency service.

2.4 Legal Considerations

The legal environment in which roundabouts operate is an important area for jurisdictions to consider when developing a roundabout program or set of guidelines.

The rules of the road that govern the operation of motor vehicles in a given State

can have a significant influence on the way a roundabout operates and on how

legal issues such as crashes involving roundabouts are handled. Local jurisdictions

that are interested in developing a roundabout program need to be aware of the

governing State regulations in effect. The following sections discuss several of the

important legal issues that should be considered. These have been based on the

provisions of the 1992 Uniform Vehicle Code (UVC) (8), which has been adopted to

varying degrees by each State, as well as the rules of the road, and commentaries

thereof, from the United Kingdom (9) and Australia (10, 11). Note that the information in the following sections does not constitute specific legal opinion; each jurisdiction should consult with its attorneys on specific legal issues.

2.4.1 Definition of “intersection”

The central legal issue around which all other issues are derived is the fundamental

relationship between a roundabout and the legal definition of an “intersection.” A

roundabout could be legally defined one of two ways:

• As a single intersection; or

• As a series of T-intersections.

The UVC does not provide clear guidance on the appropriate definition of an intersection with respect to roundabouts. The UVC generally defines an “intersection”

as the area bounded by the projection of the boundary lines of the approaching

roadways (UVC §1-132a). It also specifies that where a highway includes two roadways 9.1 m (30 ft) or more apart, each crossing shall be regarded as a separate

intersection (UVC §1-132b). This may imply that most circular intersections should

be regarded as a series of T-intersections. This distinction has ramifications in the

interpretation of the other elements identified in this section.

This guide recommends that a roundabout be specifically defined as a single intersection, regardless of the size of the roundabout. This intersection should be defined as the area bounded by the limits of the pedestrian crossing areas around the

perimeter of a single central island. Closely spaced roundabouts with multiple central islands should be defined as separate intersections, as each roundabout is

typically designed to operate independently.

It is recommended that roundabouts

be defined as a single intersection:

the area bounded by the limits of the

pedestrian crossing areas.
38 Federal Highway Administration

2.4.2 Right-of-way between vehicles

The UVC specifies that “when two vehicles approach or enter an intersection from

different highways at approximately the same time, the driver of the vehicle on the

left shall yield the right-of-way to the vehicle on the right” (UVC §11-401). This runs

contrary to the default operation of a roundabout, which assigns the right-of-way to

the vehicle on the left and any vehicle in front. This requires the use of yield signs

and yield lines at all approaches to a roundabout to clearly define right-of-way.

This guide recommends that right-of-way at a roundabout be legally defined such

that an entering vehicle shall yield the right-of-way to the vehicle on the left (France

passed such a law in 1984). This definition does not change the recommendation

for appropriately placed yield signs and yield lines.

2.4.3 Required lane position at intersections

At a typical intersection with multilane approaches, vehicles are required by the

UVC to use the right-most lane to turn right and the left-most lane to turn left,

unless specifically signed or marked lanes allow otherwise (e.g., double left-turn

lanes) (UVC §11-601). Because multilane roundabouts can be used at intersections

with more than four legs, the concept of “left turns” and “right turns” becomes

more difficult to legally define. The following language (10) is recommended:

Unless official traffic control devices indicate otherwise, drivers must make lane

choices according to the following rules:

• If a driver intends to exit the roundabout less than halfway around it, the right

lane must be used.

• If a driver intends to exit the roundabout more than halfway around it, the left

lane must be used.

The Australian Traffic Act (10) gives no guidance for straight through movements

(movements leaving the roundabout exactly halfway), and the general Australian

practice is to allow drivers to use either lane unless signed or marked otherwise.

On multilane roundabouts where the intersecting roadways are not at 90-degree

angles or there are more than four legs to the roundabout, special consideration

should be given to assisting driver understanding through advance diagrammatic

guide signs or lane markings on approaches showing the appropriate lane choices.

2.4.4 Priority within the circulatory roadway

For multilane roundabouts, the issue of priority within the circulatory roadway is

important. Any vehicle on the inner track on the circulatory roadway (e.g., a vehicle

making a left turn) will ultimately cross the outer track of the circulatory roadway to

exit. This may cause conflicts with other vehicles in the circulatory roadway.

Consistent with its lack of treatment of roundabouts, the UVC does not provide

clear guidance on priority within the circulatory roadway of a roundabout. In general, the UVC provides that all overtaking should take place on the left (UVC §11-

303). However, the UVC also specifies the following with respect to passing on the

right (UVC §11-304a):

Because of yield-to-the-right laws,

yield signs and lines must be used on

roundabout entries to assign right-ofway to the circulatory roadway.

Recommended lane assignments:

Exit less than halfway, use the right

lane. Exit more than halfway, use the

left lane. Exit exactly halfway, use

either lane.
Roundabouts: An Informational Guide • 2: Policy Considerations 39

The driver of a vehicle may overtake and pass upon the right of another vehicle only

under the following conditions.

1. When the vehicle overtaken is making or about to make a left turn;

2. Upon a roadway with unobstructed pavement of sufficient width for two or

more lines of vehicles moving lawfully in the direction being traveled by the

overtaking vehicle.

A case could be made that this provision applies to conditions within a circulatory

roadway of a multilane roundabout. Under the definition of a roundabout as a single

intersection, a vehicle making a left turn could be overtaken on the right, even

though the completion of the left turn requires exiting on the right.

International rules of the road vary considerably on this point. The United Kingdom,

for example, requires drivers to “watch out for traffic crossing in front of you on the

roundabout, especially vehicles intending to leave by the next exit. Show them

consideration.” (9, §125) This is generally interpreted as meaning that a vehicle at

the front of a bunch of vehicles within the circulatory roadway has the right-of-way,

regardless of the track it is on, and following vehicles on any track must yield to the

front vehicle as it exits. Australia, on the other hand, does not have a similar statement in its legal codes, and this was one of the factors that led Australians to favor

striping of the circulatory roadway in recent years. Further research and legal exploration need to be performed to determine the effect of this legal interpretation

on driver behavior and the safety and operation of multilane roundabouts.

For clarity, this guide makes the following recommendations:

• Overtaking within the circulatory roadway should be prohibited.

• Exiting vehicles should be given priority over circulating vehicles, provided that

the exiting vehicle is in front of the circulating vehicle.

2.4.5 Pedestrian accessibility

The legal definition of a roundabout as one intersection or a series of intersections

also has implications for pedestrians, particularly with respect to marked and unmarked crosswalks. A portion of the UVC definition of a crosswalk is as follows:

“. . . and in the absence of a sidewalk on one side of the roadway, that part of a

roadway included within the extension of the lateral lines of the existing sidewalk

at right angles to the centerline” (UVC §1-112(a)). Under the definition of a roundabout as a series of T-intersections, this portion of the definition could be interpreted to mean that there are unmarked crosswalks between the perimeter and

the central island at every approach. The recommended definition of a roundabout

as a single intersection simplifies this issue, for the marked or unmarked crosswalks around the perimeter as defined are sufficient and complete.

In all States, drivers are required to either yield or stop for pedestrians in a crosswalk (however, this requirement is often violated, and therefore it is prudent for

pedestrians not to assume that this is the case). In addition, the provisions of the

ADA also apply to roundabouts in all respects, including the design of sidewalks,

crosswalks, and ramps. Under the ADA, accessible information is required to make

the existing public right-of-way an accessible program provided by State and local

governments (28 CFR 35.150). Any facility or part of a facility that is newly constructed by a State or local government must be designed and constructed so that

Recommendations: No overtaking

within the circulatory roadway, and

exiting vehicles in front of other

circulating vehicles have priority

when exiting.
40 Federal Highway Administration

it is readily accessible to and usable by people with disabilities (28 CFR 35.151(a)).

Alterations to existing facilities must include modifications to make altered areas

accessible to individuals with disabilities (28 CFR 735.151 (b)).

Current guidelines do not specifically address ways to make roundabouts accessible. Nonetheless, these provisions mean providing information about safely crossing streets in an accessible format, including at roundabouts. At a minimum, design information should provide for:

• Locating the crosswalk;

• Determining the direction of the crosswalk;

• Determining a safe crossing time; and

• Locating the splitter island refuge.

2.4.6 Parking

Many States prohibit parking within a specified distance of an intersection; others

allow parking right up to the crosswalk. The degree to which these laws are in

place will govern the need to provide supplemental signs and/or curb markings

showing parking restrictions. To provide the necessary sight distances for safe

crossings to occur, this guide recommends that parking be restricted immediately

upstream of the pedestrian crosswalks.

The legal need to mark parking restrictions within the circulatory roadway may be

dependent on the definition of a roundabout as a single intersection or as a series

of T-intersections. Using the recommended definition of a roundabout as a single

intersection, the circulatory roadway would be completely contained within the

intersection, and the UVC currently prohibits parking within an intersection (UVC

§11-1003).

2.5 Public Involvement

Public acceptance of roundabouts has often been found to be one of the biggest

challenges facing a jurisdiction that is planning to install its first roundabout. Without the benefit of explanation or first-hand experience and observation, the public

is likely to incorrectly associate roundabouts with older, nonconforming traffic circles

that they have either experienced or heard about. Equally likely, without adequate

education, the public (and agencies alike) will often have a natural hesitation or

resistance against changes in their driving behavior and driving environment.

In such a situation, a proposal to install a roundabout may initially experience a

negative public reaction. However, the history of the first few roundabouts installed

in the United States also indicates that public attitude toward roundabouts improves significantly after construction. A recent survey conducted of jurisdictions

across the United States (3) reported a significant negative public attitude toward

roundabouts prior to construction (68 percent of the responses were negative or

very negative), but a positive attitude after construction (73 percent of the responses

were positive or very positive).

A recent survey found negative

public attitudes towards roundabouts

before construction, but positive

attitudes following construction.
Roundabouts: An Informational Guide • 2: Policy Considerations 41

A wide variety of techniques have been used successfully in the United States to

inform and educate the public about new roundabouts. Some of these include

public meetings, informational brochures and videos, and announcements in the

newspaper or on television and radio. A public involvement process should be

initiated as soon as practical, preferably early in the planning stages of a project

while other intersection forms are also being considered.

2.5.1 Public meetings

Public meetings can be a good forum for bringing the public into the design process. This allows early identification of potential problems and helps to gain overall

acceptance throughout the process. Public input may be useful at various stages

in the planning process: data collection, problem definition, generation of design

alternatives, selection of preferred alternatives, detailed design, go/no-go decision,

construction/opening, and landscape maintenance. Many jurisdictions require or

recommend public meetings with the affected neighborhood or businesses prior

to approval of the project by elected officials. Even if such meetings are not required, they can be helpful in easing concerns about a new form of intersection for

a community.

2.5.2 Informational brochures

A number of agencies, including the Maryland State Highway Administration and

the City of Montpelier, Vermont, have used informational brochures to educate the

public about roundabouts in their communities. Brochures have also been prepared for specific projects. Exhibit 2-6 shows examples from the brochures prepared for the I-70/Vail Road roundabouts in Vail, Colorado, and the Towson Roundabout in Towson, Maryland. These brochures include drawings or photographic simulations of the proposed roundabout. The brochures also typically include general

information on roundabouts (what roundabouts are, where they can be found, and

the types of benefits that can be expected). Sometimes they also include instructions on how to use the roundabout as a motorist, bicyclist, and pedestrian. The

Towson brochure included additional information on the business association in

the area, the streetscape policy of the county, and information on the construction

phases of the roundabout.

Public meetings, videos and

brochures, and media

announcements are some of

the ways to educate the public

about new roundabouts.
42 Federal Highway Administration

Exhibit 2-6. Examples of informational brochures.

(a) Vail, CO

(b) Towson, MD
Roundabouts: An Informational Guide • 2: Policy Considerations 43

2.5.3 Informational videos

A number of agencies and consulting firms have prepared videos to inform the

public about roundabouts. These videos are typically 10 to 15 minutes in length and

include footage of existing roundabouts and narration about their operational and

safety characteristics. These videos have been successfully used at public meetings as an effective means of introducing the public to roundabouts.

2.5.4 Media announcements

Given the new nature of a roundabout in many communities, the local media (newspaper, radio, and television) is likely to become involved. Such interest often occurs

early in the process, and then again upon the opening of the roundabout. Radio

reading services, telephone information services, and publications intended primarily for individuals with disabilities should be used to communicate with persons who are visually impaired when a roundabout is proposed and when it opens.

2.6 Education

One of the important issues facing a State considering the implementation of roundabouts is the need to provide adequate driver, cyclist, and pedestrian education. To

clarify the following tips and instructions, user education should begin by using

simple exhibits such as those in Chapter 1 to familiarize them with the basic physical features of a roundabout intersection. Users should also familiarize themselves

with the instructions for all other modes so that they understand the expectations

of each other. The following sections provide instructional material and model language for drivers, cyclists, and pedestrians that can be adapted to drivers manuals.

These have been adapted from similar rules of the road and drivers manuals used

for roundabouts in the United Kingdom (9), Australia (10), and the State of Victoria,

Australia (11).

2.6.1 Driver education

2.6.1.1 Approaching the roundabout

On approaching a roundabout, decide as early as possible which exit you need to

take and get into the correct lane (refer to the section below on “Turning at roundabouts”). Reduce your speed. Bicyclists are vehicles and need to share the lane at

intersections. Therefore, allow bicycles to enter the roadway from any bicycle lane.

The law gives pedestrians the right-of-way in a crosswalk. Yield to pedestrians

waiting to cross or crossing on the approach. Watch out for and be particularly

considerate of people with disabilities, children, and elderly pedestrians. Always

keep to the right of the splitter island (either painted or raised) on the approach to

the roundabout.

2.6.1.2 Entering the roundabout

Upon reaching the roundabout yield line, yield to traffic circulating from the left

unless signs or pavement markings indicate otherwise. Do not enter the roundabout beside a vehicle already circulating within the roundabout, as a vehicle near

the central island may be exiting at the next exit. Watch out for traffic already on the

roundabout, especially cyclists and motorcyclists. Do not enter a roundabout when

an emergency vehicle is approaching on another leg; allow queues to clear in front

of the emergency vehicle.

The following sample instructions

assume that readers have already

seen introductory material on

roundabouts, such as the brochures

depicted in the previous section.
44 Federal Highway Administration

2.6.1.3 Within the roundabout

Within a roundabout, do not stop except to avoid a collision; you have the right-ofway over entering traffic. Always keep to the right of the central island and travel in

a counterclockwise direction.

Where the circulatory roadway is wide enough to allow two or more vehicles to

travel side-by-side, do not overtake adjacent vehicles who are slightly ahead of

yours as they may wish to exit next. Watch out for traffic crossing in front of you

on the roundabout, especially vehicles intending to leave by the next exit. Do not

change lanes within the roundabout except to exit.

When an emergency vehicle is approaching, in order to provide it a clear path to

turn through the roundabout, proceed past the splitter island of your exit before

pulling over.

2.6.1.4 Exiting the roundabout

Maintain a slow speed upon exiting the roundabout. Always indicate your exit using your right-turn signal. For multilane roundabouts, watch for vehicles to your

right, including bicycles that may cross your path while exiting, and ascertain if

they intend to yield for you to exit. Watch for and yield to pedestrians waiting to

cross, or crossing the exit leg. Watch out for and be particularly considerate of

people with disabilities, children, and elderly pedestrians. Do not accelerate until

you are beyond the pedestrian crossing point on the exit.

2.6.1.5 Turning at roundabouts

Unless signs or pavement markings indicate otherwise:

• When turning right or exiting at the first exit around the roundabout, use the

following procedure:

– Turn on your right-turn signal on the approach.

– If there are multiple approach lanes, use only the right-hand lane.

– Keep to the outside of the circulatory roadway within the roundabout and

continue to use your right-turn signal through your exit.

– When there are multiple exit lanes use the right-hand lane.

• When going straight ahead (i.e., exiting halfway around the roundabout), use

the following procedure (see Exhibit 2-7):

– Do not use any turn signals on approach.

– If there are two approach lanes, you may use either the left– or right-hand

approach lanes.

– When on the circulatory roadway, turn on your right-turn signal once you

have passed the exit before the one you want and continue to use your rightturn signal through your exit.

– Maintain your inside (left) or outside (right) track throughout the roundabout if

the circulatory roadway is wide. This means that if you entered using the inner

(left) lane, circulate using the inside track of the circulatory roadway and exit

from here by crossing the outside track. Likewise, if you entered using the

outer (right) lane, circulate using the outside track of the circulatory roadway

and exit directly from here. Do not change lanes within the roundabout except when crossing the outer circulatory track in the act of exiting.
Roundabouts: An Informational Guide • 2: Policy Considerations 45

– When exiting the circulatory roadway from the inside track, watch out on the

outside track for leading or adjacent vehicles that continue to circulate around

the roundabout.

– When exiting the circulatory roadway from the outside track, yield to leading

or adjacent vehicles that are exiting into the same lane.

• When turning left or making a U-turn (i.e., exiting more than halfway around

the roundabout), use the following procedure (see Exhibit 2-8):

– Turn on your left turn signal.

– If there are multiple approach lanes, use only the left-hand lane.

– Keep to the inner (left) side of the circulatory roadway (nearest the central

island).

– Continue to use your left-turn signal until you have passed the exit before the

one you want, and then use your right-turn signal through your exit.

– When exiting from a multilane roundabout from the inside part of the circulatory roadway, use only the inner lane on the exit (the lane nearest the splitter

island). Watch out on the outside part of the circulatory roadway for leading or

adjacent vehicles that continue to circulate around the roundabout.

Exhibit 2-7. Driving straight

through a roundabout.

Source: The Highway Code (UK) (9), converted to right-hand drive
46 Federal Highway Administration

• When in doubt about lane choice (especially for roundabouts with legs at angles

other than 901⁄2), use the following general rules to determine which lane

you should be in (unless signs or pavement markings indicate otherwise):

– If you intend to exit the roundabout less than halfway around it, use the right

lane.

– If you intend to exit the roundabout more than halfway around it, use the left

lane.

2.6.1.6 Motorcyclists and bicyclists

Watch out for motorcyclists and bicyclists. Give them plenty of room and show

due consideration. Bicyclists may enter the approach roadway from a bicycle lane.

Bicyclists will often keep to the right on the roundabout; they may also indicate left

to show they are continuing around the roundabout. It is best to treat bicyclists as

other vehicles and not pass them while on the circulatory roadway. Motorcyclists

should not ride across the mountable truck apron next to the central island, if present.

2.6.1.7 Large vehicles

When car drivers approach a roundabout, do not overtake large vehicles. Large

vehicles (for example, trucks and buses) may have to swing wide on the approach

or within the roundabout. Watch for their turn signals and give them plenty of room,

especially since they may obscure other conflicting users.

Exhibit 2-8. Turning

 left at a roundabout.

Source: The Highway Code (UK) (9), converted to right-hand drive
Roundabouts: An Informational Guide • 2: Policy Considerations 47

To negotiate a roundabout, drivers of large vehicles may need to use the full width

of the roadway, including mountable aprons if provided. They should be careful of

all other users of the roundabouts and, prior to entering the roundabout, satisfy

themselves that other users are aware of them and will yield to them.

2.6.2 Bicyclist education

Bicyclists should likewise be educated about the operating characteristics of roundabouts. Well-designed, low-speed, single-lane roundabouts should not present much

difficulty to bicyclists. They should enter these roundabouts just as they enter a

stop sign or signal controlled intersection without auxiliary lanes (the bike lane

terminates on the approach to these intersections, too). On the approach to the

entry, a bicyclist should claim the lane. Right-turning cyclists should keep to the

right side of the entry lane; others should be near the center of the lane.

Cyclists have three options upon approaching a roundabout:

• Travel on the circulatory roadway of the roundabout like motorists. When using

a double-lane roundabout as a vehicle, obey all rules of the road for vehicles

using roundabouts. However, you may feel safer approaching in the right-hand

lane and keeping to the right in the roundabout (rather like making two through

movements to turn left at a signalized intersection). If you do keep to the right,

take extra care when crossing exits and signal left to show you are not leaving.

Watch out for vehicles crossing your path to leave or join the roundabout. Watch

out for large vehicles on the roundabout, as they need more space to maneuver.

It may be safer to wait until they have cleared the roundabout. Or,

• If you are unsure about using the roundabout, dismount and exit the approach

lane before the splitter island on the approach, and move to the sidewalk. Once

on the sidewalk, walk your bicycle like a pedestrian. Or,

• Some roundabouts may have a ramp that leads to a widened sidewalk or a

shared bicycle-pedestrian path that runs around the perimeter of the roundabout. If a ramp access is provided prior to the pedestrian crossing, you may

choose to ramp up to curb level and traverse the sidewalk or path while acting

courteously to pedestrians. A ramp may also be provided on the exit legs of a

roundabout to reenter the roadway, after verifying that it is safe to do so.

2.6.3 Pedestrian education

Pedestrians have the right-of-way within crosswalks at a roundabout; however,

pedestrians must not suddenly leave a curb or other safe waiting place and walk

into the path of a vehicle if it is so close that it is an immediate hazard. This can be

problematic if the design is such that a disabled pedestrian cannot accurately determine the gap. Specific education beyond these general instructions should be

provided for disabled pedestrians to use any information provided for them.

• Do not cross the circulatory roadway to the central island. Walk around the

perimeter of the roundabout.

• Use the crosswalks on the legs of the roundabout. If there is no crosswalk marked

on a leg of the roundabout, cross the leg about one vehicle-length away (7.5 m [25

ft]) from the circulatory roadway of the roundabout. Locate the wheelchair ramps

in the curbs. These are built in line with a grade-level opening in the median island.

This opening is for pedestrians to wait before crossing the next roadway.
48 Federal Highway Administration

• Roundabouts are typically designed to enable pedestrians to cross one direction of traffic at a time. Look and listen for approaching traffic. Choose a safe

time to cross from the curb ramp to the median opening (note that although you

have the right-of-way, if approaching vehicles are present, it is prudent to first

satisfy yourself that conflicting vehicles have recognized your presence and

right to cross, through visual or audible cues such as vehicle deceleration or

driver communication). If a vehicle slows for you to cross at a two-lane roundabout, be sure that conflicting vehicles in adjacent lanes have done likewise

before accepting the crossing opportunity.

• Most roundabouts provide a raised median island halfway across the roadway;

wait in the opening provided and choose a safe time to cross traffic approaching

from the other direction.

2.7 References

1. Brown, M. TRL State of the Art Review—The Design of Roundabouts. London: HMSO, 1995.

2. Alphand, F., U. Noelle, and B. Guichet. “Roundabouts and Road Safety: State

of the Art in France.” In Intersections without Traffic Signals II, Springer-Verlag,

Germany (W. Brilon, ed.), 1991, pp. 107–125.

3. Jacquemart, G. Synthesis of Highway Practice 264: Modern Roundabout Practice in the United States. National Cooperative Highway Research Program.

Washington, D.C: National Academy Press, 1998.

4. Department of Transport (United Kingdom). “Killing Speed and Saving Lives.”

As reported in Oregon Department of Transportation, Oregon Bicycle and Pedestrian Plan, 1995.

5. Garder, P. The Modern Roundabouts: The Sensible Alternative for Maine. Maine

Department of Transportation, Bureau of Planning, Research and Community

Services, Transportation Research Division, 1998.

6. Niederhauser, M.E., B.A. Collins, and E.J. Myers. “The Use of Roundabouts:

Comparison with Alternate Design Solution.” Compendium of Technical Papers, 67th Annual Meeting, Institute of Transportation Engineers. August 1997.

7. Federal Highway Administration (FHWA). Older Driver Highway Design Handbook. Publication No. FHWA-RD-97-135. Washington, D.C.: FHWA, January

1998.

8. National Committee on Uniform Traffic Laws and Ordinances (NCUTLO). Uniform Vehicle Code and Model Traffic Ordinance. Evanston, Illinois: NCUTLO,

1992.

9. Department of Transport (United Kingdom). The Highway Code. Department

of Transport and the Central Office of Information for Her Majesty’s Stationery

Office, 1996.

10. Australia. Traffic Act, Part 6A, 1962.

11. VicRoads. Victorian Traffic Handbook, Fourth Edition. Melbourne, Australia:

Roads Corporation, 1998.
Planning

3.1 Planning Steps 51

3.2 Considerations of Context 53

3.2.1 Decision environments 53

3.2.2 Site-specific conditions 54

3.3 Number of Entry Lanes 55

3.3.1 Single- and double-lane roundabouts 56

3.3.2 Mini-roundabouts 56

3.4 Selection Categories 58

3.4.1 Community enhancement 58

3.4.2 Traffic calming 58

3.4.3 Safety improvement 59

3.4.4 Operational improvement 62

3.4.5 Special situations 63

3.5 Comparing Operational Performance of Alternative

Intersection Types 64

3.5.1 Two-way stop-control alternative 64

3.5.2 All-way stop-control alternative 65

3.5.3 Signal control alternative 67

3.6 Space Requirements 69

3.7 Economic Evaluation 70

3.7.1 Methodology 73

3.7.2 Estimating benefits 73

3.7.3 Estimation of costs 75

3.8 References 76

3
50 Federal Highway Administration

Exhibit 3-1. Maximum daily service volumes for a four-leg roundabout. 57

Exhibit 3-2. Planning-level maximum daily service volumes for

mini-roundabouts. 57

Exhibit 3-3. Example of community enhancement roundabout. 59

Exhibit 3-4. Example of traffic calming roundabouts. 60

Exhibit 3-5. Comparison of predicted rural roundabout injury crashes

with rural TWSC intersections. 61

Exhibit 3-6. Model comparison of predicted injury crashes for

single-lane and double-lane roundabouts with rural or

urban signalized intersections. 61

Exhibit 3-7. Average delay per vehicle at the MUTCD peak hour

signal warrant threshold (excluding geometric delay). 63

Exhibit 3-8. Comparison of TWSC and single-lane roundabout capacity. 65

Exhibit 3-9. Sample hourly distribution of traffic. 66

Exhibit 3-10. Annual savings in delay of single-lane roundabout

versus AWSC, 50 percent of volume on the major street. 67

Exhibit 3-11. Annual savings in delay of single-lane roundabout versus

AWSC, 65 percent of volume on the major street. 67

Exhibit 3-12. Delay savings for roundabout vs. signal, 50 percent

volume on major street. 69

Exhibit 3-13. Delay savings for roundabout vs. signal, 65 percent

volume on major street. 69

Exhibit 3-14. Assumptions for spatial comparison of roundabouts

and comparable conventional intersections. 70

Exhibit 3-15. Area comparison: Urban compact roundabout vs.

comparable signalized intersection. 71

Exhibit 3-16. Area comparison: Urban single-lane roundabout vs.

comparable signalized intersection. 71

Exhibit 3-17. Area comparison: Urban double-lane roundabout vs.

comparable signalized intersection. 72

Exhibit 3-18. Area comparison: Urban flared roundabout vs.

comparable signalized intersection. 72

Exhibit 3-19. Estimated costs for crashes of varying levels of severity. 74
Roundabouts: An Informational Guide • 3: Planning 51

Chapter 1 presented a range of roundabout categories, and suggested typical daily

service volume thresholds below which four-leg roundabouts may be expected to

operate, without requiring a detailed capacity analysis. Chapter 2 introduced roundabout performance characteristics, including comparisons with other intersection

forms and control, which will be expanded upon in this chapter. This chapter covers

the next steps that lead up to the decision to construct a roundabout with an approximate configuration at a specific location, preceding the detailed analysis and

design of a roundabout. By confirming that there is good reason to believe that

roundabout construction is feasible and that a roundabout offers a sensible method

of accommodating the traffic demand, these planning activities make unnecessary

the expenditure of effort required in subsequent chapters.

Planning for roundabouts begins with specifying a preliminary configuration. The

configuration is specified in terms of the minimum number of lanes required on

each approach and, thus, which roundabout category is the most appropriate basis

for design: urban or rural, single-lane or double-lane roundabout. Given sufficient

space, roundabouts can be designed to accommodate high traffic volumes. There

are many additional levels of detail required in the design and analysis of a

high-capacity, multi-lane roundabout that are beyond the scope of a planning level

procedure. Therefore, this chapter focuses on the more common questions that

can be answered using reasonable assumptions and approximations.

Feasibility analysis requires an approximation of some of the design parameters

and operational characteristics. Some changes in these approximations may be

necessary as the design evolves. A more detailed methodology for performing the

operational evaluation and geometric design tasks is presented later in Chapters 4

and 6 of this guide, respectively.

3.1 Planning Steps

The following steps may be followed when deciding whether to implement a roundabout at an intersection:

• Step 1: Consider the context. What are there regional policy constraints that

must be addressed? Are there site-specific and community impact reasons why

a roundabout of any particular size would not be a good choice? (Section 3.2)

• Step 2: Determine a preliminary lane configuration and roundabout category

based on capacity requirements (Section 3.3). Exhibit 3-1 will be useful for making a basic decision on the required number of lanes. If Exhibit 3-1 indicates that

more than one lane is required on any approach, refer to Chapters 4 and 6 for

the more detailed analysis and design procedures. Otherwise, proceed with

the planning procedure.

• Step 3: Identify the selection category (Section 3.4). This establishes why a

roundabout may be the preferred choice and determines the need for specific

information.

Some of the assumptions and

approximations used in planning may

change as the design evolves, but are

sufficient at this stage to answer

many common questions.

Planning determines whether a

roundabout is even feasible, before

expending the effort required in

subsequent steps.

Chapter 3 Planning
52 Federal Highway Administration

• Step 4: Perform the analysis appropriate to the selection category. If the selection is to be based on operational performance, use the appropriate comparisons with alternative intersections (Section 3.5).

• Step 5: Determine the space requirements. Refer to Section 3.6 and Appendix

B for the right-of-way widths required to accommodate the inscribed circle diameter. Determine the space feasibility. Is there enough right-of-way to build it?

This is a potential rejection point. There is no operational reason to reject a

roundabout because of the need for additional right-of-way; however, right-of-way

acquisition introduces administrative complications that many agencies would

prefer to avoid.

• Step 6: If additional space must be acquired or alternative intersection forms

are viable, an economic evaluation may be useful (Section 3.7).

The results of the steps above should be documented to some extent. The level of

detail in the documentation will vary among agencies and will generally be influenced by the size and complexity of the roundabout. A roundabout selection study

report may include the following elements:

• It may identify the selection category that specifies why a roundabout is the

logical choice at this intersection;

• It may identify current or projected traffic control or safety problems at the intersection if the roundabout is proposed as a solution to these problems;

• It may propose a configuration, in terms of number of lanes on each approach;

• It may demonstrate that the proposed configuration can be implemented feasibly and that it will provide adequate capacity on all approaches; and

• It may identify all potential complicating factors, assess their relevance to the

location, and identify any mitigation efforts that might be required.

Agencies that require a more complete or formal rationale may also include the

following additional considerations:

• It may demonstrate institutional and community support indicating that key institutions (e.g., police, fire department, schools, etc.) and key community leaders have been consulted;

• It may give detailed performance comparisons of the roundabout with alternative control modes;

• It may include an economic analysis, indicating that a roundabout compares

favorably with alternative control modes from a benefit-cost perspective; and

• It may include detailed appendices containing traffic volume data, signal, or

all-way stop control (AWSC) warrant analysis, etc.

None of these elements should be construed as an absolute requirement for documentation. The above list is presented as a guide to agencies who choose to prepare a roundabout study report.

Suggested contents of a

roundabout selection

study report.
Roundabouts: An Informational Guide • 3: Planning 53

3.2 Considerations of Context

3.2.1 Decision environments

There are three somewhat different policy environments in which a decision may

be made to construct a roundabout at a specific location. While the same basic

analysis tools and concepts apply to all of the environments, the relative importance of the various aspects and observations may differ, as may prior constraints

that are imposed at higher policy levels.

A new roadway system: Fewer constraints are generally imposed if the location

under consideration is not a part of an existing roadway system. Right-of-way is

usually easier to acquire or commit. Other intersection forms also offer viable alternatives to roundabouts. There are generally no field observations of site-specific

problems that must be addressed. This situation is more likely to be faced by developers than by public agencies.

The first roundabout in an area: The first roundabout in any geographic area

requires an implementing agency to perform due diligence on roundabouts regarding their operational and design aspects, community impacts, user needs, and

public acceptability. On the other hand, a successfully implemented roundabout,

especially one that solves a perceived problem, could be an important factor in

gaining support for future roundabouts at locations that could take advantage of

the potential benefits that roundabouts may offer. Some important considerations

for this decision environment include:

• Effort should be directed toward gaining community and institutional support

for the selection of a site for the first roundabout in an area. Public acceptance

for roundabouts, like any new roadway facility, require agency staff to understand the potential issues and communicate these effectively with the impacted

community;

• An extensive justification effort may be necessary to gain the required support;

• A cautious and conservative approach may be appropriate; careful consideration should be given to conditions that suggest that the benefits of a roundabout might not be fully realized. Collecting data on current users of the facility

can provide important insights regarding potential issues and design needs;

• A single-lane roundabout in the near-term is more easily understood by most

drivers and therefore may have a higher probability of acceptance by the motoring public;

• The choice of design and analysis procedures could set a precedent for future

roundabout implementation; therefore, the full range of design and analysis

alternatives should be explored in consultation with other operating agencies in

the region; and

• After the roundabout is constructed, evaluating its operation and the public response could provide documentation to support future installations.

Retrofit to an existing intersection in an area where roundabouts have already gained

acceptance: This environment is one in which a solution to a site-specific problem

is being sought. Because drivers are familiar with roundabout operation, a less

intensive process may suffice. Double-lane roundabouts could be considered, and

the regional design and evaluation procedures should have already been agreed

Will the roundabout be...

• Part of a new roadway?

• The first in an area?

• A retrofit of an existing

intersection?

The first roundabout in an area

requires greater education and

justification efforts. Single-lane

roundabouts will be more easily

understood initially than

multilane roundabouts.
54 Federal Highway Administration

upon. The basic objectives of the selection process in this case are to demonstrate

the community impacts and that a roundabout will function properly during the

peak period within the capacity limits imposed by the space available; and to decide whether one is the preferred alternative. If the required configuration involves

additional right-of-way, a more detailed analysis will probably be necessary, using

the methodology described in Chapter 4.

Many agencies that are contemplating the construction of their first roundabout

are naturally reluctant to introduce complications, such as double-lane, yieldcontrolled junctions, which are not used elsewhere in their jurisdiction. It is also a

common desire to avoid intersection designs that require additional right-of-way,

because of the effort and expense involved in right-of-way acquisition. Important

questions to be addressed in the planning phase are therefore:

• Will a minimally configured roundabout (i.e., single-lane entrances and circulatory roadway) provide adequate capacity and performance for all users, or will

additional lanes be required on some legs or at some future time?

• Can the roundabout be constructed within the existing right-of-way, or will it be

necessary to acquire additional space beyond the property lines?

• Can a single-lane roundabout be upgraded in the future to accommodate growth?

If not, a roundabout alternative may require that more rigorous analysis and design

be conducted before a decision is made.

3.2.2 Site-specific conditions

Some conditions may preclude a roundabout at a specific location. Certain

site-related factors may significantly influence the design and require a more detailed investigation of some aspects of the design or operation. A number of these

factors (many of which are valid for any intersection type) are listed below:

• Physical or geometric complications that make it impossible or uneconomical to

construct a roundabout. These could include right-of-way limitations, utility conflicts, drainage problems, etc.

• Proximity of generators of significant traffic that might have difficulty negotiating the roundabout, such as high volumes of oversized trucks.

• Proximity of other traffic control devices that would require preemption, such as

railroad tracks, drawbridges, etc.

• Proximity of bottlenecks that would routinely back up traffic into the roundabout,

such as over-capacity signals, freeway entrance ramps, etc. The successful operation of a roundabout depends on unimpeded flow on the circulatory roadway. If traffic on the circulatory roadway comes to a halt, momentary intersection gridlock can occur. In comparison, other control types may continue to serve

some movements under these circumstances.

• Problems of grades or unfavorable topography that may limit visibility or complicate construction.

• Intersections of a major arterial and a minor arterial or local road where an unacceptable delay to the major road could be created. Roundabouts delay and deflect all traffic entering the intersection and could introduce excessive delay or

speed inconsistencies to flow on the major arterial.

Site-specific factors that may

significantly influence a

roundabout's design.
Roundabouts: An Informational Guide • 3: Planning 55

• Heavy pedestrian or bicycle movements in conflict with high traffic volumes.

(These conflicts pose a problem for all types of traffic control. There is very little

experience on this topic in the U.S., mostly due to a lack of existing roundabout

sites with heavy intermodal conflicts).

• Intersections located on arterial streets within a coordinated signal network. In

these situations, the level of service on the arterial might be better with a signalized intersection incorporated into the system. Chapter 8 deals with system

considerations for roundabouts.

The existence of one or more of these conditions does not necessarily preclude

the installation of a roundabout. Roundabouts have, in fact, been built at locations

that exhibit nearly all of the conditions listed above. Such factors may be resolved

in several ways:

• They may be determined to be insignificant at the specific site;

• They may be resolved by operational modeling or specific design features that

indicate that no significant problems will be created;

• They may be resolved through coordination with and support from other agencies, such as the local fire department; and

• In some cases, specific mitigation actions may be required.

All complicating factors should be resolved prior to the choice of a roundabout as

the preferred intersection alternative.

The effect of a particular factor will often depend on the degree to which roundabouts have been implemented in the region. Some conditions would not be expected to pose problems in areas where roundabouts are an established form of

control that is accepted by the public. On the other hand, some conditions, such as

heavy pedestrian volumes, might suggest that the installation of a roundabout be

deferred until this control mode has demonstrated regional acceptance. Most agencies have an understandable reluctance to introduce complications at their first

roundabout.

3.3 Number of Entry Lanes

A basic question that needs to be answered is how many entry lanes a roundabout

would require to serve the traffic demand. The capacity of a roundabout is clearly a

critical parameter and one that should be checked at the outset of any feasibility

study. Chapter 4 offers a detailed capacity computation procedure, mostly based

on experiences in other countries. Some assumptions and approximations have

been necessary in this chapter to produce a planning-level approach for deciding

whether or not capacity is sufficient.

Since this is the first of several planning procedures to be suggested in this chapter, some discussion of the assumptions and approximations is appropriate. First,

traffic volumes are generally represented for planning purposes in terms of Average Daily Traffic (ADT), or Average Annual Daily Traffic (AADT). Traffic operational

analyses must be carried out at the design hour level. This requires an assumption

of a K factor and a D factor to indicate, respectively, the proportion of the AADT
56 Federal Highway Administration

assigned to the design hour, and the proportion of the two-way traffic that is assigned to the peak direction. All of the planning-level procedures offered in this

chapter were based on reasonably typical assumed values for K of 0.1 and D of

0.58.

There are two site-specific parameters that must be taken into account in all computations. The first is the proportion of traffic on the major street. For roundabout

planning purposes, this value was assumed to lie between 0.5 and 0.67. All analyses assumed a four-leg intersection. The proportion of left turns must also be considered, since left turns affect all traffic control modes adversely. For the purposes

of this chapter, a reasonably typical range of left turns were examined. Right turns

were assumed to be 10 percent in all cases. Right turns are included in approach

volumes and require capacity, but are not included in the circulating volumes downstream because they exit before the next entrance.

The capacity evaluation is based on values of entering and circulating traffic volumes as described in Chapter 4. The AADT that can be accommodated is conservatively estimated as a function of the proportion of left turns, for cross-street

volume proportions of 50 percent and 67 percent. For acceptable roundabout operation, many sources advise that the volume-to-capacity ratio on any leg of a

roundabout not exceed 0.85 (1, 2). This assumption was used in deriving the AADT

maximum service volume relationship.

3.3.1 Single- and double-lane roundabouts

The resulting maximum service volumes are presented in Exhibit 3-1 for a range of

left turns from 0 to 40 percent of the total volume. This range exceeds the normal

expectation for left turn proportions. This procedure is offered as a simple, conservative method for estimating roundabout lane requirements. If the 24-hour volumes fall below the volumes indicated in Exhibit 3-1, a roundabout should have no

operational problems at any time of the day. It is suggested that a reasonable

approximation of lane requirements for a three-leg roundabout may be obtained

using 75 percent of the service volumes shown on Exhibit 3-1.

If the volumes exceed the threshold suggested in Exhibit 3-1, a single-lane or

double-lane roundabout may still function quite well, but a closer look at the actual

turning movement volumes during the design hour is required. The procedures for

such analysis are presented in Chapter 4.

3.3.2 Mini-roundabouts

Mini-roundabouts are distinguished from traditional roundabouts primarily by their

smaller size and more compact geometry. They are typically designed for negotiation speeds of 25 km/h (15 mph). Inscribed circle diameters generally vary from 13

m to 25 m (45 ft to 80 ft). Mini-roundabouts are usually implemented with safety in

mind, as opposed to capacity. Peak-period capacity is seldom an issue, and most

mini-roundabouts operate on residential or collector streets at demand levels well

below their capacity. It is important, however, to be able to assess the capacity of

any proposed intersection design to ensure that the intersection would function

properly if constructed.

At very small roundabouts, it is reasonable to assume that each quadrant of the

circulatory roadway can accommodate only one vehicle at a time. In other words,

The volume-to-capacity ratio

of any roundabout leg is

recommended not to

exceed 0.85.
Roundabouts: An Informational Guide • 3: Planning 57

a vehicle may not enter the circulatory roadway unless the quadrant on both sides

of the approach is empty. Given a set of demand volumes for each of the 12 standard movements at a four-leg roundabout, it is possible to simulate the roundabout

to estimate the maximum service volumes and delay for each approach. By making assumptions about the proportion of left turns and the proportion of cross street

traffic, a general estimate of the total entry maximum service volumes of the roundabout can be made, and is provided in Exhibit 3-2. AADT maximum service volumes are represented based on an assumed K value of 0.10. Note that these

volumes range from slightly more than 12,000 to slightly less than 16,000 vehicles

per day. The maximum throughput is achieved with an equal proportion of vehicles

on the major and minor roads, and with low proportions of left turns.

Exhibit 3-1. Maximum daily

service volumes for a

four-leg roundabout.

Exhibit 3-2. Planning-level

maximum daily service volumes

for mini-roundabouts.

For three-leg roundabouts, use

75 percent of the maximum

AADT volumes shown.

0

10,000

20,000

30,000

40,000

50,000

60,000

0% 10% 20% 30% 40%

Left Turn Percentage

Maximum AADT (K=0.10)

1 Lane (50% Minor) 1 Lane (33% Minor) 2 Lanes (50% Minor) 2 Lanes (33% Minor)

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

10% 30% 50%

Percent Left Turns

AADT Capacity (vehicles)

25% Cross Traffic 50% Cross Traffic
58 Federal Highway Administration

3.4 Selection Categories

There are many locations at which a roundabout could be selected as the preferred

traffic control mode. There are several reasons why this is so, and each reason

creates a separate selection category. Each selection category, in turn, requires

different information to demonstrate the desirability of a roundabout. The principal

selection categories will be discussed in this section, along with their information

requirements.

A wide range of roundabout policies and evaluation practices exists among operating agencies within the U.S. For example, the Florida Department of Transportation

requires a formal “justification report” to document the selection of a roundabout

as the most appropriate traffic control mode at any intersection on their State highway system. On the other hand, private developers may require no formal rationalization of any kind. It is interesting to note that the Maryland Department of Transportation requires consideration of a roundabout as an alternative at all intersections proposed for signalization.

It is reasonable that the decision to install a roundabout should require approximately the same level of effort as the alternative control mode. In other words, if a

roundabout is proposed as an alternative to a traffic signal, then the analysis effort

should be approximately the same as that required for a signal. If the alternative is

stop sign control, then the requirements could be relaxed.

The following situations present an opportunity to demonstrate the desirability of

installing a roundabout at a specific location.

3.4.1 Community enhancement

Roundabouts have been proposed as a part of a community enhancement project

and not as a solution to capacity problems. Such projects are often located in commercial and civic districts, as a gateway treatment to convey a change of environment and to encourage traffic to slow down. Traffic volumes are typically well below the thresholds shown in Exhibit 3-1; otherwise, one of the more operationally

oriented selection categories would normally be more appropriate.

Roundabouts proposed for community enhancement require minimal analysis as a

traffic control device. The main focus of the planning procedure should be to demonstrate that they would not introduce traffic problems that do not exist currently.

Particular attention should be given to any complications that would imply either

operational or safety problems. The urban compact category may be the most

appropriate roundabout for such applications. Exhibit 3-3 provides an example of a

roundabout installed primarily for community enhancement.

3.4.2 Traffic calming

The decision to install a roundabout for traffic calming purposes should be supported by a demonstrated need for traffic calming along the intersecting roadways.

Most of the roundabouts in this category will be located on local roads. Examples

of conditions that might suggest a need for traffic calming include:

• Documented observations of speeding, high traffic volumes, or careless driving

activities;

The planning focus for

community enhancement

roundabouts should be to

demonstrate that they will not

create traffic problems that do

not now exist.

Conditions that traffic calming

roundabouts may address.
Roundabouts: An Informational Guide • 3: Planning 59

Exhibit 3-3. Example of

community enhancement

roundabout.

Naples, FL

• Inadequate space for roadside activities, or a need to provide slower, safer conditions for non-automobile users; or

• New construction (road opening, traffic signal, new road, etc.) which would potentially increase the volumes of “cut-through” traffic.

Capacity should be an issue when roundabouts are installed for traffic calming

purposes only because traffic volumes on local streets will usually be well below

the level that would create congestion. If this is not the case, another primary

selection category would probably be more suitable. The urban mini-roundabout or

urban compact roundabout are most appropriate for traffic calming purposes. Exhibit 3-4 provides an example of roundabouts installed primarily for traffic calming.

3.4.3 Safety improvement

The decision to install a roundabout as a safety improvement should be based on a

demonstrated safety problem of the type susceptible to correction by a roundabout. A review of crash reports and the type of accidents occurring is essential.

Examples of safety problems include:

• High rates of crashes involving conflicts that would tend to be resolved by a

roundabout (right angle, head-on, left/through, U-turns, etc.);

• High crash severity that could be reduced by the slower speeds associated with

roundabouts;

Safety issues that roundabouts

may help correct.
60 Federal Highway Administration

• Site visibility problems that reduce the effectiveness of stop sign control (in this

case, landscaping of the roundabout needs to be carefully considered); and

• Inadequate separation of movements, especially on single-lane approaches.

Chapter 5 should be consulted for a more detailed analysis of the safety characteristics of roundabouts. There are currently a small number of roundabouts and therefore a relatively small crash record data base in the U.S. Therefore, it has not been

possible to develop a national crash model for this intersection type. Roundabout

crash prediction models have been developed for the United Kingdom (3). Crash

models for conventional intersections in the United States are available (4, 5). Although crash data reporting may not be consistent between the U.K. and the U.S.,

comparison is plausible. The two sets of models have a key common measure of

effectiveness in terms of injury and fatal crash frequency.

Therefore, for illustrative purposes, Exhibit 3-5 provides the results of injury crash

prediction models for various ADT volumes of roundabouts versus rural TWSC intersections (6). The comparison shown is for a single-lane approach, four-leg roundabout with single-lane entries, and good geometric design. For the TWSC rural

intersection model, the selected variables include rolling terrain, the main road as

major collector, and a design speed of 80 km/h (50 mph). Rural roundabouts may

experience approximately 66 percent fewer injury crashes than rural TWSC intersections for 10,000 entering ADT, and approximately 64 percent fewer crashes for

20,000 ADT. At urban roundabouts, the reduction will probably be smaller.

Also for illustration, Exhibit 3-6 provides the results of injury crash prediction models for various average daily traffic volumes at roundabouts versus rural and urban

signalized intersections (6). The selected variables of the crash model for signalized

(urban/suburban) intersections include multiphase fully-actuated signal, with a speed

of 80 km/h (50 mph) on the major road. The 20,000 entering ADT is applied to

single-lane roundabout approaches with four-legs. The 40,000 ADT is applied to

double-lane roundabout approaches without flaring of the roundabout entries. In

comparison to signalized intersections, roundabouts may experience approximately

Exhibit 3-4. Example of traffic

calming roundabouts.

Naples, FL
Roundabouts: An Informational Guide • 3: Planning 61

33 percent fewer injury crashes in urban and suburban areas and 56 percent fewer

crashes in rural areas for 20,000 entering ADT. For 40,000 entering ADT, this reduction may only be about 15 percent in urban areas. Therefore, it is likely that roundabout safety may be comparable to signalized intersections at higher ADT (greater

than 50,000).

These model comparisons are an estimation of mean crash frequency or average

safety performance from a random sample of four-leg intersections from different

countries and should be supplemented by engineering judgment and attention to

safe design for all road users.

Exhibit 3-5. Comparison of

predicted roundabout injury

crashes with rural TWSC

intersections.

Source: (6)

Exhibit 3-6. Comparison of

predicted injury crashes for

single-lane and double-lane

roundabouts with rural or urban

signalized intersections.

Source: (6)

Roundabouts have fewer annual

injury crashes than rural two-way

stop-controlled intersections, and

the total number of crashes at

roundabouts is relatively

insensitive to minor street

demand volumes.

Roundabouts have fewer injury

accidents per year than signalized

intersections, particularly in rural

areas. At volumes greater than

50,000 ADT, urban roundabout

safety may be comparable to that

of urban signalized intersections.

0.0

0.50

1.00

1.50

2.0

2.50

3.0

3.50

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

ADT Minor/ADT 

Injury Accidents/Year

TWSC (10,000 ADT) TWSC (20,000 ADT) Roundabout (10,000 ADT) Roundabout (20,000 ADT)

0.0

0.50

1.00

1.50

2.0

2.50

3.0

3.50

0 0.1 0.2 0.3 0.4 0.5 0.6

Rural Intersection (20,000 ADT)

Roundabout (20,000 ADT) Roundabout (40,000 ADT)

Urban Intersection (20,000 ADT) Urban Intersection (40,000 ADT)

Injury Accidents/Year
62 Federal Highway Administration

3.4.4 Operational improvement

A roundabout may be considered as a logical choice if its estimated performance is

better than alternative control modes, usually either stop or signal control. The

performance evaluation models presented in the next chapter provide a sound

basis for comparison, but their application may require more effort and resources

than an agency is prepared to devote in the planning stage. To simplify the selection process, the following assumptions are proposed for a planning-level comparison of control modes:

1. A roundabout will always provide a higher capacity and lower delays than AWSC

operating with the same traffic volumes and right-of-way limitations.

2. A roundabout is unlikely to offer better performance in terms of lower overall

delays than TWSC at intersections with minor movements (including cross street

entry and major street left turns) that are not experiencing, nor predicted to

experience, operational problems under TWSC.

3. A single-lane roundabout may be assumed to operate within its capacity at any

intersection that does not exceed the peak-hour volume warrant for signals.

4. A roundabout that operates within its capacity will generally produce lower delays than a signalized intersection operating with the same traffic volumes and

right-of-way limitations.

The above assumptions are documented in the literature (7) or explained by the

analyses in Section 3.5. Collectively, they provide a good starting point for further

analysis using procedures in Chapter 4. Although a roundabout may be the optimal

control type from a vehicular operation standpoint, the relative performance of this

control alternative for other modes should also be taken into consideration, as

explained in Chapter 4.

3.4.4.1 Roundabout performance at flow thresholds for peak hour signal

warrants

There are no warrants for roundabouts included in the Manual of Uniform Traffic

Control Devices (MUTCD) (8), and it may be that roundabouts are not amenable to

a warranting procedure. In other words, each roundabout should be justified on its

own merits as the most appropriate intersection treatment alternative. It is, however, useful to consider the case in which the traffic volumes just meet the MUTCD

warrant thresholds for traffic signals. For purposes of this discussion, the MUTCD

peak hour warrant will be applied with a peak hour factor (PHF) of 0.9. Thus, the

evaluation will reflect the performance in the heaviest 15 minutes of the peak hour.

Roundabout delays were compared with the corresponding values for TWSC, AWSC,

and signals. A single-lane roundabout was assumed because the capacity of a

single lane roundabout was adequate for all cases at the MUTCD volume warrant

thresholds. SIDRA analysis software was used to estimate the delay for the various control alternatives because SIDRA was the only program readily available at

the time this guide was developed that modeled all of the control alternatives (9).

The MUTCD warrant thresholds are given in terms of the heaviest minor street

volume and sum of the major street volumes. Individual movement volumes may

be obtained from the thresholds by assuming a directional factor, D, and left turn

proportions. A “D” factor of 0.58 was applied to this example. Left turns on all

approaches were assumed to be 10 to 50 percent of the total approach volume. In

General delay and capacity

comparisons between roundabouts and other forms of

intersection control.
Roundabouts: An Informational Guide • 3: Planning 63

determining the MUTCD threshold volumes, two lanes were assumed on the major street and one lane on the minor street.

Based on these assumptions, the average delays per vehicle for signals and roundabouts are presented in Exhibit 3-7. These values represent the approach delay as

perceived by the motorist. They do not include the geometric delay incurred within

the roundabout. It is clear from this figure that roundabout control delays are substantially lower than signal delays, but in neither case are the delays excessive.

Similar comparisons are not presented for TWSC, because the capacity for minor

street vehicles entering the major street was exceeded in all cases at the signal

Roundabout approach delay is

relatively insensitive to total

major street volume, but is

sensitive to the left-turn

percentage.

warrant thresholds. AWSC was found to be feasible only under a limited range of

conditions: a maximum of 20 percent left turns can be accommodated when the

major street volume is low and only 10 percent can be accommodated when

the major street volume is high. Note that the minor street volume decreases

as the major street volume increases at the signal warrant threshold.

This analysis of alternative intersection performance at the MUTCD peak hour volume signal warrant thresholds indicates that the single-lane roundabout is very

competitive with all other forms of intersection control.

3.4.5 Special situations

It is important that the selection process not discourage the construction of a roundabout at any location where a roundabout would be a logical choice. Some flexibility must be built into the process by recognizing that the selection categories above

are not all-inclusive. There may still be other situations that suggest that a roundabout would be a sensible control choice. Many of these situations are associated

with unusual alignment or geometry where other solutions are intractable.

Exhibit 3-7. Average delay per

vehicle at the MUTCD peak hour

signal warrant threshold (excluding geometric delay).

0

2

4

6

8

10

12

14

16

18

20

600 700 800 900 1000 1100 1200 1300 1400 1500

Total Major Street Volume (veh/h)

Average Delay (s/veh)

Signal (10% left turns) Signal (50% left turns)

Roundabout (10% left turns) Roundabout (50% left turns)
64 Federal Highway Administration

3.5 Comparing Operational Performance of Alternative Intersection Types

If a roundabout is being considered for operational reasons, then it may be compared

with other feasible intersection control alternatives such as TWSC, AWSC, or signal control. This section provides approximate comparisons suitable for planning.

3.5.1 Two-way stop-control alternative

The majority of intersections in the U.S. operate under TWSC, and most of those

intersections operate with minimal delay. The installation of a roundabout at a TWSC

intersection that is operating satisfactorily will be difficult to justify on the basis of

performance improvement alone, and one of the previously described selection

categories is likely to be more appropriate.

The two most common problems at TWSC intersections are congestion on the

minor street caused by a demand that exceeds capacity, and queues that form on

the major street because of inadequate capacity for left turning vehicles yielding to

opposing traffic. Roundabouts may offer an effective solution to traffic problems at

TWSC intersections with heavy left turns from the major route because they provide more favorable treatment to left turns than other control modes. “T” intersections are especially good candidates in this category because they tend to have

higher left turning volumes.

On the other hand, the problems experienced by low-volume cross street traffic at

TWSC intersections with heavy through volumes on the major street are very difficult to solve by any traffic control measure. Roundabouts are generally not the

solution to this type of problem because they create a significant impediment to

the major movements. This situation is typical of a residential street intersection

with a major arterial. The solution in most cases is to encourage the residential

traffic to enter the arterial at a collector road with an intersection designed to accommodate higher entering volumes. The proportion of traffic on the major street

is an important consideration in the comparison of a roundabout with a conventional four-leg intersection operating under TWSC. High proportions of minor street

traffic tend to favor roundabouts, while low proportions favor TWSC.

An example of this may be seen in Exhibit 3-8, which shows the AADT capacity for

planning purposes as a function of the proportion of traffic on the major street. The

assumptions in this exhibit are the same as those that have been described previously in Section 3.3. Constant proportions of 10 percent right turns (which were

ignored in roundabout analysis) and 20 percent left turns were used for all movements. As expected, the roundabout offers a much higher capacity at lower proportions of major street traffic. When the major and minor street volumes are equal,

the roundabout capacity is approximately double that of the TWSC intersection. It

is interesting to note that the two capacity values converge at the point where the

minor street proportion becomes negligible. This effect confirms the expectation

that a roundabout will have approximately the same capacity as a stop-controlled

intersection when there is no cross street traffic.

Roundabouts may offer an

effective solution at TWSC

intersections with heavy left turns

from the major street.

Roundabouts work better

when the proportion of minor

street traffic is higher.
Roundabouts: An Informational Guide • 3: Planning 65

3.5.2 All-way stop-control alternative

When cross street traffic volumes are heavy enough to meet the MUTCD warrants

for AWSC control, roundabouts become an especially attractive solution because

of their higher capacities and lower delays. The selection of a roundabout as an

alternative to AWSC should emphasize cost and safety considerations, because

roundabouts always offer better performance for vehicles than AWSC, given the

same traffic conditions. Roundabouts that are proposed as alternatives to stop

control would typically have single-lane approaches.

A substantial part of the benefit of a roundabout compared to an all-way stop intersection is obtained during the off-peak periods, because the restrictive stop control applies for the entire day. The MUTCD does not permit stop control on a part-time

basis. The extent of the benefit will depend on the amount of traffic at the intersection and on the proportion of left turns. Left turns degrade the operation of all traffic

control modes, but they have a smaller effect on roundabouts than on stop signs or

signals.

The planning level analysis that began earlier in this chapter may be extended to

estimate the benefits of a roundabout compared to AWSC. Retaining the previous

assumptions about the directional and temporal distribution factors for traffic volumes (i.e., K=0.1, D=0.58), it is possible to analyze both control modes throughout

an entire 24-hour day. Only one additional set of assumptions is required. It is

necessary to construct an assumed hourly distribution of traffic throughout the day

that conforms to these two factors.

A reasonably typical sample distribution for this purpose is illustrated in Exhibit 3-9,

which would generally represent inbound traffic to employment centers, because

of the larger peak in the AM period, accompanied by smaller peaks in the noontime

and PM periods. Daytime off-peak periods have 4 percent of the AADT per hour,

and late-night off-peak periods (midnight to 6 AM) have 1 percent.

A substantial part of the delayreduction benefit of roundabouts,

compared to AWSC intersections,

comes during off-peak periods.

Exhibit 3-8. Comparison of TWSC

and single-lane roundabout capacity.

Roundabout capacity decreases

as the proportion of minor

street entering traffic decreases.

Roundabouts and TWSC

intersections have about the

same capacity when the minor

street proportion is less than

10 percent.

0

5,000

10,000

15,000

20,000

25,000

30,000

50% 60% 70% 80% 90%

Major Street Traffic Proportion

AADT Capacity

TWS Roundabout
66 Federal Highway Administration

The outbound direction may be added as a mirror image of the inbound direction,

keeping the volumes the same as the inbound during the off-peak periods and

applying the D factor of 0.58 during the AM and PM peaks. This distribution was

used in the estimation of the benefits of a roundabout compared to the AWSC

mode. It was also used later for comparison with traffic signal operations. For purposes of estimating annual delay savings, a total of 250 days per year is assumed.

This provides a conservative estimate by eliminating weekends and holidays.

The comparisons were performed using traffic operations models that are described

in Chapter 4 of this guide. The SIDRA model was used to analyze both the roundabout and AWSC operation, because SIDRA was the only model readily available at

the time this guide was developed that treated both of these types of control.

SIDRA provides an option to either include or omit the geometric delay experienced within the intersection. The geometric delay was included for purposes of

estimating annual benefits. It was excluded in Section 3.4.4.1 that dealt with

driver-perceived approach delay.

The results of this comparison are presented in Exhibit 3-10 and Exhibit 3-11 in

terms of potential annual savings in delay of a single-lane roundabout over an AWSC

intersection with one lane on all approaches, as a function of the proportion of left

turning traffic for single-lane approaches for volume distributions of 50 percent and

65 percent on the major street, respectively. Each exhibit has lines representing 10

percent, 25 percent, and 33 percent left turn proportions.

Note that the potential annual benefit is in the range of 5,000 to 50,000 vehicle-hours

per year. The benefit increases substantially with increasing AADT and left turn

proportions. The comparison terminates in each case when the capacity of the

AWSC operation is exceeded. No comparisons were made beyond 18,000 AADT,

because AWSC operation is not practical beyond that level.

Exhibit 3-9. Sample hourly

distribution of traffic.

0

2

4

6

8

10

12

1:00

3:00

5:00

7:00

9:00

11:00

13:00

15:00

17:00

19:00

21:00

23:00

Time Ending

Percent of AADT
Roundabouts: An Informational Guide • 3: Planning 67

3.5.3 Signal control alternative

When traffic volumes are heavy enough to warrant signalization, the selection process becomes somewhat more rigorous. The usual basis for selection here is that

a roundabout will provide better operational performance than a signal in terms of

stops, delay, fuel consumption, and pollution emissions. For planning purposes,

this may generally be assumed to be the case provided that the roundabout is

operating within its capacity. The task then becomes to assess whether any roundabout configuration can be made to work satisfactorily. If not, then a signal or

grade separation are remaining alternatives. As in the case of stop control, intersections with heavy left turns are especially good roundabout candidates.

The delay-reduction benefit of

roundabouts, compared to AWSC,

increases as left-turn volumes, major

street proportion, and AADT increase.

Exhibit 3-10. Annual savings in

delay of single-lane roundabout

versus AWSC, 50 percent of volume

on the major street.

Exhibit 3-11. Annual savings in

delay of single-lane roundabout

versus AWSC, 65 percent of volume

on the major street.

0

10,000

20,000

30,000

40,000

50,000

60,000

10,000 12,000 14,000 16,000 18,000

AADT

Annual Delay Reduction (veh-h/yr)

10% left turns 25% left turns 33% left turns

0

10,000

20,000

30,000

40,000

50,000

60,000

10,000 12,000 14,000 16,000 18,000

AADT

Annual Delay Reduction (veh-h/yr)

10% left turns 25% left turns 33% left turns
68 Federal Highway Administration

The graphical approximation presented earlier for capacity estimation should be

useful at this stage. The results should be considered purely as a planning level

estimate, and it must be recognized that this estimate will probably change during

the design phase. Users of this guide should also consult the most recent version

of the Highway Capacity Manual (HCM) (10) as more U.S. data and consensus on

modeling U.S. roundabout performance evolves.

As in the case of AWSC operations, some of the most important benefits of a

roundabout compared to a traffic signal will accrue during the off-peak periods. The

comparison of delay savings discussed previously has therefore been extended to

deal with traffic signals as well as stop signs. The same temporal distribution of

traffic volumes used for the roundabout-AWSC comparison was assumed.

The signal timing design was prepared for each of the conditions to accommodate

traffic in the heaviest peak period. The traffic actuated controller was allowed to

respond to fluctuations in demand during the rest of the day using its own logic.

This strategy is consistent with common traffic engineering practice. All approaches

were considered to be isolated and free of the influence of coordinated systems.

Left turn protection was provided for the whole day for all approaches with a volume cross-product (i.e., the product of the left turn and opposing traffic volumes)

of 60,000 or greater during the peak period. When left turn protection was provided, the left turns were also allowed to proceed on the solid green indication (i.e.,

protected-plus-permitted operation).

The results of this comparison are presented in Exhibit 3-12 for 50 percent major

street traffic and Exhibit 3-13 for 65 percent major street traffic. Both cases include

AADT values up to 34,000 vehicles per day. Single-lane approaches were used for

both signals and roundabouts with AADTs below 25,000 vehicles per day. Two-lane

approaches were assumed beyond that point. All signalized approaches were assumed to have left turn bays.

Benefits may continue to accrue beyond the 34,000 AADT level but the design

parameters for both the signal and the roundabout are much more difficult to generalize for planning level analyses. When AADTs exceed 34,000 vehicles per day,

performance evaluation should be carried out using the more detailed procedures

presented in Chapter 4 of this guide.

The selection of a roundabout as an alternative to signal control will be much simpler if a single-lane roundabout is estimated to have adequate capacity. If, on the

other hand, it is determined that one or more legs will require more than one entry

lane, some preliminary design work beyond the normal planning level will generally

be required to develop the roundabout configuration and determine the space requirements.
Roundabouts: An Informational Guide • 3: Planning 69

3.6 Space Requirements

Roundabouts that are designed to accommodate vehicles larger than passenger

cars or small trucks typically require more space than conventional intersections.

However, this may be more than offset by the space saved compared with turning

lane requirements at alternative intersection forms. The key indicator of the required space is the inscribed circle diameter. A detailed design is required to determine the space requirements at a specific site, especially if more than one lane is

needed to accommodate the entering and circulating traffic. This is, however, another case in which the use of assumptions and approximations can produce

When volumes are evenly split

between major and minor

approaches, the delay savings

of roundabouts versus signals

are especially notable on

two-lane approaches with

high left turn proportions.

When the major street approaches

dominate, roundabout delay is lower

than signal delay, particularly at the

upper volume limit for single-lane

approaches and when there is a

high proportion of left turns.

Exhibit 3-12. Delay savings for

roundabout vs. signal, 50 percent

volume on major street.

Exhibit 3-13. Delay savings for

roundabout vs. signal, 65 percent

volume on major street.

The design templates in Appendix B

may be used to determine initial

space requirements for the appropriate roundabout category.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

10,000 13,000 16,000 19,000 22,000 25,000 28,000 31,000 34,000

AADT

Annual Delay Reduction (veh-h/yr)

10% left turns 25% left turns 33% left turns

TWO-LANE APPROACH 

SINGLE-LANE APPROACH

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

10,000 13,000 16,000 19,000 22,000 25,000 28,000 31,000 34,000

AADT

Annual Delay Reduction (veh-h/yr)

10% left turns 25% left turns 33% left turns

TWO-LANE APPROACH 

SINGLE-LANE APPROACH
70 Federal Highway Administration

preliminary values that are adequate for planning purposes. For initial space requirements, the design templates in Appendix B for the most appropriate of the six

roundabout categories for the specific site may be consulted.

One important question is whether or not the proposed roundabout will fit within

the existing property lines, or whether additional right-of-way will be required. Four

examples have been created to demonstrate the spatial effects of comparable

intersection types, and the assumptions are summarized in Exhibit 3-14. Note that

there are many combinations of turning volumes that would affect the actual lane

configurations and design storage lengths. Therefore, these examples should not

be used out of context.

As can be seen in Exhibit 3-15 through Exhibit 3-18, roundabouts typically require

more area at the junction than conventional intersections. However, as capacity

needs increase the size of the roundabout and comparable conventional (signalized) intersection, the increase in space requirements are increasingly offset by a

reduction in space requirements on the approaches. This is because the widening

or flaring required for a roundabout can be accomplished in a shorter distance than

is typically required to develop left turn lanes and transition tapers at conventional

intersections.

As can be seen in Exhibit 3-18, flared roundabouts offer the most potential for

reducing spatial requirements on the approaches as compared to conventional intersections. This effect of providing capacity at the intersections while reducing

lane requirements between intersections, known as “wide nodes and narrow roads,”

is discussed further in Chapter 8.

3.7 Economic Evaluation

Economic evaluation is an important part of any public works planning process. For

roundabout applications, economic evaluation becomes important when comparAlthough roundabouts typically

require more area at the junction

compared to conventional

intersections, they may not need as

much area on the approaches.

Exhibit 3-14. Assumptions for

spatial comparison of

roundabouts and comparable

 conventional intersections.

Roundabout Type Conventional Intersection

Main Street Side Street Main Street Side Street

Category Approach Lanes Approach Lanes Approach Lanes Approach Lanes

Urban compact 1 1 1 1

Urban single-lane 1 1 1 + LT pocket 1

Urban double-lane 2 1 2 + LT pocket 1 + LT pocket

Urban double-lane 1 flared to 2 1 2 + LT pocket 1 + LT pocket

 with flaring

Note: LT = left turn
Roundabouts: An Informational Guide • 3: Planning 71

Exhibit 3-15. Area comparison:

Urban compact roundabout

vs. comparable signalized

intersection.

Exhibit 3-16. Area comparison:

Urban single-lane roundabout

vs. comparable signalized

intersection.
72 Federal Highway Administration

Urban flared roundabouts in

particular illustrate the “wide

nodes, narrow roads” concept

discussed further in Chapter 8.

Exhibit 3-17. Area comparison:

Urban double-lane roundabout

vs. comparable signalized

intersection.

Exhibit 3-18. Area comparison:

Urban flared roundabout vs.

comparable signalized

 intersection.
Roundabouts: An Informational Guide • 3: Planning 73

ing roundabouts against other forms of intersections and traffic control, such as

comparing a roundabout with a signalized intersection.

The most appropriate method for evaluating public works projects of this type is

usually the benefit-cost analysis method. The following sections discuss this method

as it typically applies to roundabout evaluation, although it can be generalized for

most transportation projects.

3.7.1 Methodology

The benefit-cost method is elaborated on in detail in a number of standard references, including the ITE Transportation Planning Handbook (11) and various American Association of State Highway and Transportation Officials (AASHTO) publications (12, 13). The basic premise of this method of evaluation is to compare the

incremental benefit between two alternatives to the incremental costs between

the same alternatives. Assuming Alternatives A and B, the equation for calculating

the incremental benefit-cost ratio of Alternative B relative to Alternative A is given

in Equation 3-1.

(3-1)

Benefit-cost analysis typically takes two forms. For assessing the viability of a

number of alternatives, each alternative is compared individually with a no-build

alternative. If the analysis for Alternative A relative to the no-build alternative indicates a benefit-cost ratio exceeding 1.0, Alternative A has benefits that exceed its

costs and is thus a viable project.

For ranking alternatives, the incremental benefit-cost ratio analysis is used to compare the relative benefits and costs between alternatives. Projects should not be

ranked based on their benefit-cost ratio relative to the no-build alternative. After

eliminating any alternatives that are not viable as compared to the no-build alternative, alternatives are compared in a pair-wise fashion to establish the priority between projects.

Since many of the input parameters may be estimated, a rigorous analysis should

consider varying the parameter values of key assumptions to verify that the recommended alternative is robust, even under slightly varying assumptions, and under

what circumstances it may no longer be preferred.

3.7.2 Estimating benefits

Benefits for a public works project are generally comprised of three elements:

safety benefits, operational benefits, and environmental benefits. Each benefit is

typically quantified on an annualized basis and so is readily usable in a benefit-cost

analysis. The following sections discuss these in more detail.

Rank alternatives based on

their incremental benefit-cost

ratio, not on their ratio relative

to the no-build alternative.

Benefits consist of:

• Safety benefits

• Operational benefits

• Environmental benefits

B/CB A = BenefitsB

 – BenefitsA

CostsB – CostsA
74 Federal Highway Administration

3.7.2.1 Safety benefits

Safety benefits are defined as the assumed savings to the public due to a reduction in crashes within the project area. The general procedure for determining safety

benefits is as follows:

• Quantify the existing safety history in the study area in terms of a crash rate for

each level of severity (fatal, injury, property damage). This rate, expressed in

terms of crashes per million entering vehicles, is computed by dividing the number of crashes of a given severity that occurred during the “before” period by

the number of vehicles that entered the intersection during the same period.

This results in a “before” crash rate for each level of severity.

• Estimate the change in crashes of each level of severity that can be reasonably

expected due to the proposed improvements. As documented elsewhere in

this guide, roundabouts tend to have proportionately greater reductions in fatal

and injury crashes than property damage crashes.

• Determine a new expected crash rate (an “after” crash rate) by multiplying the

“before” crash rates by the expected reductions. It is best to use local data to

determine appropriate crash reduction factors due to geometric or traffic control changes, as well as the assumed costs of various severity levels of crashes.

• Estimate the number of “after” crashes of each level of severity for the life of

the project by multiplying the “after” crash rate by the expected number of

entering vehicles over the life of the project.

• Estimate a safety benefit by multiplying the expected number of “after” crashes

of each level of severity by the average cost of each crash and then annualizing

the result. The values in Exhibit 3-19 can provide a starting point, although local

data should be used where available.

Exhibit 3-19. Estimated costs

for crashes of varying levels of

severity.

Crash Severity Economic Cost (1997 dollars)

Death (per death) $980,000

Injury (per injury) $34,100

Property Damage Only (per crash) $6,400

Source: National Safety Council (14)

3.7.2.2 Operational benefits

The operational benefits of a project may be quantified in terms of the overall

reduction in person-hours of delay to the public. Delay has a cost to the public in

terms of lost productivity, and thus a value of time can typically be assigned to

changes in estimated delay to quantify benefits associated with delay reduction.

The calculation of annual person-hours of delay can be performed with varying

levels of detail, depending on the availability of data. For example, the vehicle-hours

of delay may be computed as follows. The results should be converted to

person-hours of delay using appropriate vehicle-occupancy factors (including transit), then adding pedestrian delay if significant.

Quantify operational benefits

 in terms of vehicle-hours

 of delay.
Roundabouts: An Informational Guide • 3: Planning 75

• Estimate the delay per vehicle for each hour of the day. If turning movements are available for multiple hours, this estimate can be computed directly. If only the peak hour is available, the delay for an off-peak hour can be

approximated by proportioning the peak hour turning movements by total

entering vehicles.

• Determine the daily vehicle-hours of delay by multiplying the estimated delay per vehicle for a given hour by the total entering vehicles during that hour

and then aggregating the results over the entire day. If data is available,

these calculations can be separated by day of week or by weekday, Saturday, and Sunday.

• Determine annual vehicle-hours of delay by multiplying the daily vehicle-hours

of delay by 365. If separate values have been calculated by day of week, first

determine the weekday vehicle-hours of delay and then multiply by 52.1

(365 divided by 7). It may be appropriate to use fewer than 365 days per year

because the operational benefits will not usually apply equally on all days.

3.7.2.3 Environmental benefits

The environmental benefits of a project are most readily quantified in terms of

reduced fuel consumption and improved air quality. Of these, reductions in fuel

consumption and the benefits associated with those reductions are typically

the simplest to determine.

One way to determine fuel consumption is to use the same procedure for estimating delay, as described previously. Fuel consumption is an output of several

of the models in use today, although the user is cautioned to ensure that the

model is appropriately calibrated for current U.S. conditions. Alternatively, one

can estimate fuel consumption by using the estimate of annual vehicle-hours of

delay and then multiplying that by an assumed fuel consumption rate during

idling, expressed as liters per hour (gallons per hour) of idling. The resulting

estimate can then be converted to a cost by assuming an average cost of fuel,

expressed in dollars per liter (dollars per gallon).

3.7.3 Estimation of costs

Costs for a public works project are generally comprised of two elements: capitalized construction costs and operations and maintenance (O&M) costs. Although O&M costs are typically determined on an annualized basis, construction costs are typically a near-term activity that must be annualized. The following sections discuss these in more detail.

3.7.3.1 Construction costs

Construction costs for each alternative should be calculated using normal preliminary engineering cost estimating techniques. These costs should include

the costs of any necessary earthwork, paving, bridges and retaining walls, signing and striping, illumination, and signalization.
76 Federal Highway Administration

To convert construction costs into an annualized value for use in the benefit-cost

analysis, a capital recovery factor (CRF) should be used, shown in Equation 3-2.

This converts a present value cost into an annualized cost over a period of n years

using an assumed discount rate of i percent.

(3-2)

where: i = discount rate

n = number of periods (years)

3.7.3.2 Operation and maintenance (O&M) costs

Operation and maintenance costs vary significantly between roundabouts and other

forms of intersection control beyond the basic elements. Common elements include signing and pavement marking maintenance and power for illumination, if

provided.

Roundabouts typically have a slightly higher illumination power and maintenance

costs compared to signalized or sign-controlled intersections due to a larger number of illumination poles. Roundabouts have slightly higher signing and pavement

marking maintenance costs due to a higher number of signs and pavement markings. Roundabouts also introduce additional cost associated with the maintenance

of any landscaping in and around the roundabout.

Signalized intersections have considerable additional cost associated with power

for the traffic signal and maintenance costs such as bulb replacement, detection

maintenance, etc. Power costs vary considerably from region to region and over

time and should be verified locally. For general purposes, an annual cost of $3,000

for providing power to a signalized intersection is a reasonable approximation.

3.8 References

1. Austroads. Guide to Traffic Engineering Practice, Part 6—Roundabouts. Sydney,

Australia: Austroads, 1993.

2. Brilon, W., N. Wu, and L. Bondzio. “Unsignalized Intersections in Germany—A

State of the Art 1997.” In Proceedings of the Third International Symposium on

Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.

3. Maycock, G., and R.D. Hall. Crashes at four-arm roundabouts. TRRL Laboratory

Report LR 1120. Crowthorne, England: Transport and Road Research Laboratory, 1984.

4. Vogt, A. Crash Models for Rural Intersections: 4-Lane by 2-Lane Stop-Controlled

and 2-Lane by 2-Lane Signalized. Washington, D.C.: Federal Highway Administration, 1999.

5. Bauer, K.M., and D.W. Harwood. Statistical Models of At-Grade Intersection

Crashes. Report No. FHWA-RD-99-094. Washington, D.C.: Federal Highway Administration, 1999.

Signalized intersections also have

O&M costs for:

• Signal power

• Bulb replacement

• Detection maintenance

Roundabout O&M costs are

typically slightly higher than

signalized intersections for:

• Illumination

• Signing

• Pavement marking

• Landscaping

CRF = i(1 + i)

n

 i(1 + i)

n – 1
Roundabouts: An Informational Guide • 3: Planning 77

6. Bared, J.G., and K. Kennedy. “Safety Impacts of Roundabouts,” Chapter 28,

The Traffic Safety Toolbox: A Primer on Traffic Safety, Institute of Transportation

Engineers, 2000.

7. Florida Department of Transportation. Florida Roundabout Guide. Florida Department of Transportation, March 1996.

8. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control

Devices. Washington, D.C.: FHWA, 1988.

9. Akçelik, R., and M. Besley. SIDRA 5 User Guide. Melbourne, Australia: Australian Road Research Board, January 1999.

10. Transportation Research Board. Highway Capacity Manual. Special Report 209.

Washington, D.C.: Transportation Research Board, National Research Council,

July 1999 (draft).

11. Institute of Transportation Engineers. Transportation Planning Handbook (J.

Edwards, Jr., ed.). Englewood Cliffs, N.J.: Prentice Hall, 1992.

12. American Association of State Highway Officials (AASHO). A Policy on Design

of Urban Highways and Arterial Streets. Washington, D.C.: AASHO, 1973.

13. American Association of State Highway & Transportation Officials (AASHTO).

A Manual on User Benefit Analysis of Highway and Bus Transit Improvements.

Washington, D.C.: AASHTO, 1977.

14. National Safety Council. Accident Facts, 1998 Edition.
4 Operation

4.1 Traffic Operation at Roundabouts 82

4.1.1 Driver behavior and geometric elements 82

4.1.2 Concept of roundabout capacity 83

4.2 Data Requirements 83

4.3 Capacity 86

4.3.1 Single-lane roundabout capacity 86

4.3.2 Double-lane roundabout capacity 88

4.3.3 Capacity effect of short lanes at flared entries 88

4.3.4 Comparison of single-lane and double-lane roundabouts 89

4.3.5 Pedestrian effects on entry capacity 90

4.3.6 Exit capacity 91

4.4 Performance Analysis 91

4.4.1 Degree of saturation 92

4.4.2 Delay 92

4.4.3 Queue length 94

4.4.4 Field observations 96

4.5 Computer Software for Roundabouts 96

4.6 References 98

Exhibit 4-1. Conversion factors for passenger car equivalents (pce). 84

Exhibit 4-2. Traffic flow parameters. 85

Exhibit 4-3. Approach capacity of a single-lane roundabout. 87

Exhibit 4-4. Approach capacity of a double-lane roundabout. 88
80 Federal Highway Administration

Exhibit 4-5. Capacity reduction factors for short lanes. 89

Exhibit 4-6. Capacity comparison of single-lane and double-lane roundabouts. 89

Exhibit 4-7. Capacity reduction factor M for a single-lane roundabout assuming

pedestrian priority. 90

Exhibit 4-8. Capacity reduction factor M for a double-lane roundabout assuming

pedestrian priority. 91

Exhibit 4-9. Control delay as a function of capacity and entering flow. 93

Exhibit 4-10. 95th-percentile queue length estimation. 95

Exhibit 4-11. Summary of roundabout software products for operational analysis. 97
Roundabouts: An Informational Guide • 4: Operation 81

Roundabouts produce both control

delay and geometric delay.

This chapter presents methods for analyzing the operation of an existing or planned

roundabout. The methods allow a transportation analyst to assess the operational

performance of a facility, given information about the usage of the facility and its

geometric design elements. An operational analysis produces two kinds of estimates: (1) the capacity of a facility, i.e., the ability of the facility to accommodate

various streams of users, and (2) the level of performance, often measured in terms

of one or more measures of effectiveness, such as delay and queues.

The Highway Capacity Manual (1) (HCM) defines the capacity of a facility as “the

maximum hourly rate at which persons or vehicles can reasonably be expected to

traverse a point or uniform section of a lane or roadway during a given time period

under prevailing roadway, traffic, and control conditions.” While capacity is a specific measure that can be defined and estimated, level of service (LOS) is a qualitative measure that “characterizes operational conditions within a traffic stream and

their perception by motorists and passengers.” To quantify level of service, the

HCM defines specific measures of effectiveness for each highway facility type.

Control delay is the measure of effectiveness that is used to define level of service

at intersections, as perceived by users. In addition to control delay, all intersections

cause some drivers to also incur geometric delays when making turns. A systems

analysis of a roadway network may include geometric delay because of the slower

vehicle paths required for turning through intersections. An example speed profile

is shown in Chapter 6 to demonstrate the speed reduction that results from geometric delay at a roundabout.

While an operational analysis can be used to evaluate the performance of an existing roundabout during a base or future year, its more common function in the U.S.

may be to evaluate new roundabout designs.

This chapter:

• Describes traffic operations at roundabouts;

• Lists the data required to evaluate the performance of a roundabout;

• Presents a method to estimate the capacity of five of the six basic roundabout configurations presented in this guide;

• Describes the measures of effectiveness used to determine the performance

of a roundabout and a method to estimate these measures; and

• Briefly describes the computer software packages available to implement the

capacity and performance analysis procedures.

Appendix A provides background information on the various capacity relationships.

Chapter 4 Operation
82 Federal Highway Administration

4.1 Traffic Operation at Roundabouts

4.1.1 Driver behavior and geometric elements

A roundabout brings together conflicting traffic streams, allows the streams to

safely merge and traverse the roundabout, and exit the streams to their desired

directions. The geometric elements of the roundabout provide guidance to drivers

approaching, entering, and traveling through a roundabout.

Drivers approaching a roundabout must slow to a speed that will allow them to

safely interact with other users of the roundabout, and to negotiate the roundabout. The width of the approach roadway, the curvature of the roadway, and the

volume of traffic present on the approach govern this speed. As drivers approach

the yield line, they must check for conflicting vehicles already on the circulating

roadway and determine when it is safe and prudent to enter the circulating stream.

The widths of the approach roadway and entry determine the number of vehicle

streams that may form side by side at the yield line and govern the rate at which

vehicles may enter the circulating roadway. The size of the inscribed circle affects

the radius of the driver’s path, which in turn determines the speed at which drivers

travel on the roundabout. The width of the circulatory roadway determines the

number of vehicles that may travel side by side on the roundabout.

The British (2), French (3), and German (4) analytical procedures are based on empirical relationships that directly relate capacity to both traffic characteristics and

roundabout geometry. The British empirical relationships reveal that small sublane

changes in the geometric parameters produce significant changes in capacity.

For instance, if some approaches are flared or have additional short lanes, these

provide considerably more capacity for two reasons. First, wider entries require

wider circulatory roadway widths. This provides for more opportunities for the circulatory traffic to bunch together, thus increasing the number of acceptable opportunities to enter, thereby increasing capacity. Second, the typical size of groups of

drivers entering into acceptable opportunities in the circulatory traffic is quite small,

so short lanes can be very effective in increasing group sizes, because the short

lane is frequently able to be filled.

The British (2) use the inscribed circle diameter, the entry width, the approach

(road) half width, the entry radius, and the sharpness of the flare to define the

performance of a roundabout. The sharpness of the flare, S, is a measure of the

rate at which the extra width is developed in the entry flare. Large values of S

correspond to short, severe flares, and small values of S correspond to long, gradual

flares (5).

The results of the extensive empirical British research indicate that approach half

width, entry width, average effective flare length and entry angle have the most

significant effect on entry capacity. Roundabouts fit into two general classes: those

with a small inscribed circle diameter of less than 50 m (165 ft.) and those with a

diameter above 50 m. The British relationships provide a means of including both of

these roundabout types. The inscribed circle diameter has a relatively small effect

for inscribed diameters of 50 m (165 ft) or less. The entry radius has little effect on

capacity provided that it is 20 m (65 ft) or more. The use of perpendicular entries (70

Approach speed is governed by:

• Approach roadway width

• Roadway curvature

• Approach volume

Geometric elements that affect

entry capacity include:

• Approach half width

• Entry width

• Entry angle

• Average effective flare

length
Roundabouts: An Informational Guide • 4: Operation 83

degrees or more) and small entry radii (less than 15 m [50 ft]) will reduce capacity.

The presence of the geometric parameters in the British and French models allow

designers to manipulate elements of their design to determine both their operational and safety effects. German research has not been able to find the same

influence of geometry, although this may be due to the relatively narrow range of

geometries in Germany (4).

Thus, the geometric elements of a roundabout, together with the volume of traffic

desiring to use a roundabout at a given time, may determine the efficiency with

which a roundabout operates.

4.1.2 Concept of roundabout capacity

The capacity of each entry to a roundabout is the maximum rate at which vehicles

can reasonably be expected to enter the roundabout from an approach during a

given time period under prevailing traffic and roadway (geometric) conditions. An

operational analysis considers a precise set of geometric conditions and traffic flow

rates defined for a 15-minute analysis period for each roundabout entry. While consideration of Average Annual Daily Traffic volumes (AADT) across all approaches is

useful for planning purposes as provided in Exhibit 1-13 and Chapter 3, analysis of

this shorter time period is critical to assessing the level of performance of the

roundabout and its individual components.

The capacity of the entire roundabout is not considered, as it depends on many

terms. However, Exhibit 1-13 provides threshold average daily traffic volumes for

the various categories of roundabouts, assuming four legs. Below these thresholds, a four-legged roundabout with roadways intersecting perpendicularly should

have adequate capacity (provided the traffic volumes are reasonably balanced and

the geometry does not deviate substantially from those shown on the design templates in Exhibits 1-7 through 1-12). The focus in this chapter on the roundabout

entry is similar to the operational analysis methods used for other forms of

unsignalized intersections and for signalized intersections. In each case, the capacity of the entry or approach is computed as a function of traffic on the other (conflicting) approaches, the interaction of these traffic streams, and the intersection

geometry.

For a properly designed roundabout, the yield line is the relevant point for capacity

analysis. The approach capacity is the capacity provided at the yield line. This is

determined by a number of geometric parameters in addition to the entry width.

On multilane roundabouts it is important to balance the use of each lane, because

otherwise some lanes may be overloaded while others are underused. Poorly designed exits may influence driver behavior and cause lane imbalance and congestion at the opposite leg.

4.2 Data Requirements

The analysis method described in this chapter requires the specification of traffic

volumes for each approach to the roundabout, including the flow rate for each directional movement. Volumes are typically expressed in passenger car vehicles per

hour (vph), for a specified 15-minute analysis period. To convert other vehicle types

to passenger car equivalents (pce), use the conversion factors given in Exhibit 4-1.

Perpendicular entries and small

entry radii reduce capacity;

inscribed circle diameters of 50

m (165 ft) or less have little

effect on capacity.

Roundabout capacity defined.

Operational analyses consider

15-minute volumes, as opposed

to the daily volumes used in

planning analyses.

The approach capacity is the

capacity provided at the yield

line.

Different size vehicles have

different capacity impacts;

passenger cars are used as the

basis for comparison.
84 Federal Highway Administration

Car 1.0

Single-unit truck or bus 1.5

Truck with trailer 2.0

Bicycle or motorcycle 0.5

Source: (6), (7)

Passenger Car

Vehicle Type Equivalent (pce)

Exhibit 4-1. Conversion factors

for passenger car equivalents

(pce).

Traffic volume data for an urban roundabout should be collected for each directional

movement for at least the morning and evening peak periods, since the various

movements, and thus approach and circulating volumes, may peak at different times.

At rural roundabouts, the analyst should check the requirements of the agency

with the jurisdiction of the site. The reader is referred to the Manual of Transportation Engineering Studies (8) for a complete discussion of traffic volume data collection methods. Typically, intersection volume counts are made at the intersection

stop bar, with an observer noting the number of cars that pass that point over a

specified time period. However, particularly with respect to cases in which demand exceeds capacity (when queues do not dissipate within the analysis period),

it is important to note that the stop bar counts reflect only the volume that is

served, not the demand volume. In this case, care must be taken to collect data

upstream of the end of a queue so that true demand volumes are available for

analysis.

The relationship between the standard origin-to-destination turning movements at

an intersection and the circulating and entry flows at a roundabout is important, yet

is often complicated to compute, particularly if an intersection has more than four

approaches. For conventional intersctions, traffic flow data are accumulated by directional turning movement, such as for the northbound left turn. For roundabouts,

however, the data of interest for each approach are the entry flow and the circulating flow. Entry flow is simply the sum of the through, left, and right turn movements on an approach. Circulating flow is the sum of the vehicles from different

movements passing in front of the adjacent uptstream splitter island. At existing

roundabouts, these flows can simply be measured in the field. Right turns are

included in approach volumes and require capacity, but are not included in the

circulating volumes downstream because they exit before the next entrance.

For proposed or planned four-legged roundabouts, Equations 4-1 through 4-4 can

be applied to determine conflicting (circulating) flow rates, as shown graphically in

Exhibit 4-2.

VEB,circ = VWB,LT + VSB,LT + V SB,TH + VNB,U-turn + VWB,U-turn + VSB,U-turn (4-1)

VWB,circ = VEB,LT + VNB,LT + VNB,TH + VSB,U-turn + VEB,U-turn + VNB,U-turn (4-2)

VNB,circ = VEB,LT + VEB,TH + VSB,LT + VWB,U-turn + VSB,U-turn + VEB,U-turn (4-3)

VSB,circ = VWB,LT + VWB,TH + VNB,LT + VEB,U-turn+ VNB,U-turn + VWB,U-turn (4-4)

Determining circulating

volumes as a function of

turning movement volumes.

Entry flow and circulating flow

for each approach are the

volumes of interest for

roundabout capacity analysis,

rather than turning

movement volumes.
Roundabouts: An Informational Guide • 4: Operation 85

Exhibit 4-2. Traffic flow

parameters.

While this method is mathematically correct, it is somewhat sensitive to errors and

inconsistencies in the input data. It is important that the counts at all of the locations in the roundabout be made simultaneously. Inconsistencies in the data from

counts taken on different days can produce meaningless results, including negative volumes. At a minimum, the sum of the entering and exiting volumes should

be checked and adjustments should be made if necessary to ensure that the same

amount of traffic enters and leaves the roundabout.

For existing roundabouts, when approach, right-turn, circulating, and exit flows are

counted, directional turning movements can be computed as shown in the following example. Equation 4-5 shows the through movement flow rate for the eastbound approach as a function of the entry flow rate for that approach, the exit flow

rate for the opposing approach, the right turn flow rate for the subject approach,

the right turn flow rate for the approach on the right, and the circulating flow rate

for the approach on the right. Other through movement flow rates can be estimated using a similar relationship.

VEB,TH = VEB,entry + VWB,exit - VEB,RT - VNB,RT - VNB,circ (4-5)

The left turn flow rate for an approach is a function of the entry flow rate, the

through flow rate, and the right turn flow rate for that same approach, as shown in

Equation 4-6. Again, other movements’ flows are estimated using similar equations.

VEB,LT = VEB,entry - VEB,TH - VEB,RT (4-6)

NORTH

WB,RT

WB,TH

WB,L T

WB,U

SB,RT

NB,RT

SB,TH

NB,TH

SB,L T

NB,L T

SB,U NB,U

EB,RT

EB,TH

EB,L T

EB,U

SBCIRC

EBCIRC

NBCIRC

WBCIRC
86 Federal Highway Administration

4.3 Capacity

The maximum flow rate that can be accommodated at a roundabout entry depends on two factors: the circulating flow on the roundabout that conflicts with the

entry flow, and the geometric elements of the roundabout.

When the circulating flow is low, drivers at the entry are able to enter the roundabout without significant delay. The larger gaps in the circulating flow are more

useful to the entering drivers and more than one vehicle may enter each gap. As

the circulating flow increases, the size of the gaps in the circulating flow decrease,

and the rate at which vehicles can enter also decreases. Note that when computing the capacity of a particular leg, the actual circulating flow to use may be less

than demand flows, if the entry capacity of one leg contributing to the circulating

flow is less than demand on that leg.

The geometric elements of the roundabout also affect the rate of entry flow. The

most important geometric element is the width of the entry and circulatory roadways, or the number of lanes at the entry and on the roundabout. Two entry lanes

permit nearly twice the rate of entry flow as does one lane. Wider circulatory roadways allow vehicles to travel alongside, or follow, each other in tighter bunches and

so provide longer gaps between bunches of vehicles. The flare length also affects

the capacity. The inscribed circle diameter and the entry angle have minor effects

on capacity.

As at other forms of unsignalized intersection, when traffic flows on an approach

exceed approximately 85 percent of capacity, delays and queue lengths vary significantly about their mean values (with standard deviations of similar magnitude

as the means). For this reason, the analysis procedures in some countries (Australia, Germany, and the United Kingdom), and this guide, recommend that roundabouts be designed to operate at no more than 85 percent of their estimated capacity.

As performance data become available for roundabouts designed according to the

procedures in this guide in the United States, they will provide a basis for development of operational performance procedures specifically calibrated for U.S. conditions. Therefore, analysts should consult future editions of the Highway Capacity

Manual.

Roundabouts should be

designed to operate at no more

than 85 percent of their

estimated capacity. Beyond this

threshold, delays and queues

vary significantly from their

mean values.

4.3.1 Single-lane roundabout capacity

Exhibit 4-3 shows the expected capacity for a single-lane roundabout for both the

urban compact and urban/rural single-lane designs. The exhibit shows the variation

of maximum entry flow as a function of the circulating flow on the roundabout. The

calculation of the circulating flow was described previously. The capacity forecast

shown in the chart is valid for single-lane roundabouts with inscribed circle diameters of 25 m to 55 m (80 ft to 180 ft). The capacity forecast is based on simplified

British regression relationships in Appendix A, which may also be derived with a

gap-acceptance model by incorporating limited priority behavior.

Roundabout approach capacity

is dependent on the conflicting

circulating flow and the

roundabout’s geometric

elements.
Roundabouts: An Informational Guide • 4: Operation 87

Exhibit 4-3. Approach capacity

of a single-lane roundabout.

The slope of the upper line

changes because circulating

flow downstream from a

roundabout entry should not

exceed 1,800 veh/h.

Note that in any case, the flow rate downstream of the merge point (between the

entry and the next exit) should not be allowed to exceed 1,800 veh/h. Exceeding

this threshold may indicate the need for a double-lane entry.

The urban compact design is expected to have a reduced capacity, but has significant benefits of reduced vehicle speeds through the roundabout (per the German

equations in Appendix A). This increases safety for pedestrians and bicyclists compared with the larger single lane roundabouts. Mini-roundabout capacities may be

approximated using the daily maximum service volumes provided for them in Chapter 3, but in any case should not exceed the capacity of the urban compact design.

Circulating flow should not

exceed 1,800 veh/h at any

point in a single-lane

roundabout. Exit flows

exceeding 1,200 veh/h may

indicate the need for a

double-lane exit.

0

w (veh/h)

800
88 Federal Highway Administration

4.3.2 Double-lane roundabout capacity

Exhibit 4-4 shows the expected capacity of a double-lane roundabout that is based

on the design templates for the urban/rural double-lane roundabouts. The capacity

forecast shown in the chart is valid for double-lane roundabouts with inscribed

circle diameters of 40 m to 60 m (130 ft to 200 ft). The capacity forecast is based on

simplified British regression relationships in Appendix A, which may also be derived with a gap-acceptance model by incorporating limited priority behavior. Larger

inscribed diameter roundabouts are expected to have slightly higher capacities at

moderate to high circulating flows.

Exhibit 4-4. Approach

capacity of a double-lane

roundabout.

4.3.3 Capacity effect of short lanes at flared entries

By flaring an approach, short lanes may be added at the entry to improve the performance. If an additional short lane is used, it is assumed that the circulatory road

width is also increased accordingly. The capacity of the entry is based on the assumption that all entry lanes will be effectively used. The capacity is given by the

product of the appropriate factor in Exhibit 4-5 and the capacity of a two-lane roundabout in Exhibit 4-4. Refer to Appendix A for a derivation of these factors (9).

When flared approaches are

used, the circulatory road width

must be widened.

See Appendix A for further

information on the effects of short

lanes at flared entries.

0

400

800

1200

1600

2000

2400

2800

0 400 800 1200 1600 2000 2400 2800 3200 3600

Circulatory Flow (veh/h)

Maximum Entry Flow (veh/h)
Roundabouts: An Informational Guide • 4: Operation 89

4.3.4 Comparison of single-lane and double-lane roundabouts

Exhibit 4-6 shows a comparison of the expected capacity for both the single-lane

and double-lane roundabouts. Again, it is evident that the number of lanes, or the

size of the entry and circulating roadways, has a significant effect on the entry

capacity.

Exhibit 4-5. Capacity reduction

factors for short lanes.

The use of short lanes can

nearly double approach

capacity, without requiring a

two-lane roadway prior to the

roundabout.

Exhibit 4-6. Capacity

comparison of single-lane and

double-lane roundabouts.

0 * 0.500

1 0.707

2 0.794

4 0.871

6 0.906

8 0.926

10 0.939

Number of vehicle spaces in

the short lane, nf

Factor (applied to double-lane

approach capacity)

Source (10)

*Used for the case of a single lane entry to a double-lane roundabout.
90 Federal Highway Administration

Exhibit 4-7. Capacity reduction

factor M for a single-lane

roundabout assuming

pedestrian priority.

4.3.5 Pedestrian effects on entry capacity

Pedestrians crossing at a marked crosswalk that gives them priority over entering

motor vehicles can have a significant effect on the entry capacity. In such cases, if

the pedestrian crossing volume and circulating volume are known, the vehicular

capacity should be factored (multiply by M) according to the relationship shown in

Exhibit 4-7 or Exhibit 4-8 for single-lane and double-lane roundabouts, respectively.

Note that the pedestrian impedance decreases as the conflicting vehicle flow increases. The Highway Capacity Manual (1) provides additional guidance on the capacity of pedestrian crossings and should be consulted if the capacity of the crosswalk itself is an issue.

The effects of conflicting

pedestrians on approach

capacity decrease as conflicting

vehicular volumes increase, as

entering vehicles become more

likely to have to stop regardless

of whether pedestrians are

present.

Source: (10)
Roundabouts: An Informational Guide • 4: Operation 91

4.3.6 Exit capacity

An exit flow on a single lane of more than 1,400 veh/h, even under good operating

conditions for vehicles (i.e., tangential alignment, and no pedestrians and bicyclists)

is difficult to achieve. Under normal urban conditions, the exit lane capacity is in the

range of 1,200 to 1,300 veh/h. Therefore, exit flows exceeding 1,200 veh/h may

indicate the need for a double-lane exit (11).

4.4 Performance Analysis

Three performance measures are typically used to estimate the performance of a

given roundabout design: degree of saturation, delay, and queue length. Each measure provides a unique perspective on the quality of service at which a roundabout

will perform under a given set of traffic and geometric conditions. Whenever possible, the analyst should estimate as many of these parameters as possible to

obtain the broadest possible evaluation of the performance of a given roundabout

design. In all cases, a capacity estimate must be obtained for an entry to the roundabout before a specific performance measure can be computed.

Exhibit 4-8. Capacity

reduction factor M for a

double-lane roundabout

assuming pedestrian priority.

Source: (10)

Key performance measures for

roundabouts:

• Degree of saturation

• Delay

• Queue length
92 Federal Highway Administration

4.4.1 Degree of saturation

Degree of saturation is the ratio of the demand at the roundabout entry to the

capacity of the entry. It provides a direct assessment of the sufficiency of a given

design. While there are no absolute standards for degree of saturation, the Australian design procedure suggests that the degree of saturation for an entry lane should

be less than 0.85 for satisfactory operation. When the degree of saturation exceeds this range, the operation of the roundabout will likely deteriorate rapidly,

particularly over short periods of time. Queues may form and delay begins to increase exponentially.

4.4.2 Delay

Delay is a standard parameter used to measure the performance of an intersection. The Highway Capacity Manual (1) identifies delay as the primary measure of

effectiveness for both signalized and unsignalized intersections, with level of service determined from the delay estimate. Currently, however, the Highway Capacity Manual only includes control delay, the delay attributable to the control device.

Control delay is the time that a driver spends queuing and then waiting for an

acceptable gap in the circulating flow while at the front of the queue. The formula

for computing this delay is given in Equation 4-7 (12, based on 13; see also 14).

Exhibit 4-9 shows how control delay at an entry varies with entry capacity and

circulating flow. Each curve for control delay ends at a volume-to-capacity ratio of

1.0, with the curve projected beyond that point as a dashed line.

(4-7)

where: d = average control delay, sec/veh;

v x = flow rate for movement x, veh/h;

cmx= capacity of movement x, veh/h; and

T = analysis time period, h (T = 0.25 for a 15-minute period).
Roundabouts: An Informational Guide • 4: Operation 93

Note that as volumes approach capacity, control delay increases exponentially,

with small changes in volume having large effects on delay. An accurate analysis of

delay under conditions near or over saturation requires consideration of the following factors:

• The effect of residual queues. Roundabout entries operating near or over capacity can generate significant residual queues that must be accounted for between consecutive time periods. The method presented above does not account for these residual queues. These factors are accounted for in the delay

formulae developed by Kimber and Hollis (15); however, these formulae are

difficult to use manually.

• The metering effect of upstream oversaturated entries. When an upstream entry is operating over capacity, the circulating volume in front of a downstream

entry is less than the true demand. As a result, the capacity of the downstream

entry is higher than what would be predicted from analyzing actual demand.

For most design applications where target degrees of saturation are no more than

0.85, the procedures presented in this section are sufficient. In cases where it is

desired to more accurately estimate performance in conditions near or over capacity, the use of software that accounts for the above factors is recommended.

Geometric delay is the additional time that a single vehicle with no conflicting

flows spends slowing down to the negotiation speed, proceeding through the intersection, and accelerating back to normal operating speed. Geometric delay may

Exhibit 4-9. Control delay as a

function of capacity and

entering flow.

0

10

20

30

40

50

60

0 400 800 1200 1600 2000 2400

Entering flow (veh/h)

Delay (sec/veh)

400 veh/h 800 veh/h 1200 veh/h 1600 veh/h 2000 veh/h 2400 veh/h

Capacity
94 Federal Highway Administration

be an important consideration in network planning (possibly affecting route travel

times and choices) or when comparing operations of alternative intersection types.

While geometric delay is often negligible for through movements at a signalized or

stop-controlled intersection, it can be more significant for turning movements such

as those through a roundabout. Calculation of geometric delay requires an estimate of the proportion of vehicles that must stop at the yield line, as well as knowledge of the roundabout geometry as it affects vehicle speeds during entry, negotiation, and exit. Procedures for calculating the number of stops and geometric

delay are given in the Australian design guide (16).

4.4.3 Queue length

Queue length is important when assessing the adequacy of the geometric design

of the roundabout approaches.

The average queue length (L vehicles) can be calculated by Little’s rule, as shown in

Equation 4-8 (17):

L = v • d / 3600 (4-8)

where: v = entry flow, veh/h

d = average delay, seconds/veh

Average queue length is equivalent to the vehicle-hours of delay per hour on an

approach. It is useful for comparing roundabout performance with other intersection forms, and other planning procedures that use intersection delay as an input.

For design purposes, Exhibit 4-10 shows how the 95th-percentile queue length

varies with the degree of saturation of an approach (18, 19). The x-axis of the graph

is the degree of saturation, or the ratio of the entry flow to the entry capacity.

Individual lines are shown for the product of T and entry capacity. To determine the

95th-percentile queue length during time T, enter the graph at the computed degree of saturation. Move vertically until the computed curve line is reached. Then

move horizontally to the left to determine the 95th-percentile queue length. Alternatively, Equation 4-8 can be used to approximate the 95th-percentile queue. Note

that the graph and equation are only valid where the volume-to-capacity ratio immediately before and immediately after the study period is no greater than 0.85 (in

other words, the residual queues are negligible).
Roundabouts: An Informational Guide • 4: Operation 95

Exhibit 4-10. 95th-percentile

queue length estimation.

(4-9)

where: Q95= 95th percentile queue, veh,

vx = flow rate for movement x, veh/h,

cm,x= capacity of movement x, veh/h, and

T = analysis time period, h (0.25 for 15-minute period).

Source: (19)
96 Federal Highway Administration

4.4.4 Field observations

The analyst may evaluate an existing roundabout to determine its performance and

whether changes to its design are needed. Measurements of vehicle delay and

queuing can be made using standard traffic engineering techniques. In addition,

the analyst can perform a qualitative assessment of the roundabout performance.

The following list indicates conditions for which corrective design measures should

be taken (20). If the answers to these questions are negative, no corrective actions

need be taken.

• Do drivers stop unnecessarily at the yield point?

• Do drivers stop unnecessarily within the circulating roadway?

• Do any vehicles pass on the wrong side of the central island?

• Do queues from an external bottleneck back up into the roundabout from an exit

road?

• Does the actual number of entry lanes differ from those intended by the design?

• Do smaller vehicles encroach on the truck apron?

• Is there evidence of damage to any of the signs in the roundabout?

• Is there any pedestrian activity on the central island?

• Do pedestrians and cyclists fail to use the roundabout as intended?

• Are there tire marks on any of the curb surfaces to indicate vehicle contact?

• Is there any evidence of minor accidents, such as broken glass, pieces of rim,

etc., on the approaches or the circulating roadway?

• Is there any gravel or other debris collected in nontraveled areas that could be a

hazard to bicycles or motorcyclists?

• Are the vehicle speeds appropriate?

4.5 Computer Software for Roundabouts

While the analytical procedures of different countries are not very complex, they

are repetitive and time consuming, so most of these procedures have been implemented in software. A summary of current (as of 1999) software products and the

analytical procedures that they implement is presented in Exhibit 4-11. The reader is

also advised to consult the latest version of the U.S. Highway Capacity Manual.

While the procedures provided in this chapter are recommended for most applications covered by this guide, models such as ARCADY, RODEL, SIDRA, KREISEL, or

GIRABASE may be consulted to determine the effects of geometric parameters,

particularly for multilane roundabouts outside the realm of this guide, or for finetuning designs to improve performance. Note that many of these models represent different underlying data or theories and will thus produce different results.

Chapter 8 provides some information on microscopic simulation modeling which

may be useful alternatives analysis in systems context.

Points to consider for a qualitative

assessment of roundabout

performance.
Roundabouts: An Informational Guide • 4: Operation 97

Name Scope Application and Qualities (1999 versions)

Exhibit 4-11.

Summary of

roundabout

software products

for operational

analysis.

British method (50 percent confidence limits). Capacity, delay, and

queuing. Includes projected number of crashes per year. Data were

collected at extensive field studies and from experiments involving

drivers at temporary roundabouts. Empirical relationships were developed from the data and incorporated into ARCADY. This model

reflects British driving behavior and British roundabout designs. A

prime attribute is that the capacities it predicts have been measured.

British method (user-specified confidence limits). Capacity, delay, and

queuing. Includes both an evaluation mode (geometric parameters

specified) and a design mode (performance targets specified). Includes

a crash prediction model. RODEL uses the British empirical equations. It also assists the user in developing an appropriate roundabout

for the traffic conditions.

Australian method, with analytical extensions. Capacity, delay, queue,

fuel, and environmental measures. Also evaluates two-way stop-controlled, all-way stop controlled, and signalized intersections. It also

gives roundabout capacities from U.S. HCM 1997 and German procedures. SIDRA is based on gap acceptance processes. It uses field

data for the gap acceptance parameters to calibrate the model. There

has been limited field evaluation of the results although experience

has shown that the results fit Australian and U.S. single-lane (21) roundabout conditions satisfactorily. An important attribute is that the user

can alter parameters to easily reflect local driving.

U.S. HCM 1997 method. Limited to capacity estimation based on

entering and circulating volume. Optional gap acceptance parameter

values provide both a liberal and conservative estimate of capacity.

The data used to calibrate the models were recorded in the U.S. The

two curves given reflect the uncertainty from the results. The upperbound average capacities are anticipated at most roundabouts. The

lower bound results reflect the operation that might be expected until

roundabouts become more common.

Developed in Germany. Offers many user-specified options to implement the full range of procedures found in the literature from U.S.

(including this chapter), Europe, Britain, and Australia. KREISEL gives

the average capacity from a number of different procedures. It provides a means to compare these procedures.

French method. Capacity, delay, and queuing projections based on

regression. Sensitive to geometric parameters. Gives average values.

All configurations

All configurations

Single-lane

roundabouts

with a limited

range of

volumes

All configurations

and other control

types

All configurations

All configurations

including multiple

roundabout

interactions

ARCADY

RODEL

SIDRA

HCS-3

KREISEL

GIRABASE
98 Federal Highway Administration

4.6 References

1. Transportation Research Board. Highway Capacity Manual. Special Report 209.

Washington, D.C.: Transportation Research Board, National Research Council,

July 1999 (draft).

2. Kimber, R.M. The traffic capacity of roundabouts. TRRL Laboratory Report LR

942. Crowthorne, England: Transport and Road Research Laboratory, 1980.

3. Guichet, B. “Roundabouts In France: Development, Safety, Design, and Capacity.” In Proceedings of the Third International Symposium on Intersections

Without Traffic Signals (M. Kyte, ed.), Portland, Oregon, U.S.A. University of

Idaho, 1997.

4. Brilon, W., N. Wu, and L. Bondzio. “Unsignalized Intersections in Germany—

A State of the Art 1997.” In Proceedings of the Third International Symposium

on Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A.

University of Idaho, 1997.

5. Ourston & Doctors, Inc. Roundabout Design Guidelines. 1995.

6. Jessen, G.D. Ein Richtlinienvorschlag für die Behandlung der Leistungsfähigkeit

von Knotenpunkten ohne Signalregelung (A guideline suggested for capacity

calculations for unsignalized intersections). Strassenverkehrstechnik, Nr. 7/8,

1968.

7. Harders, J. Grenz- und Folgezeitlücken als Grundlage für die Berechnung der

Leistungsfähigkeit von Landstrassen (Critical gaps and follow-up times or capacity calculations at rural roads). Schriftenreihe Strassenbau und

Strassenverkehrstechnik, Vol. 216, 1976.

8. Institute of Transportation Engineers. Manual of Transportation Engineering

Studies (H.D. Robertson, J.E. Hummer, and D.C. Nelson, ed.). Englewood

Cliffs, N.J.: Prentice-Hall, 1994.

9. Wu, N. “Capacity of Shared/Short Lanes at Unsignalized Intersections.” In

Proceedings of the Third International Symposium on Intersections without

Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho,

1997.

10. Brilon, W., B. Stuwe, and O. Drews. Sicherheit und Leistungsfähigkeit von

Kreisverkehrsplätzen (Safety and Capacity of Roundabouts). Research Report.

Ruhr-University Bochum, 1993.

11. Brilon, W. Letter to Principal Investigator. September 18, 1999.

12. Transportation Research Board. Highway Capacity Manual. Special Report 209.

Washington, D.C.: Transportation Research Board, National Research Council,

1994.

13. Akçelic, R., and R.J. Troutbeck. “Implementation of the Australian roundabout

analysis method in SIDRA.” In Highway Capacity and Level of Service: Proceedings of the International Symposium on Highway Capacity (U. Brannolte,

ed.), Karlsruhe, Germany. Rotterdam, Germany: Balkema Publisher, 1991, pp.

17–34.
Roundabouts: An Informational Guide • 4: Operation 99

14. Transportation Research Board. Review of International Practices Used to

Evaluate Unsignalized Intersections. Transportation Research Circular 468.

Washington, D.C.: Transportation Research Board, National Research Council,

April 1997.

15. Kimber, R.M. and E.M. Hollis. Traffic queues and delays at road junctions. TRRL

Laboratory Report LR 909. Crowthorne, England: Transport and Road Research

Laboratory, 1979.

16. Austroads. Guide to Traffic Engineering Practice, Part 6—Roundabouts. Sydney,

Australia: Austroads, 1993.

17. Little, J.D.C. A Proof of the Queueing Formula L = W . λ. Operations Research

9, S. 383–387, 1961.

18. Heidemann, D. “Queue lengths and waiting-time distributions at priority intersections.” In Transportation Research B, Vol 25B, (4) pp. 163–174, 1991.

19. Wu, N. “An Approximation for the Distribution of Queue Lengths at Unsignalized

Intersections.” In Proceedings of the Second International Symposium on Highway Capacity (ed. R. Akçelik), Sydney, Australia. Australian Road Research Board,

1994.

20. Florida Department of Transportation. Florida Roundabout Guide. Florida Department of Transportation, March 1996.

21. Flannery, A., L. Elefteriadou, P. Koza, and J. McFadden. “Safety, delay and

capacity of single-lane roundabouts in the United States.” In Transportation

Research Record 1646. Washington, D.C.: Transportation Research Board, National Research Council, 1998, pp. 63–70.
5 Safety

5.1 Introduction 103

5.2 Conflicts 104

5.2.1 Vehicle conflicts 105

5.2.2 Pedestrian conflicts 108

5.2.3 Bicycle conflicts 110

5.3 Crash Statistics 111

5.3.1 Comparisons to previous intersection treatment 111

5.3.2 Collision types 113

5.3.3 Pedestrians 117

5.3.4 Bicyclists 120

5.4 Crash Prediction Models 122

5.5 References 125

Exhibit 5-1. Vehicle conflict points for “T” Intersections with single-lane

approaches. 105

Exhibit 5-2. Vehicle conflict point comparison for intersections with

single-lane approaches. 106

Exhibit 5-3. Improper lane-use conflicts in double-lane roundabouts. 107

Exhibit 5-4. Improper turn conflicts in double-lane roundabouts. 108

Exhibit 5-5. Vehicle-pedestrian conflicts at signalized intersections. 109

Exhibit 5-6. Vehicle-pedestrian conflicts at single-lane roundabouts. 109

Exhibit 5-7. Bicycle conflicts at conventional intersections (showing two

left-turn options). 110
102 Federal Highway Administration

Exhibit 5-8. Bicycle conflicts at roundabouts. 111

Exhibit 5-9. Average annual crash frequencies at 11 U.S. intersections

converted to roundabouts. 112

Exhibit 5-10. Mean crash reductions in various countries. 112

Exhibit 5-11. Reported proportions of major crash types at roundabouts. 113

Exhibit 5-12. Comparison of collision types at roundabouts. 114

Exhibit 5-13. Graphical depiction of collision types at roundabouts. 115

Exhibit 5-14. Crash percentage per type of user for urban roundabouts in 15 towns

in western France. 116

Exhibit 5-15. British crash rates for pedestrians at roundabouts and signalized

intersections. 117

Exhibit 5-16. Percentage reduction in the number of crashes by mode at 181

converted Dutch roundabouts. 117

Exhibit 5-17. British crash rates (crashes per million trips) for bicyclists and

motorcyclists at roundabouts and signalized intersections. 120

Exhibit 5-18. A comparison of crashes between signalized and roundabout

intersections in 1998 in 15 French towns. 120
Roundabouts: An Informational Guide • 5: Safety 103

Chapter 5 Safety

Roundabouts may improve the safety of intersections by eliminating or altering conflict types, by reducing speed differentials at intersections, and by forcing drivers to

decrease speeds as they proceed into and through the intersection. Though roundabout crash records in the United States are limited, the experiences of other countries can be used to help design roundabouts in this country. Understanding the

sensitivity of geometric element parameters, along with the crash experience, will

assist the designer in optimizing the safety of all vehicle occupants, pedestrians, and

bicyclists.

5.1 Introduction

Many studies have found that one of the benefits of roundabout installation is the

improvement in overall safety performance. Several studies in the U.S., Europe, and

Australia have found that roundabouts perform better in terms of safety than other

intersection forms (1, 2, 3, 4). In particular, single-lane roundabouts have been found

to perform better than two-way stop-controlled (TWSC) intersections in the U.S. (5).

Although the frequency of reported crashes is not always lower at roundabouts, the

reduced injury rates are usually reported (6). Safety is better at small and medium

capacity roundabouts than at large or multilane roundabouts (1, 7). While overall

crash frequencies have been reduced, the crash reductions are most pronounced for

motor vehicles, less pronounced for pedestrians, and equivocal for bicyclists, depending on the study and bicycle design treatments (4, 6, 7). Crash statistics for

various user groups are reported in Section 5.3.

The reasons for the increased safety level at roundabouts are:

• Roundabouts have fewer conflict points in comparison to conventional intersections. The potential for hazardous conflicts, such as right angle and left turn

head-on crashes is eliminated with roundabout use. Single-lane approach roundabouts produce greater safety benefits than multilane approaches because of

fewer potential conflicts between road users, and because pedestrian crossing

distances are short.

• Low absolute speeds associated with roundabouts allow drivers more time to

react to potential conflicts, also helping to improve the safety performance of

roundabouts.

• Since most road users travel at similar speeds through roundabouts, i.e., have

low relative speeds, crash severity can be reduced compared to some traditionally controlled intersections.

• Pedestrians need only cross one direction of traffic at a time at each approach

as they traverse roundabouts, as compared with unsignalized intersections. The

conflict locations between vehicles and pedestrians are generally not affected

by the presence of a roundabout, although conflicting vehicles come from a

more defined path at roundabouts (and thus pedestrians have fewer places to

check for conflicting vehicles). In addition, the speeds of motorists entering and

exiting a roundabout are reduced with good design. As with other crossings

Roundabouts may improve

intersection safety by:

• Eliminating or altering

conflicts

• Decreasing speeds into and

through the intersection

• Decreasing speed

differentials
104 Federal Highway Administration

requiring acceptance of gaps, roundabouts still present visually impaired pedestrians with unique challenges, as described in Chapter 2.

For the design of a new roundabout, safety can be optimized not only by relying on

recorded past performance of roundabouts in general, but primarily by applying all

design knowledge proven to impact safety. For optimum roundabout safety and

operational performance the following should be noted:

• Minimizing the number of potential conflicts at any geometric feature should

reduce the multiple vehicle crash rate and severity.

• Minimizing the potential relative speed between two vehicles at the point of

conflict will minimize the multiple vehicle crash rate and severity (it may also

optimize capacity). To reduce the potential relative speed between vehicles,

either the absolute speeds of both vehicles need to be reduced or the angle

between the vehicle paths needs to be reduced. Commuter bicyclist speeds

can range from 20 to 25 km/h (12 to 15 mph) and designs that constrain the

speeds of motor vehicles to similar values will minimize the relative speeds and

improve safety. Lower absolute speeds will also assist pedestrian safety.

• Limiting the maximum change in speed between successive horizontal geometric elements will minimize the single vehicle crash rate and severity.

5.2 Conflicts

The frequency of crashes at an intersection is related to the number of conflict points

at an intersection, as well as the magnitude of conflicting flows at each conflict point.

A conflict point is a location where the paths of two motor vehicles, or a vehicle and

a bicycle or pedestrian queue, diverge, merge, or cross each other.

Besides conflicts with other road users, the central island of a roundabout presents a particular hazard that may result in over-representation of single-vehicle

crashes that tend to occur during periods of low traffic volumes. At cross intersections, many such violations may go unrecorded unless a collision with another

vehicle occurs.

The following sections present a variety of conflicts among vehicles, bicycles, and

pedestrians. Both legal conflicts (queuing at an intersection, merging into a traffic

stream) and conflicts prohibited by law or by traffic control devices (failure to yield

to pedestrians, running a stop sign) have been included for completeness. Even

though traffic control devices can significantly reduce many conflicts, they can not

eliminate them entirely due to violations of those devices. Many of the most serious crashes are caused by such violations.

As with crash analyses, conflict analyses are more than the simple enumeration of

the number of conflicts. A conflict analysis should account for the following factors:

• Existence of conflict point;

Conflict points occur where one

vehicle path crosses, merges or

diverges with, or queues behind

the path of another vehicle,

pedestrian, or bicycle.

Conflicts can arise from both

legal and illegal maneuvers;

many of the most serious

crashes are caused by failure to

observe traffic control devices.
Roundabouts: An Informational Guide • 5: Safety 105

• Exposure, measured by the product of the two conflicting stream volumes at a

given conflict point;

• Severity, based on the relative velocities of the conflicting streams (speed and

angle); and

• Vulnerability, based on the ability for a member of each conflicting stream to

survive a crash.

5.2.1 Vehicle conflicts

5.2.1.1 Single-lane roundabouts

Exhibit 5-1 presents a diagram of vehicle-vehicle conflict points for a traditional

three-leg (“T”) intersection and a three-leg roundabout. As the figure shows, the

number of vehicle-vehicle conflict points for roundabouts decreases from nine to

six for three-leg intersections. Note that these diagrams do not take into account

the ability to separate conflicts in space (through the use of separate left or right

turning lanes) or time (through the use of traffic control devices such as stop signs

or traffic signals).

Roundabouts bring the

simplicity of a “T” intersection

to intersections with more than

three legs.

Exhibit 5-1. Vehicle conflict

points for “T” Intersections

with single-lane approaches.

Exhibit 5-2 presents similar diagrams for a traditional four-leg (“X” or “cross”) intersection and a four-leg roundabout. As the figure shows, the number of vehiclevehicle conflict points for roundabouts decreases from 32 to 8 for four-leg intersections.
106 Federal Highway Administration

Exhibit 5-2. Vehicle conflict

point comparison for intersections with single-lane approaches.

A four-leg single-lane roundabout has 75% fewer vehicle

conflict points—compared to a

conventional intersection.

Conflicts can be divided into three basic categories, in which the degree of severity

varies, as follows:

• Queuing conflicts. These conflicts are caused by a vehicle running into the back

of a vehicle queue on an approach. These types of conflicts can occur at the

back of a through-movement queue or where left-turning vehicles are queued

waiting for gaps. These conflicts are typically the least severe of all conflicts

because the collisions involve the most protected parts of the vehicle and the

relative speed difference between vehicles is less than in other conflicts.

• Merge and diverge conflicts. These conflicts are caused by the joining or separating of two traffic streams. The most common types of crashes due to merge

conflicts are sideswipes and rear-end crashes. Merge conflicts can be more severe than diverge conflicts due to the more likely possibility of collisions to the

side of the vehicle, which is typically less protected than the front and rear of the

vehicle.

• Crossing conflicts. These conflicts are caused by the intersection of two traffic

streams. These are the most severe of all conflicts and the most likely to involve

injuries or fatalities. Typical crash types are right-angle crashes and head-on crashes.

As Exhibit 5-1 and Exhibit 5-2 show, a roundabout reduces vehicular crossing conflicts for both three- and four-leg intersections by converting all movements to right

turns. Again, separate turn lanes and traffic control (stop signs or signalization) can

often reduce but not eliminate the number of crossing conflicts at a traditional

intersection by separating conflicts in space and/or time. However, the most severe crashes at signalized intersections occur when there is a violation of the traffic control device designed to separate conflicts by time (e.g., a right-angle collision due to running a red light, and vehicle-pedestrian collisions). Therefore, the

ability of single-lane roundabouts to reduce conflicts through physical, geometric

features has been demonstrated to be more effective than the reliance on driver

obedience of traffic control devices.

Crossing conflicts are the most

severe and carry the highest

public cost.

Diverging

Crossing

Merging
Roundabouts: An Informational Guide • 5: Safety 107

5.2.1.2 Double-lane roundabouts

In general, double-lane roundabouts have some of the same safety performance

characteristics as their simpler single-lane counterparts. However, due to the presence of additional entry lanes and the accompanying need to provide wider circulatory and exit roadways, double lane roundabouts introduce additional conflicts

not present in single-lane roundabouts. This makes it important to use the minimum required number of entry, circulating and exit lanes, subject to capacity considerations. For example, according to United Kingdom roundabout crash models,

for a 10,000 entering Average Daily Traffic (ADT), flaring the entry width from one to

two lanes is likely to increase injury crashes by 25 percent (8).

The number of vehicular and pedestrian conflicts points in both conventional intersections and roundabouts increases considerably when they have additional approach lanes. The designer is encouraged to graphically determine conflicts for a

particular location, as this information can raise awareness of design issues and

may be useful in public presentations.

The types of conflicts present in multilane roundabouts that do not exist in singlelane roundabouts occur when drivers use the incorrect lane or make an improper

turn. These types of conflicts are depicted in Exhibit 5-3 and Exhibit 5-4, respectively. While these types of conflicts can also be present in other intersection forms,

they can be prevalent with drivers who are unfamiliar with roundabout operation.

The conflicts depicted in Exhibit 5-4, in particular, can be created by not providing a

proper design geometry that allows vehicles to travel side-by-side throughout the

entire roundabout (see Chapter 6). Crashes resulting from both types of conflicts

can also be reduced through proper driver education.

Double-lane roundabouts have

some of the same safety

performance characteristics as

single-lane roundabouts, but

introduce additional conflicts.

Incorrect lane use and incorrect

turns are multilane roundabout

conflicts that do not exist in

single-lane roundabouts.

Exhibit 5-3. Improper lane-use

conflicts in double-lane

roundabouts.
108 Federal Highway Administration

As with single-lane roundabouts, the most severe vehicular crossing conflicts are

eliminated and replaced by less severe merging conflicts. The additional conflicts

unique to multilane roundabouts are generally low-speed sideswipe conflicts that

typically have low severity. Therefore, although the number of conflict points increases

at multilane roundabouts when compared to a single lane roundabouts, the overall

severity of conflicts is generally less than alternative intersection control.

5.2.2 Pedestrian conflicts

Vehicle-pedestrian conflicts can be present at every intersection, even those with

minimal pedestrian volume. The following sections examine pedestrian conflicts at

signalized intersections and at roundabouts.

Signalized intersections offer the opportunity to reduce the likelihood of pedestrian-vehicle conflicts through the use of signal phasing that allows only a few

movements to move legally at any given time. Exhibit 5-5 summarizes the typical

pedestrian conflicts present on one approach to a signalized intersection. As the

exhibit shows, a pedestrian crossing at a typical signalized intersection (permitted

or protected-permitted left turns, right turns on red allowed) faces four potential

vehicular conflicts, each coming from a different direction:

• Crossing movements on red (typically high-speed, illegal)

• Right turns on green (legal)

• Left turns on green (legal for protected-permitted or permitted left turn phasing)

• Right turns on red (typically legal)

In terms of exposure, the illegal movements should be accorded a lower weight

than legal conflicts. However, they may be accorded an offsetting higher weight in

terms of severity. For an intersection with four single-lane approaches, this results

in a total of 16 pedestrian-vehicle conflicts.

Exhibit 5-4. Improper turn

conflicts in double-lane

roundabouts.

Types of pedestrian crossing

conflicts present at signalized

intersections.
Roundabouts: An Informational Guide • 5: Safety 109

Exhibit 5-5. Vehicle-pedestrian

conflicts at signalized intersections.

Pedestrians at roundabouts, on the other hand, face two conflicting vehicular movements on each approach, as depicted in Exhibit 5-6:

• Conflict with entering vehicles; and

• Conflict with exiting vehicles.

At conventional and roundabout intersections with multiple approach lanes, an additional conflict is added with each additional lane that a pedestrian must cross.

The direction conflicting

vehicles will arrive from is more

predictable for pedestrians at

roundabouts.

Exhibit 5-6. Vehicle-pedestrian

conflicts at single-lane roundabouts.
110 Federal Highway Administration

5.2.3 Bicycle conflicts

Bicycles face similar conflicts as motor vehicles at both signalized intersections

and roundabouts. However, because bicyclists typically ride on the right side of the

road between intersections, they face additional conflicts due to overlapping paths

with motor vehicles. Conflicts unique to bicyclists occur on each approach to conventional four-leg intersections, as depicted in Exhibit 5-7 (showing left turns like

motor vehicles or left turns like pedestrians).

Exhibit 5-7. Bicycle conflicts

at conventional intersections

(showing two left-turn options).

At roundabouts, bicycles may be provided the option of traveling as a vehicle or as

a pedestrian. As a result, the conflicts experienced by bicyclists are dependent on

how they choose to negotiate the roundabout, as shown in Exhibit 5-8. When traveling as a vehicle at a single-lane roundabout, an additional conflict occurs at the

point where the bicyclist merges into the traffic stream; the remainder are similar

to those for motor vehicles. At double-lane and larger roundabouts where bicycles

are typically traveling on the outside part of the circulatory roadway, bicyclists face

a potential conflict with exiting vehicles where the bicyclist is continuing to circulate around the roundabout. Bicyclists may feel compelled to “negotiate” the circle

(e.g., by indicating their intentions to drivers with their arms) while avoiding conflicts where possible. Bicyclists are less visible and therefore more vulnerable to

the merging and exiting conflicts that happen at double-lane roundabouts.

When traveling as a pedestrian, an additional conflict for bicyclists occurs at the point

where the bicyclist gets onto the sidewalk, at which point the bicyclist continues

around the roundabout like a pedestrian. On shared bicycle-pedestrian paths or on

sidewalks, if bicyclists continue to ride, additional bicycle-pedestrian conflicts occur

wherever bicycle and pedestrian movements cross (not shown on the exhibit).

Bicycles can be provided with

the option of traveling as either

a vehicle or a pedestrian

through a roundabout.
Roundabouts: An Informational Guide • 5: Safety 111

5.3 Crash Statistics

 This section summarizes the overall safety performance of roundabouts in various

countries (including the U.S.) and then examines the detailed collision types experienced in France and Queensland, Australia. Pedestrian and bicycle crash statistics are discussed separately, including design issues for visually impaired pedestrians.

5.3.1 Comparisons to previous intersection treatment

Exhibit 5-9 shows the crash frequencies (average annual crashes per roundabout)

experienced at eleven intersections in the U.S. that were converted to roundabouts.

As the exhibit shows, both types of roundabouts showed a reduction in both injury

and property-damage crashes after installation of a roundabout. It should be noted

that due to the small size of the data sample, the only result that is statistically

significant is the injury crash reduction for small and moderate roundabouts.

Exhibit 5-8. Bicycle conflicts

at roundabouts (showing two

left-turn options).

Bicycle-pedestrian conflicts can

also occur on shared pathways

adjacent to the roundabout.
112 Federal Highway Administration

Percent Change 5

Exhibit 5-9. Average annual

crash frequencies at 11 U.S.

intersections converted to

roundabouts.

Notes:

1. Mostly single-lane roundabouts with an inscribed circle diameter of 30 to 35 m (100 to 115 ft).

2. Multilane roundabouts with an inscribed circle diameter greater than 50 m (165 ft).

3. Inj. = Injury crashes

4. PDO = Property Damage Only crashes

5. Only injury crash reductions for small/moderate roundabouts were statistically significant.

Source: (9)

Compared to results from Australia, France, and the United Kingdom, these crash

frequencies are quite high. Annual crash frequencies in France, Australia, and United

Kingdom of 0.15, 0.6, and 3.31 injury crashes per roundabout, respectively, have

been reported (1, 10). The reader should note that the UK has many high-volume,

multilane roundabouts.

In spite of the higher frequencies, injury crash rates, which account for traffic volume exposure, are significantly lower at U.S. roundabout sites. In a recent study of

eight single-lane roundabouts in Maryland and Florida, the injury crash rate was

found to be 0.08 crashes per million entering vehicles (5). By comparison, the

injury crash rate was reported to be 0.045 crashes per million entering vehicles in

France and 0.275 crashes per million entering vehicles in the United Kingdom (1, 10).

Experiences in the United States show a reduction in crashes after building a roundabout of about 37 percent for all crashes and 51 percent for injury crashes. These

values correspond with international studies with much larger sample sizes, as

shown in Exhibit 5-10.

Small/Moderate1 8 4.8 2.0 2.4 2.4 0.5 1.6 -51% 73% -32%

Large2 3 21.5 5.8 15.7 15.3 4.0 11.3 -29% -31% -10%

Total 11 9.3 3.0 6.0 5.9 1.5 4.2 -37% -51% -29%

Type of

Roundabout Sites

Before

Roundabout Roundabout

Total Inj.3 PDO4 Total Inj. PDO Total Inj. PDO

Exhibit 5-10. Mean crash

reductions in various countries.

Country

Mean Reduction (%)

All Crashes Injury Crashes

Australia 41 - 61% 45 - 87%

France 57 - 78%

Germany 36%

Netherlands 47%

United Kingdom 25 - 39%

United States 37% 51%

Source: (2), France: (11)
Roundabouts: An Informational Guide • 5: Safety 113

Crash Type of Entering- Single

Country Description Roundabout circulating Rear-end Vehicle

The findings of these studies show that injury crashes are reduced more dramatically than crashes involving property damage only. This again is in part due to the

configuration of roundabouts, which eliminates severe crashes such as left turn,

head-on, and right angle collisions. Most of these studies also show that crash

reduction in rural areas is much higher than in urban areas.

Note that the geometry of many studied sites may not necessarily conform to

good roundabout design. Improved design principles, such as an emphasis on achieving consistent speeds, may result in better safety performance. It should also be

noted that these crash reductions are generally for sites where roundabouts were

selected to replace problem intersections. Therefore, they do not necessarily represent a universal safety comparison with all other intersection types.

Collisions at roundabouts tend to be less severe than at conventional intersections. Most crashes reported at roundabouts are a result of drivers failing to yield

on entry, referred to as entering-circulating crashes. In addition, rear-end collisions

and single vehicle crashes have been reported in many studies. Exhibit 5-11 shows

the percentage of the three main crash types reported in different countries.

Caveats for comparing the

results of crash studies.

1. Percentages do not necessarily sum to 100% because only three major crash categories are shown.

Source: (10)

Exhibit 5-11. Reported

proportions of major crash

types at roundabouts.

Australia All crashes Single and 51% 22% 18%

multilane

France Injury crashes Single and 37% 13% 28%

multilane

Germany All crashes Single lane 30% 28% 17%

Switzerland All crashes Single and 46% 13% 35%

multilane

United Kingdom Injury crashes Single and 20 - 71% 7 - 25% 8 - 30%

multilane

Type of Crash1

5.3.2 Collision types

It is instructive for designers to examine details of collision types and location at

roundabouts. Statistics are available for roundabouts designed according to local

practices in France, Queensland (Australia), and the United Kingdom. It should be

noted that the reported frequencies are to some extent related to the specific

design standards and reporting processes used in these countries.

Exhibit 5-12 presents a summary of the percentage of crashes by collision type.

The numbered items in the list correspond to the numbers indicated on the diagrams given in Exhibit 5-13 as reported in France. The French data illustrate collision types for a sample of 202 injury crashes from 179 urban and suburban roundabouts in France for the period 1984–1988 (12). For comparison purposes, data
114 Federal Highway Administration

from Queensland, Australia (13) and the United Kingdom (1) have been superimposed onto the same classification system.

The results in Exhibit 5-12 are instructive for a number of reasons:

• A variety of collision types can take place at roundabouts. A designer should be

aware of these collision types when making decisions about alignment and

location of fixed objects. It is recommended that these collision types be adopted

as conflict types in the U.S. to conduct traffic conflict analysis and report crashes

at roundabouts.

• Although reporting methodologies may vary somewhat, crash experience varies from country to country. This may be due to a combination of differences in

driver behavior, and design features.

1. Failure to yield at entry (entering-circulating) 36.6% 50.8% 71.1%

2. Single-vehicle run off the circulatory roadway 16.3% 10.4% 8.2%2

3. Single vehicle loss of control at entry 11.4% 5.2% 2

4. Rear-end at entry 7.4% 16.9% 7.0%3

5. Circulating-exiting 5.9% 6.5%

6. Pedestrian on crosswalk 5.9% 3.5%4

7. Single vehicle loss of control at exit 2.5% 2.6% 2

8. Exiting-entering 2.5%

9. Rear-end in circulatory roadway 0.5% 1.2%

10. Rear-end at exit 1.0% 0.2%

11. Passing a bicycle at entry 1.0%

12. Passing a bicycle at exit 1.0%

13. Weaving in circulatory roadway 2.5% 2.0%

14. Wrong direction in circulatory roadway 1.0%

15. Pedestrian on circulatory roadway 3.5% 4

16. Pedestrian at approach outside crosswalk 1.0% 4

Other collision types 2.4% 10.2%

Other sideswipe crashes 1.6%

Notes:

1. Data are for “small” roundabouts (curbed central islands > 4 m [13 ft] diameter, relatively large ratio of

inscribed circle diameter to central island size)

2. Reported findings do not distinguish among single-vehicle crashes.

3. Reported findings do not distinguish among approaching crashes.

4. Reported findings do not distinguish among pedestrian crashes.

Sources: France (12), Australia (13), United Kingdom (1)

Queensland United

Collision Type France (Australia) Kingdom1

Exhibit 5-12. Comparison of

collision types at roundabouts.
Roundabouts: An Informational Guide • 5: Safety 115

Exhibit 5-13. Graphical

depiction of collision types at

roundabouts.

Source (8)
116 Federal Highway Administration

Three of the predominant types of collision are: (1) failures to yield at entry to

circulating vehicles, (2) single vehicle run-off the circulatory roadway, and (3) single

vehicle run-into the central island. A more recent crash study (14) confirmed a high

proportion of single vehicle crashes: 49 percent in rural areas, versus 21 percent in

urban areas. According to crash models from the United Kingdom, single vehicle

crashes range between 20 and 40 percent depending on traffic and design characteristics of sites. In the United Kingdom models, separation by urban and rural

areas is not provided.

To reduce the severity of single vehicle crashes, special attention should be accorded to improving visibility and avoiding or removing any hard obstacles on the

central island and splitter islands in both urban and rural environments. A French

study (14) identified a number of major obstacles that caused fatalities and injuries:

trees, guardrail, concrete barriers, fences, walls, piers, sign or light poles, landscaping pots or hard decorative objects, and steep cross-slopes on the central

island.

In rural areas, the benefit of lighting has not yet been quantified. In France, only 36

percent of the rural sites are lighted. At these sites, 46 percent of all crashes, and

49 percent of single vehicle crashes occur at night (14).

The French study (7) in 15 towns of 202 urban roundabout crashes compared with

all crossroads reported the percentage of crashes by user type, as shown in Exhibit 5-14. The percentage of crashes concerning pedestrians was similar to all

crossroads. However, the percentage of crashes involving bicycles and mopeds

was larger—15.4 percent for urban crossroads overall versus 24.2 percent for roundabouts, i.e., almost 60 percent more.

Exhibit 5-14. Crash percentage per type of user for urban

roundabouts in 15 towns in

western France.

Pedestrians 6.3% 5.6%

Bicycles 3.7% 7.3%

Mopeds 11.7% 16.9%

Motor cycles 7.4% 4.8%

Cars 65.7% 61.2%

Utility vehicles 2.0% 0.6%

Heavy goods vehicles 2.0% 3.0%

Bus/coach 0.8% 0.6%

Miscellaneous 0.4% 0.0%

Total 100.0% 100.0%

Source: (7)

User All Crossroads Roundabouts
Roundabouts: An Informational Guide • 5: Safety 117

5.3.3 Pedestrians

As was described previously, vehicular injury crashes normally decrease when roundabouts are installed at an existing intersection. The safety benefits of roundabouts

have been found to generally carry over to pedestrians as well, as shown in British

statistics of Exhibit 5-15. This may be due to the reduced speeds at roundabouts as

compared with the previous intersection forms.

Exhibit 5-15. British crash

rates for pedestrians at

roundabouts and

signalized intersections.

For pedestrians, the risk of being involved in a severe collision is lower at roundabouts than at other forms of intersections, due to the slower vehicle speeds.

Likewise, the number of conflict points for pedestrians is lower at roundabouts

than at other intersections, which can lower the frequency of collisions. The splitter

island between entry and exit allows pedestrians to resolve conflicts with entering

and exiting vehicles separately.

A Dutch study of 181 intersections converted to roundabouts (4) found reductions

(percentage) in all pedestrian crashes of 73 percent and in pedestrian injury crashes

of 89 percent. In this study, all modes shared in the safety benefits to greater

(passenger cars) or lesser extents (bicycles), as shown in Exhibit 5-16.

Exhibit 5-16. Percentage

reduction in the number of

crashes by mode at 181

converted Dutch roundabouts.

Mini-roundabout 0.31

Conventional roundabout 0.45

Flared roundabout 0.33

Signals 0.67

Source: (1, 15)

Pedestrian Crashes

Intersection Type per Million Trips

Passenger car 63% 95%

Moped 34% 63%

Bicycle 8% 30%

Pedestrian 73% 89%

Total 51% 72%

Source: (4)

Mode All Crashes Injury Crashes
118 Federal Highway Administration

A risk analysis of 59 roundabouts and 124 signalized intersections was carried out

on crash data in Norway between 1985 and 1989. Altogether, 33 crashes involving

personal injury were recorded at the 59 roundabouts. Only 1 of these crashes

involved a pedestrian, compared with the signalized intersections, where pedestrians were involved in 20 percent of the personal injury crashes (57 of 287 injury

crashes) (16).

Further, there is no quantitative evidence of increased safety for pedestrians at

roundabouts with striped (zebra) crossings, where pedestrians have priority. Therefore, striped crossings have generally not been used in other countries. However,

in the U.S., it is recommended that all crosswalks be striped except at rural locations with low pedestrian volumes. Although this is not their intended function,

striped crosswalks may further alert approaching drivers to a change in their appropriate speed near the yield point.

Crash data have not been collected to indicate whether a pedestrian has a disability, and no studies have focused specifically on the safety of visually impaired pedestrians at roundabouts. This is an area requiring further research.

5.3.3.1 Information access for blind or visually impaired pedestrians

Roundabout crossing skills may be difficult for disabled pedestrians to perform

without assistance. For example, audible pedestrian-activated signals may be considered on an approach, although this treatment is not typical. Any leg of any roundabout could be equipped with a pedestrian-activated signal at the pedestrian crossing, if a balanced design requires providing assistance to pedestrians at that location. For example, motorized volume that is too heavy at times to provide a sufficient number of gaps acceptable for pedestrians may warrant a pedestrian signal

equipped with audible devices to assist people with visual disabilities.

When crossing a roundabout, there are several areas of difficulty for pedestrians

who are blind or visually impaired. It is desirable that a visually impaired pedestrian

with good travel skills should be able to arrive at an unfamiliar intersection and cross

it with pre-existing skills and without special, intersection-specific training. Roundabouts pose problems at several points of the crossing experience, from the perspective of their access to information:

• The first task of the visually impaired pedestrian is to locate the crosswalk. This

can be difficult if the roundabout is not properly landscaped and if the curb edge

of the ramp is not marked with a detectable warning surface (see Chapter 6).

The crosswalk direction must also be unambiguous.

Zebra-stripe markings are

recommended at most

roundabouts to indicate

pedestrian crossings.

Safety of visually impaired

pedestrians at roundabouts

requires further research.

Challenges that roundabouts

pose to visually impaired

pedestrians.

• Depending upon whether the visually impaired pedestrian is crossing the roundabout in a clockwise or counterclockwise direction, they must listen for a safe

gap to cross either the entrance or exit lane(s). The primary problem is the sound

of traffic on the roundabout, which may mask the sound of cars approaching the
Roundabouts: An Informational Guide • 5: Safety 119

Chapters 6 and 7 provide

suggestions on designing

roundabouts to accommodate

persons with disabilities.

crosswalk. While crossing the exit lane poses the greater hazard to the pedestrian who is visually impaired because of the higher speed of the vehicles, crossing the entrance may also pose significant problems. Entering traffic, while slower,

may also be intimidating as it may not be possible to determine by sound alone

whether a vehicle has actually stopped or intends to stop. Sighted pedestrians

often rely upon communication through eye contact in these situations; however, that is not a useful or reliable technique for the pedestrian who is visually

impaired. Both these problems are further exacerbated at roundabouts with

multilane entrances and exits. In these roundabouts, a stopped car in the near

lane may mask the sounds of other traffic. It may also block the view of the

driver in the far lane of the cane or guide dog of a person who is visually impaired who begins to cross (this is also a problem for children and people using

wheelchairs on any crossing of a multilane road).

• The third task is locating the splitter island pedestrian refuge. If this refuge is

not ramped, curbed, or equipped with detectable warnings, it is not detectable

by a pedestrian who is visually impaired.

• Crossing the remaining half of the crossing (see the second bullet above).

• Locating the correct walkway to either continue their path or locate the adjacent

crosswalk to cross the next leg of the roundabout.

Unless these issues are addressed by a design, the intersection is “inaccessible”

and may not be permissible under the ADA. Chapters 6 and 7 provide specific

suggestions to assist in providing the above information. However, more research

is required to develop the information jurisdictions need to determine where roundabouts may be appropriate and what design features are required for people with

disabilities. Until specific standards are adopted, engineers and jurisdictions must

rely on existing related research and professional judgment to design pedestrian

features so that they are usable by pedestrians with disabilities.

Possible design remedies for the difficulties faced by pedestrians include tight entries, raised speed tables with detectable warnings, treatments for visually impaired pedestrians to locate crosswalks, raised pavement markers with yellow flashing lights to alert drivers of crossing pedestrians, pedestrian crossings with actuated signals set sufficiently upstream of the yield line to minimize the possibility of

exiting vehicle queues spilling back into the circulatory roadway (6). However, the

safety of these treatments at roundabouts has not been tested in the United States.
120 Federal Highway Administration

Exhibit 5-17. British crash

rates (crashes per million trips)

for bicyclists and motorcyclists

at roundabouts and signalized

intersections.

A French study (7) compared the crashes in 1988 in 15 towns in the west of France

at both signalized intersections and roundabouts, as shown in Exhibit 5-18. The

conclusions from the analysis were:

• There were twice as many injury crashes per year at signalized intersections

than at roundabouts;

• Two-wheel vehicles were involved in injury crashes more often (+77 percent) at

signalized intersections than on roundabouts;

• People were more frequently killed and seriously injured per crash (+25 percent) on roundabouts than at signalized intersections;

• Proportionally, two-wheel vehicle users were more often involved in crashes (16

percent) on roundabouts than at signalized intersections. Furthermore, the consequences of such crashes were more serious.

5.3.4 Bicyclists

As shown in Exhibit 5-17, at British roundabouts bicyclists fare worse in terms of

crashes at roundabouts than at signalized intersections.

Mini-roundabout 3.11 2.37

Conventional roundabout 2.91 2.67

Flared roundabout 7.85 2.37

Signals 1.75 2.40

Source: (1, 15)

Intersection Type Bicyclists Motorcyclists

Exhibit 5-18. A comparison of

crashes between signalized and

roundabout intersections in

1998 in 15 French towns.

Number of crossroads 1,238 179

Number of personal injuries 794 59

Number of crashes involving 2-wheel vehicles 278 28

Personal injury crashes/year/crossroad 0.64 0.33

2-wheel vehicle crashes/year/crossroad 0.23 0.13

Crashes to 2-wheel vehicles per 100 crashes 35.0 40.7

Serious crashes/year/crossroad 0.14 0.089

Serious crashes to 2-wheel vehicles/year/crossroad 0.06 0.045

Serious crashes/100 crashes 21.9 27.1

Serious crashes to 2-wheel vehicles/100 crashes

to a 2-wheel vehicle 27.0 33.3

Source: (7)

Signalized

Crossroads Roundabouts
Roundabouts: An Informational Guide • 5: Safety 121

All European countries report that a more careful design is necessary to enhance

bicyclists’ safety. The type of bicycle crashes depends on the bicycle facilities provided at the roundabout. If there are no bicycle facilities, or if there is a bike lane on

the outer area of the circulatory roadway, crashes typically occur between entering

cars and circulating bicyclists as well as between cars heading into an exit and

circulating bicyclists. Improperly placed signs on the splitter island may also be a

contributing factor.

As a result, most European countries have the following policies:

• Avoid bike lanes on the outer edge of the circulatory roadway.

• Allow bicyclists to mix with vehicle traffic without any separate facility in the

circulatory roadway when traffic volumes are low, on single lane roundabouts

operating at lower speeds (e.g., up to 8,000 vehicles per day in the Netherlands

(4)).

• Introduce separated bicycle facilities outside the circulatory roadway when vehicular and bicycle volumes are high. These separated bicycle facilities cross the

exits and entries at least one car length from the edge of the circulatory roadway lane, adjacent to the pedestrian crossings. In some countries, bicyclists

have priority over entering and exiting cars, especially in urban areas (e.g., Germany). Other countries prefer to give priority to car traffic showing a yield sign

to bicyclists (e.g., Netherlands). The latter solution (i.e., separate bicycle facilities with vehicular traffic priority at the crossing points) is the standard solution

for rural areas in most European countries.

Speed is a fundamental risk factor in the safety of bicyclists and pedestrians. Typical bicyclist speeds are in the range of 20 to 25 km/h (12 to 15 mph), and designs

that constrain the speeds of vehicles to similar values will minimize the relative

speeds and thereby improve safety. Design features that slow traffic such as tightening entry curvature and entry width, and radial alignment of the legs of a roundabout, such as with the urban compact design, are considered safe treatments for

bicyclists (17).

In the Netherlands, a 90 percent decrease in injury crashes was experienced with

separate bicycle paths around roundabouts where bicyclists do not have right-ofway at the crossings (17).

A bicycle crash prediction model from Sweden has been validated against data for

Swedish, Danish, and Dutch roundabouts (18). The model provides reasonable results for roundabouts with up to 12,000 vehicles per day and 4,000 bicycles per

day. The model tends to over-predict crashes (i.e., is conservative) for roundabouts

carrying more than 12,000 vehicles per day that are also designed with separate

bicycle paths with crossings on the approach legs. It is calibrated for crossroad

intersections as well as roundabouts. To obtain the expected cycling crashes per

year at roundabouts, the value derived from the general junction model is factored

by 0.71, implying that bicycle crashes at roundabouts are 71 percent less frequent

than at junctions in general. However, the reader is cautioned when extrapolating

European bicycling experience to the U.S., as drivers in Europe are more accustomed to interacting with bicyclists.

Typical European practice is to

provide separated bicycle

facilities outside the circulatory

roadway when vehicular and

bicycle volumes are high.
122 Federal Highway Administration

5.4 Crash Prediction Models

Crash prediction models have been developed for signalized intersections in the

U.S., as discussed previously in Chapter 3. However, no crash prediction models

exist yet for U.S. roundabouts and driver behavior. Given the relatively recent introduction of roundabouts to the U.S. and driver unfamiliarity with them, crash prediction models from other countries should be used cautiously. As reported earlier in

Section 5.3, crash statistics vary from country to country, both in terms of magnitude and in terms of collision types. Consequently, the application of a crash prediction model from another country may not accurately predict crash frequencies

at U.S. locations. Nonetheless, these crash prediction models from other countries can be useful in understanding the relative effects of various geometric features on the number of crashes that might be expected. The user is thus cautioned

to use these models only for comparative purposes and for obtaining insights into

the refinement of individual geometric elements, not to use them for predicting

absolute numbers of crashes under U.S. conditions.

Crash models relating crash frequency to roundabout characteristics are available

from the United Kingdom. The sample consisted of 84 four-leg roundabouts of all

sizes, small to large and with various number of approach lanes and entry lanes

(flared or parallel entries) (1). Approach speeds were also evenly represented between 48 to 64 km/h (30 to 40 mph) and 80 to 113 km/h (50 to 70 mph). Crash data

were collected for periods of 4 to 6 years, a total of 1,427 fatal, serious, and slight

injuries only. The proportion of crashes with one casualty was 83.7 percent, and

those with two casualties was 12.5 percent. The models are based on generalized

linear regression of the exponential form, which assumes a Poisson distribution.

Their goodness of fit is expressed in terms of scaled deviations that are moderately reliable. No additional variables, other than those listed below, could further

improve the models significantly (see also (8)).

The British crash prediction equations (1), for each type of crash are listed in Equations 5-1 through 5-5. Note that these equations are only valid for roundabouts

with four legs. However, the use of these models for relative comparisons may still

be reasonable.

Entry-Circulating: (5-1)

where: A = personal injury crashes (including fatalities) per year per roundabout

approach;

Qe = entering flow (1,000s of vehicles/day)

Qc = circulating flow (1,000s of vehicles/day)

Ce = entry curvature = 1/Re

e = entry width (m)

v = approach width (m)

R = ratio of inscribed circle diameter/central island diameter

Pm = proportion of motorcycles (%)

θ = angle to next leg, measured centerline to centerline (degrees)

Crash prediction models have

not been developed for U.S.

roundabouts.
Roundabouts: An Informational Guide • 5: Safety 123

Approaching: (5-2)

where: A = personal injury crashes (including fatalities) per year at roundabout

approach or leg;

Qe = entering flow (1,000s of vehicles/day)

Ce = entry curvature = 1/Re

Re = entry path radius for the shortest vehicle path (m)

e = entry width (m)

Single Vehicle: (5-3)

where: A = personal injury crashes (including fatalities) per year at roundabout

approach or leg

Qe = entering flow (1,000s of vehicles/day)

Ce = entry curvature = 1/Re

Re = entry path radius for the shortest vehicle path (m)

V = approach width (m)

Ca = approach curvature = 1/Ra

Ra = approach radius (m), defined as the radius of a curve between 50 m

(164 ft) and 500 m (1,640 ft) of the yield line

Other (Vehicle): (5-4)

where: A = personal injury crashes (including fatalities) per year at roundabout

approach or leg

Qec = product Qe 

• Qc

Qe = entering flow (1,000s of vehicles/day)

Qc = circulating flow (1,000s of vehicles/day)

Pm = proportion of motorcycles

Pedestrian: (5-5)

where: A = personal injury crashes (including fatalities) per year at roundabout

approach or leg

Qep = product (Qe + Qex). Qp

Qe = entering flow (1,000s of vehicles/day)

Qex = exiting flow (1,000s of vehicles/day)

Qp = pedestrian crossing flow (1,000s of pedestrians/day)

According to the U.K. crash models, the major physical factors that were statistically significant are entry width, circulatory width, entry path radius, approach curvature, and angle between entries. Some of the effects of these parameters are as

follows:

• Entry width: For a total entry flow of 20,000 vehicles per day, widening an entry

from one lane to two lanes is expected to cause 30 percent more injury crashes.

At 40,000 vehicles per day, widening an entry from two lanes to three lanes will

cause a 15 percent rise in injury crashes. Moreover, the models could not take

into account the added hazard to bicyclists and pedestrians who will have to

travel longer exposed distances. (8)
124 Federal Highway Administration

• Circulatory width: Widening the circulatory roadway has less impact on crashes

than entry width. Crashes are expected to rise about 5 percent for a widening of

two meters. (8)

• Entry path radius: Entry-circulating collision type increases with entry path radius (for the fastest path), while single vehicle and approach collision types

decrease. For a double-lane approach, an optimum entry path radius is 50 to 70

m (165 to 230 ft). (8)

• Approach curvature: Approach curvature is safer when the approach curve is to

the right and less so when the curve is to the left. This implies that a design is

slightly safer when reverse curves are provided to gradually slow drivers before

entry. For a double-lane approach roundabout with entering flow of 50,000 vehicles per day, changing a straight approach to a right-turning curve of 200 m

(650 ft) radius reduces crash frequency by 5 percent. (8)

• Angle between entries: As the angle between entries decreases, the frequency

of crashes increases. For example, an approach with an angle of 60 degrees to

the next leg of the roundabout increases crash frequency by approximately 35

percent over approaches at 90-degree angles. Therefore, the angle between

entries should be maximized to improve safety.

An approach suggested in Australia (13) differs from the British approach in that the

independent variables are based on measures related to driver behavior. For instance, the collision rate for single vehicle crashes was found to be:

(5-6)

and

(5-7)

where: Asp= the number of single vehicle crashes per year per leg for vehicle path

segments prior to the yield line.

Asa = the number of single vehicle crashes per year per leg for vehicle path

segments after the yield line.

Q = the average annual daily traffic in the direction considered—one way

traffic only (veh/d)

L = the length of the driver’s path on the horizontal geometric element (m).

S = the 85th-percentile speed on the horizontal geometric element (km/h).

∆S = the decrease in the 85th-percentile speed at the start on the horizontal geometric element (km/h). This indicates the speed change from

the previous geometric element.

R = the vehicle path radius on the geometric element (m).

These equations demonstrate a direct relationship between the number of crashes,

overall speed magnitudes, and the change in speed between elements. Therefore,

this equation can be used to estimate the relative differences in safety benefits

between various geometric configurations by estimating vehicle speeds through

the various parts of a roundabout.

Maximize angles between

entries.
Roundabouts: An Informational Guide • 5: Safety 125

5.5 References

1. Maycock, G., and R.D. Hall. Crashes at four-arm roundabouts. TRRL Laboratory

Report LR 1120. Crowthorne, England: Transport and Road Research Laboratory, 1984.

2. Garder, P. The Modern Roundabouts: The Sensible Alternative for Maine. Maine

Department of Transportation, Bureau of Planning, Research and Community

Services, Transportation Research Division, 1998.

3. Brilon, W. and B. Stuwe. “Capacity and Design of Traffic Circles in Germany.” In

Transportation Research Record 1398. Washington, D.C.: Transportation Research

Board, National Research Council, 1993.

4. Schoon, C.C., and J. van Minnen. Accidents on Roundabouts: II. Second study

into the road hazard presented by roundabouts, particularly with regard to cyclists and moped riders. R-93-16. The Netherlands: SWOV Institute for Road

Safety Research, 1993.

5. Flannery, A. and T.K. Datta. “Modern Roundabouts and Traffic Crash Experience

in the United States.” In Transportation Research Record 1553. Washington, D.C.:

Transportation Research Board, National Research Council, 1996.

6. Brown, M. TRL State of the Art Review—The Design of Roundabouts. London:

HMSO, 1995.

7. Alphand, F., U. Noelle, and B. Guichet. “Roundabouts and Road Safety: State of

the Art in France.” In Intersections without Traffic Signals II, Springer-Verlag,

Germany (W. Brilon, ed.), 1991, pp. 107–125.

8. Bared, J.G., and K. Kennedy. “Safety Impacts of Modern Roundabouts,” Chapter

28, The Traffic Safety Toolbox: A Primer on Traffic Safety, Institute of Transportation Engineers, 2000.

9. Jacquemart, G. Synthesis of Highway Practice 264: Modern Roundabout Practice in the United States. National Cooperative Highway Research Program. Washington, D.C: National Academy Press, 1998.

10. Brilon, W. and L. Bondzio. White Paper: Summary of International Statistics on

Roundabout Safety (unpublished), July 1998.

11. Guichet, B. “Roundabouts In France: Development, Safety, Design, and Capacity.” In Proceedings of the Third International Symposium on Intersections Without Traffic Signals (M. Kyte, ed.), Portland, Oregon, U.S.A. University of Idaho,

1997.

12. Centre d’Etude des Transports Urbains (CETUR). “Safety of Roundabouts in

Urban and Suburban Areas.” Paris, 1992.

13. Arndt, O. “Road Design Incorporating Three Fundamental Safety Parameters.”

Technology Transfer Forum 5 and 6, Transport Technology Division, Main Roads

Department, Queensland, Australia, August 1998.

14. SETRA/CETE de l’Ouest. “Safety Concerns on Roundabouts.” 1998.
126 Federal Highway Administration

15. Crown, B. “An Introduction to Some Basic Principles of U.K. Roundabout Design.” Presented at the ITE District 6 Conference on Roundabouts, Loveland,

Colorado, October 1998.

16. Seim, K. “Use, Design and Safety of Small Roundabouts in Norway.” In “Intersections Without Traffic Signals II”, Springer-Verlag, Germany (W. Brilon, ed.),

1991, pp.270–281.

17. Van Minnen, J. “Safety of Bicyclists on Roundabouts Deserves Special Attention.” SWOV Institute of Road Safety Research in the Netherlands, Research

Activities 5, March 1996.

18. Brude, U., and J. Larsson. The Safety of Cyclists at Roundabouts—A Comparison Between Swedish, Danish and Dutch Results. Swedish National Road and

Transport Research Institute (VTI), Nordic Road & Transport Research No. 1,

1997.
Roundabouts: An Informational Guide • 6: Geometric Design 127

6 Geometric Design

6.1 Introduction 130

6.1.1 Geometric elements 130

6.1.2 Design process 130

6.2 General Design Principles 132

6.2.1 Speeds through the roundabout 132

6.2.2 Design vehicle 142

6.2.3 Nonmotorized design users 144

6.2.4 Alignment of approaches and entries 144

6.3 Geometric Elements 145

6.3.1 Inscribed circle diameter 145

6.3.2 Entry width 147

6.3.3 Circulatory roadway width 149

6.3.4 Central island 150

6.3.5 Entry curves 152

6.3.6 Exit curves 154

6.3.7 Pedestrian crossing location and treatments 155

6.3.8 Splitter islands 157

6.3.9 Stopping sight distance 159

6.3.10 Intersection sight distance 161

6.3.11 Vertical considerations 164

6.3.12 Bicycle provisions 167
128 Federal Highway Administration

6.3.13 Sidewalk treatments 168

6.3.14 Parking considerations and bus stop locations 169

6.3.15 Right-turn bypass lanes 170

6.4 Double-Lane Roundabouts 172

6.4.1 The natural vehicle path 172

6.4.2 Vehicle path overlap 174

6.4.3 Design method to avoid path overlap 174

6.5 Rural Roundabouts 176

6.5.1 Visibility 177

6.5.2 Curbing 177

6.5.3 Splitter islands 177

6.5.4 Approach curves 178

6.6 Mini-Roundabouts 179

6.7 References 181

Exhibit 6-1. Basic geometric elements of a roundabout. 131

Exhibit 6-2. Roundabout design process. 131

Exhibit 6-3. Sample theoretical speed profile (urban compact roundabout). 133

Exhibit 6-4. Recommended maximum entry design speeds. 133

Exhibit 6-5. Fastest vehicle path through single-lane roundabout. 134

Exhibit 6-6. Fastest vehicle path through double-lane roundabout. 135

Exhibit 6-7. Example of critical right-turn movement. 135

Exhibit 6-8. Side friction factors at various speeds (metric units). 137

Exhibit 6-9. Side friction factors at various speeds (U.S. customary units). 137

Exhibit 6-10. Speed-radius relationship (metric units). 138

Exhibit 6-11. Speed-radius relationship (U.S. customary units). 138

Exhibit 6-12. Vehicle path radii. 139

Exhibit 6-13. Approximated R4

 values and corresponding R1 values

(metric units). 141

Exhibit 6-14. Approximated R4

 values and corresponding R1 values

(U.S. customary units). 141

Exhibit 6-15. Through-movement swept path of WB-15 (WB-50) vehicle. 143

Exhibit 6-16. Left-turn and right-turn swept paths of WB-15 (WB-50) vehicle. 143

Exhibit 6-17. Key dimensions of nonmotorized design users. 144

Exhibit 6-18. Radial alignment of entries. 145

Exhibit 6-19. Recommended inscribed circle diameter ranges. 146

Exhibit 6-20. Approach widening by adding full lane. 148
Roundabouts: An Informational Guide • 6: Geometric Design 129

Exhibit 6-21. Approach widening by entry flaring. 148

Exhibit 6-22. Minimum circulatory lane widths for two-lane

roundabouts. 150

Exhibit 6-23. Example of central island with a traversable apron. 151

Exhibit 6-24. Single-lane roundabout entry design. 153

Exhibit 6-25. Single-lane roundabout exit design. 154

Exhibit 6-26. Minimum splitter island dimensions. 157

Exhibit 6-27. Minimum splitter island nose radii and offsets. 158

Exhibit 6-28. Design values for stopping sight distances. 159

Exhibit 6-29. Approach sight distance. 160

Exhibit 6-30. Sight distance on circulatory roadway. 160

Exhibit 6-31. Sight distance to crosswalk on exit. 161

Exhibit 6-32. Intersection sight distance. 162

Exhibit 6-33. Computed length of conflicting leg of intersection

sight triangle. 163

Exhibit 6-34. Sample plan view. 164

Exhibit 6-35. Sample approach profile. 165

Exhibit 6-36. Sample central island profile. 165

Exhibit 6-37. Typical circulatory roadway section. 166

Exhibit 6-38. Typical section with a truck apron. 166

Exhibit 6-39. Possible provisions for bicycles. 168

Exhibit 6-40. Sidewalk treatments. 169

Exhibit 6-41. Example of right-turn bypass lane. 170

Exhibit 6-42. Configuration of right-turn bypass lane with

acceleration lane. 171

Exhibit 6-43. Configuration of right-turn bypass lane with

yield at exit leg. 172

Exhibit 6-44. Sketched natural paths through a

double-lane roundabout. 173

Exhibit 6-45. Path overlap at a double-lane roundabout. 174

Exhibit 6-46. One method of entry design to avoid path overlap at

double-lane roundabouts. 175

Exhibit 6-47. Alternate method of entry design to avoid path overlap

at double-lane roundabouts. 175

Exhibit 6-48. Extended splitter island treatment. 178

Exhibit 6-49. Use of successive curves on high speed approaches. 179

Exhibit 6-50. Example of mini-roundabout. 180
130 Federal Highway Administration

6.1 Introduction

Designing the geometry of a roundabout involves choosing between trade-offs of

safety and capacity. Roundabouts operate most safely when their geometry forces

traffic to enter and circulate at slow speeds. Horizontal curvature and narrow pavement widths are used to produce this reduced-speed environment. Conversely,

the capacity of roundabouts is negatively affected by these low-speed design elements. As the widths and radii of entry and circulatory roadways are reduced, so

also the capacity of the roundabout is reduced. Furthermore, many of the geometric parameters are governed by the maneuvering requirements of the largest vehicles expected to travel through the intersection. Thus, designing a roundabout is

a process of determining the optimal balance between safety provisions, operational performance, and large vehicle accommodation.

While the basic form and features of roundabouts are uniform regardless of their

location, many of the design techniques and parameters are different, depending

on the speed environment and desired capacity at individual sites. In rural environments where approach speeds are high and bicycle and pedestrian use may be

minimal, the design objectives are significantly different from roundabouts in urban environments where bicycle and pedestrian safety are a primary concern. Additionally, many of the design techniques are substantially different for single-lane

roundabouts than for roundabouts with multiple entry lanes.

This chapter is organized so that the fundamental design principles common among

all roundabout types are presented first. More detailed design considerations specific to multilane roundabouts, rural roundabouts, and mini-roundabouts are given

in subsequent sections of the chapter.

6.1.1 Geometric elements

Exhibit 6-1 provides a review of the basic geometric features and dimensions of a

roundabout. Chapter 1 provided the definitions of these elements.

6.1.2 Design process

The process of designing roundabouts, more so than other forms of intersections,

requires a considerable amount of iteration among geometric layout, operational

analysis, and safety evaluation. As described in Chapters 4 and 5, minor adjustments in geometry can result in significant changes in the safety and/or operational performance. Thus, the designer often needs to revise and refine the initial

layout attempt to enhance its capacity and safety. It is rare to produce an optimal

geometric design on the first attempt. Exhibit 6-2 provides a graphical flowchart for

the process of designing and evaluating a roundabout.

Roundabout design

involves trade-offs among

safety, operations,

and accommodating

large vehicles.

Some roundabout features are

uniform, while others vary

depending on the location and

size of the roundabout.

Roundabout design is an

iterative process.

Chapter 6 Geometric Design
Roundabouts: An Informational Guide • 6: Geometric Design 131

Exhibit 6-1. Basic geometric

elements of a roundabout.

Exhibit 6-2. Roundabout design

process.

CHARACTERISTICS PLANNING OPERATIONS GEOMETRIC DESIGN SAFETY FINAL DESIGN

Identify

Roundabout

As Potential

Design Option

Evaluate

Appropriateness

Preliminary

Capacity

Analysis

Initial

Layout

Detailed

PerformanceAnalysis

Adjustas Necessary

Adjustas Necessary

Adjustas

Necessary

Review Safety

of Final

GeometricPlan

Signing,

Striping,Lighting,

Landscaping,and

Construction

Staging

PerformSafety Audit

of Signing,Striping,

Lighting,and

Landscape Plans

Check

Safety

Parameters
132 Federal Highway Administration

Because roundabout design is such an iterative process, in which small changes in

geometry can result in substantial changes to operational and safety performance,

it may be advisable to prepare the initial layout drawings at a sketch level of detail.

Although it is easy to get caught into the desire to design each of the individual

components of the geometry such that it complies with the specifications provided in this chapter, it is much more important that the individual components are

compatible with each other so that the roundabout will meet its overall performance objectives. Before the details of the geometry are defined, three fundamental elements must be determined in the preliminary design stage:

1. The optimal roundabout size;

2. The optimal position; and

3. The optimal alignment and arrangement of approach legs.

6.2 General Design Principles

This section describes the fundamental design principles common among all categories of roundabouts. Guidelines for the design of each geometric element are

provided in the following section. Further guidelines specific to double-lane roundabouts, rural roundabouts, and mini-roundabouts are given in subsequent sections.

Note that double-lane roundabout design is significantly different from single-lane

roundabout design, and many of the techniques used in single-lane roundabout

design do not directly transfer to double-lane design.

6.2.1 Speeds through the roundabout

Because it has profound impacts on safety, achieving appropriate vehicular speeds

through the roundabout is the most critical design objective. A well-designed roundabout reduces the relative speeds between conflicting traffic streams by requiring

vehicles to negotiate the roundabout along a curved path.

6.2.1.1 Speed profiles

Exhibit 6-3 shows the operating speeds of typical vehicles approaching and negotiating a roundabout. Approach speeds of 40, 55, and 70 km/h (25, 35, and 45 mph,

respectively) about 100 m (325 ft) from the center of the roundabout are shown.

Deceleration begins before this time, with circulating drivers operating at approximately the same speed on the roundabout. The relatively uniform negotiation speed

of all drivers on the roundabout means that drivers are able to more easily choose

their desired paths in a safe and efficient manner.

6.2.1.2 Design speed

International studies have shown that increasing the vehicle path curvature decreases the relative speed between entering and circulating vehicles and thus usually results in decreases in the entering-circulating and exiting-circulating vehicle

crash rates. However, at multilane roundabouts, increasing vehicle path curvature

creates greater side friction between adjacent traffic streams and can result in

more vehicles cutting across lanes and higher potential for sideswipe crashes (2).

Thus, for each roundabout, there exists an optimum design speed to minimize

crashes.

Increasing vehicle path

curvature decreases relative

speeds between entering and

circulating vehicles, but also

increases side friction between

adjacent traffic streams in

multilane roundabouts.

The most critical design objective

is achieving appropriate vehicular

speeds through the roundabout.
Roundabouts: An Informational Guide • 6: Geometric Design 133

Exhibit 6-3. Sample

theoretical speed profile (urban

compact roundabout).

Recommended maximum entry design speeds for roundabouts at various intersection site categories are provided in Exhibit 6-4.

Recommended Maximum

Site Category Entry Design Speed

Mini-Roundabout 25 km/h (15 mph)

Urban Compact 25 km/h (15 mph)

Urban Single Lane 35 km/h (20 mph)

Urban Double Lane 40 km/h (25 mph)

Rural Single Lane 40 km/h (25 mph)

Rural Double Lane 50 km/h (30 mph)

Exhibit 6-4. Recommended

maximum entry design speeds.
134 Federal Highway Administration

Exhibit 6-5. Fastest vehicle

path through single-lane

roundabout.

6.2.1.3 Vehicle paths

To determine the speed of a roundabout, the fastest path allowed by the geometry

is drawn. This is the smoothest, flattest path possible for a single vehicle, in the

absence of other traffic and ignoring all lane markings, traversing through the entry, around the central island, and out the exit. Usually the fastest possible path is

the through movement, but in some cases it may be a right turn movement.

A vehicle is assumed to be 2 m (6 ft) wide and to maintain a minimum clearance of

0.5 m (2 ft) from a roadway centerline or concrete curb and flush with a painted

edge line (2). Thus the centerline of the vehicle path is drawn with the following

distances to the particular geometric features:

• 1.5 m (5 ft) from a concrete curb,

• 1.5 m (5 ft) from a roadway centerline, and

• 1.0 m (3 ft) from a painted edge line.

Exhibits 6-5 and 6-6 illustrate the construction of the fastest vehicle paths at a

single-lane roundabout and at a double-lane roundabout, respectively. Exhibit 6-7

provides an example of an approach at which the right-turn path is more critical

than the through movement.

Roundabout speed is determined by the fastest path

allowed by the geometry.

Through movements are usually

the fastest path, but sometimes

right turn paths are more

critical.
Roundabouts: An Informational Guide • 6: Geometric Design 135

Exhibit 6-6. Fastest vehicle

path through double-lane

roundabout.

Exhibit 6-7. Example of critical

right-turn movement.
136 Federal Highway Administration

The entry path radius should

not be significantly larger than

the circulatory radius.

Draw the fastest path for all

roundabout approaches.

As shown in Exhibits 6-5 and 6-6, the fastest path for the through movement is a

series of reverse curves (i.e., a curve to the right, followed by a curve to the left,

followed by a curve to the right). When drawing the path, a short length of tangent

should be drawn between consecutive curves to account for the time it takes for

a driver to turn the steering wheel. It may be initially better to draw the path freehand, rather than using drafting templates or a computer-aided design (CAD) program. The freehand technique may provide a more natural representation of the

way a driver negotiates the roundabout, with smooth transitions connecting curves

and tangents. Having sketched the fastest path, the designer can then measure

the minimum radii using suitable curve templates or by replicating the path in CAD

and using it to determine the radii.

The design speed of the roundabout is determined from the smallest radius along

the fastest allowable path. The smallest radius usually occurs on the circulatory

roadway as the vehicle curves to the left around the central island. However, it is

important when designing the roundabout geometry that the radius of the entry

path (i.e., as the vehicle curves to the right through entry geometry) not be significantly larger than the circulatory path radius.

The fastest path should be drawn for all approaches of the roundabout. Because

the construction of the fastest path is a subjective process requiring a certain

amount of personal judgment, it may be advisable to obtain a second opinion.

6.2.1.4 Speed-curve relationship

The relationship between travel speed and horizontal curvature is documented in

the American Association of State Highway and Transportation Officials’ document,

A Policy on Geometric Design of Highways and Streets, commonly known as the

Green Book (4). Equation 6-1 can be used to calculate the design speed for a given

travel path radius.

V Re f = + 127 ( ) (6-1a, metric) V Re f = + 15 ( ) (6-1b, U.S. customary)

where: V = Design speed, km/h where: V = Design speed, mph

R = Radius, m R = Radius, ft

e = superelevation, m/m e = superelevation, ft/ft

f = side friction factor f = side friction factor

Superelevation values are usually assumed to be +0.02 for entry and exit curves

and -0.02 for curves around the central island. For more details related to

superelevation design, see Section 6.3.11.

Values for side friction factor can be determined in accordance with the AASHTO

relation for curves at intersections (see 1994 AASHTO Figure III-19 (4)). The coefficient of friction between a vehicle’s tires and the pavement varies with the vehicle’s

speed, as shown in Exhibits 6-8 and 6-9 for metric and U.S. customary units,

respectively.
Roundabouts: An Informational Guide • 6: Geometric Design 137

0.00

0.10

0.20

0.30

0.40

0.50

0.60

5 10 15 20 25 30 35 40

Speed (mph)

Side Friction Factor

Exhibit 6-9. Side friction

factors at various speeds

(U.S. customary units).

Exhibit 6-8. Side friction

factors at various speeds

(metric units).
138 Federal Highway Administration

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400

Radius (ft)

Speed (mph)

e=+0.02 e=-0.02

Exhibit 6-10. Speed-radius

relationship (metric units).

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Radius (m)

Speed (km/h)

e=+0.02 e=-0.02

Using the appropriate friction factors corresponding to each speed, Exhibits 6-10

and 6-11 present charts in metric and U.S. customary units, respectively, showing

the speed-radius relationship for curves for both a +0.02 superelevation and -0.02

superelevation.

Exhibit 6-11. Speed-radius

relationship

(U.S. customary units.)
Roundabouts: An Informational Guide • 6: Geometric Design 139

6.2.1.5 Speed consistency

In addition to achieving an appropriate design speed for the fastest movements,

another important objective is to achieve consistent speeds for all movements.

Along with overall reductions in speed, speed consistency can help to minimize

the crash rate and severity between conflicting streams of vehicles. It also simplifies the task of merging into the conflicting traffic stream, minimizing critical

gaps, thus optimizing entry capacity. This principle has two implications:

1. The relative speeds between consecutive geometric elements should be

minimized; and

2. The relative speeds between conflicting traffic streams should be minimized.

As shown in Exhibit 6-12, five critical path radii must be checked for each approach. R1 , the entry path radius, is the minimum radius on the fastest through

path prior to the yield line. R2 , the circulating path radius, is the minimum radius

on the fastest through path around the central island. R3 , the exit path radius, is

the minimum radius on the fastest through path into the exit. R4 , the left-turn

path radius, is the minimum radius on the path of the conflicting left-turn movement. R5 , the right-turn path radius, is the minimum radius on the fastest path of

a right-turning vehicle. It is important to note that these vehicular path radii are

not the same as the curb radii. First the basic curb geometry is laid out, and then

the vehicle paths are drawn in accordance with the procedures described in Section 6.2.1.3.

Exhibit 6-12. Vehicle path radii.
140 Federal Highway Administration

On the fastest path, it is desirable for R1 to be smaller than R2 , which in turn should

be smaller than R3 . This ensures that speeds will be reduced to their lowest level at

the roundabout entry and will thereby reduce the likelihood of loss-of-control crashes.

It also helps to reduce the speed differential between entering and circulating traffic, thereby reducing the entering-circulating vehicle crash rate. However, in some

cases it may not be possible to achieve an R1

 value less than R2 within given rightof-way or topographic constraints. In such cases, it is acceptable for R1

 to be greater

than R2 , provided the relative difference in speeds is less than 20 km/h (12 mph)

and preferably less than 10 km/h (6 mph).

At single-lane roundabouts, it is relatively simple to reduce the value of R1 . The

curb radius at the entry can be reduced or the alignment of the approach can be

shifted further to the left to achieve a slower entry speed (with the potential for

higher exit speeds that may put pedestrians at risk). However, at double-lane roundabouts, it is generally more difficult as overly small entry curves can cause the

natural path of adjacent traffic streams to overlap. Path overlap happens when the

geometry leads a vehicle in the left approach lane to naturally sweep across the

right approach lane just before the approach line to avoid the central island. It may

also happen within the circulatory roadway when a vehicle entering from the righthand lane naturally cuts across the left side of the circulatory roadway close to the

central island. When path overlap occurs at double-lane roundabouts, it may reduce capacity and increase crash risk. Therefore, care must be taken when designing double-lane roundabouts to achieve ideal values for R1 , R2

, and R3 . Section 6.4

provides further guidance on eliminating path overlap at double-lane roundabouts.

The exit radius, R3 , should not be less than R1 or R2 in order to minimize loss-ofcontrol crashes. At single-lane roundabouts with pedestrian activity, exit radii may

still be small (the same or slightly larger than R2

) in order to minimize exit speeds.

However, at double-lane roundabouts, additional care must be taken to minimize

the likelihood of exiting path overlap. Exit path overlap can occur at the exit when a

vehicle on the left side of the circulatory roadway (next to the central island) exits

into the right-hand exit lane. Where no pedestrians are expected, the exit radii

should be just large enough to minimize the likelihood of exiting path overlap. Where

pedestrians are present, tighter exit curvature may be necessary to ensure sufficiently low speeds at the downstream pedestrian crossing.

The radius of the conflicting left-turn movement, R4 , must be evaluated in order to

ensure that the maximum speed differential between entering and circulating traffic is no more than 20 km/h (12 mph). The left-turn movement is the critical traffic

stream because it has the lowest circulating speed. Large differentials between

entry and circulating speeds may result in an increase in single-vehicle crashes

due to loss of control. Generally, R4

 can be determined by adding 1.5 m (5 ft) to the

central island radius. Based on this assumption, Exhibits 6-13 and 6-14 show approximate R4

 values and corresponding maximum R1 values for various inscribed

circle diameters in metric and U.S. customary units, respectively.

The natural path of a vehicle is

the path that a driver would

take in the absence of other

conflicting vehicles.
Roundabouts: An Informational Guide • 6: Geometric Design 141

Exhibit 6-14. Approximated R4

values and corresponding R1

values (U.S. customary units).

Approximate R4 Value Maximum R1 Value

Radius

(ft)

Speed

(mph)

Radius

 (ft)

Speed

(mph)

Inscribed Circle

Diameter (m)

Single-Lane Roundabout

100 35 13 165 25

115 45 14 185 26

130 55 15 205 27

150 65 15 225 28

150 50 15 205 27

165 60 16 225 28

180 65 16 225 28

200 75 17 250 29

215 85 18 275 30

230 90 18 275 30

Double-Lane Roundabout

Finally, the radius of the fastest possible right-turn path, R5 , is evaluated. Like R1 ,

the right-turn radius should have a design speed at or below the maximum design

speed of the roundabout and no more than 20 km/h (12 mph) above the conflicting

R4 design speed.

Exhibit 6-13. Approximated R4

values and corresponding R1

values (metric units).

Double-Lane Roundabout

Single-Lane Roundabout

Approximate R4 Value Maximum R1 Value

Inscribed Circle

Diameter (m)

Radius

(m)

Speed

(km/h)

Radius

(m)

Speed

(km/h)

30 11 21 54 41

35 13 23 61 43

40 16 25 69 45

45 19 26 73 46

45 15 24 65 44

50 17 25 69 45

55 20 27 78 47

60 23 28 83 48

65 25 29 88 49

70 28 30 93 50
142 Federal Highway Administration

6.2.2 Design vehicle

Another important factor determining a roundabout’s layout is the need to accommodate the largest motorized vehicle likely to use the intersection. The turning path requirements of this vehicle, termed hereafter the design vehicle, will

dictate many of the roundabout’s dimensions. Before beginning the design process, the designer must be conscious of the design vehicle and possess the

appropriate vehicle turning templates or a CAD-based vehicle turning path program to determine the vehicle’s swept path.

The choice of design vehicle will vary depending upon the approaching roadway

types and the surrounding land use characteristics. The local or State agency with

jurisdiction of the associated roadways should usually be consulted to identify

the design vehicle at each site. The AASHTO A Policy on Geometric Design of

Highways and Streets provides the dimensions and turning path requirements

for a variety of common highway vehicles (4). Commonly, WB-15 (WB-50) vehicles are the largest vehicles along collectors and arterials. Larger trucks, such

as WB-20 (WB-67) vehicles, may need to be addressed at intersections on interstate freeways or State highway systems. Smaller design vehicles may often be

chosen for local street intersections.

In general, larger roundabouts need to be used to accommodate large vehicles

while maintaining low speeds for passenger vehicles. However, in some cases,

land constraints may limit the ability to accommodate large semi-trailer combinations while achieving adequate deflection for small vehicles. At such times, a

truck apron may be used to provide additional traversable area around the central

island for large semi-trailers. Truck aprons, though, provide a lower level of operation than standard nonmountable islands and should be used only when there is

no other means of providing adequate deflection while accommodating the design vehicle.

Exhibits 6-15 and 6-16 demonstrate the use of a CAD-based computer program

to determine the vehicle’s swept path through the critical turning movements.

The design vehicle dictates many

of the roundabout’s dimensions.
Roundabouts: An Informational Guide • 6: Geometric Design 143

Exhibit 6-15. Throughmovement swept path of

WB-15 (WB-50) vehicle.

Exhibit 6-16. Left-turn and

right-turn swept paths of

 WB-15 (WB-50) vehicle.
144 Federal Highway Administration

6.2.3 Nonmotorized design users

Like the motorized design vehicle, the design criteria of nonmotorized potential

roundabout users (bicyclists, pedestrians, skaters, wheelchair users, strollers, etc.)

should be considered when developing many of the geometric elements of a roundabout design. These users span a wide range of ages and abilities that can have a

significant effect on the design of a facility.

The basic design dimensions for various design users are given in Exhibit 6-17 (5).

6.2.4 Alignment of approaches and entries

In general, the roundabout is optimally located when the centerlines of all approach

legs pass through the center of the inscribed circle. This location usually allows the

geometry to be adequately designed so that vehicles will maintain slow speeds

through both the entries and the exits. The radial alignment also makes the central

island more conspicuous to approaching drivers.

If it is not possible to align the legs through the center point, a slight offset to the

left (i.e., the centerline passes to the left of the roundabout’s center point) is acceptable. This alignment will still allow sufficient curvature to be achieved at the

entry, which is of supreme importance. In some cases (particularly when the inscribed circle is relatively small), it may be beneficial to introduce a slight offset of

the approaches to the left in order to enhance the entry curvature. However, care

must be taken to ensure that such an approach offset does not produce an excessively tangential exit. Especially in urban environments, it is important that the exit

Roundabouts are optimally located

when all approach centerlines

pass through the center of the

inscribed circle.

User Dimension Affected Roundabout Features

Exhibit 6-17. Key dimensions

of nonmotorized design users.

Bicycles

Length 1.8 m (5.9 ft) Splitter island width at crosswalk

Minimum operating width 1.5 m (4.9 ft) Bike lane width

Lateral clearance on each side 0.6 m (2.0 ft); Shared bicycle-pedestrian path

width

1.0 m (3.3 ft)

to obstructions

Pedestrian (walking)

Width 0.5 m (1.6 ft) Sidewalk width, crosswalk width

Wheelchair

Minimum width 0.75 m (2.5 ft) Sidewalk width, crosswalk width

Operating width 0.90 m (3.0 ft) Sidewalk width, crosswalk width

Person pushing stroller

Length 1.70 m (5.6 ft) Splitter island width at crosswalk

Skaters

Typical operating width 1.8 m (6 ft) Sidewalk width

Source: (5)
Roundabouts: An Informational Guide • 6: Geometric Design 145

geometry produce a sufficiently curved exit path in order to keep vehicle speeds

low and reduce the risk for pedestrians.

It is almost never acceptable for an approach alignment to be offset to the right of

the roundabout’s center point. This alignment brings the approach in at a more

tangential angle and reduces the opportunity to provide sufficient entry curvature.

Vehicles will be able to enter the roundabout too fast, resulting in more loss-ofcontrol crashes and higher crash rates between entering and circulating vehicles.

Exhibit 6-18 illustrates the preferred radial alignment of entries.

In addition, it is desirable to equally space the angles between entries. This provides optimal separation between successive entries and exits. This results in optimal angles of 90 degrees for four-leg roundabouts, 72 degrees for five-leg roundabouts, and so on. This is consistent with findings of the British accident prediction

models described in Chapter 5.

6.3 Geometric Elements

This section presents specific parameters and guidelines for the design of each

geometric element of a roundabout. The designer must keep in mind, however,

that these components are not independent of each other. The interaction between

the components of the geometry is far more important than the individual pieces.

Care must be taken to ensure that the geometric elements are all compatible with

each other so that the overall safety and capacity objectives are met.

6.3.1 Inscribed circle diameter

The inscribed circle diameter is the distance across the circle inscribed by the

outer curb (or edge) of the circulatory roadway. As illustrated in Exhibit 6-1, it is the

sum of the central island diameter (which includes the apron, if present) and twice

the circulatory roadway. The inscribed circle diameter is determined by a number

of design objectives. The designer often has to experiment with varying diameters

before determining the optimal size at a given location.

Exhibit 6-18. Radial alignment

of entries.

Approach alignment should not

be offset to the right of the

roundabout’s center point.
146 Federal Highway Administration

At single-lane roundabouts, the size of the inscribed circle is largely dependent

upon the turning requirements of the design vehicle. The diameter must be large

enough to accommodate the design vehicle while maintaining adequate deflection

curvature to ensure safe travel speeds for smaller vehicles. However, the circulatory roadway width, entry and exit widths, entry and exit radii, and entry and exit

angles also play a significant role in accommodating the design vehicle and providing deflection. Careful selection of these geometric elements may allow a smaller

inscribed circle diameter to be used in constrained locations. In general, the inscribed circle diameter should be a minimum of 30 m (100 ft) to accommodate a

WB-15 (WB-50) design vehicle. Smaller roundabouts can be used for some local

street or collector street intersections, where the design vehicle may be a bus or

single-unit truck.

At double-lane roundabouts, accommodating the design vehicle is usually not a

constraint. The size of the roundabout is usually determined either by the need to

achieve deflection or by the need to fit the entries and exits around the circumference with reasonable entry and exit radii between them. Generally, the inscribed

circle diameter of a double-lane roundabout should be a minimum of 45 m (150 ft).

In general, smaller inscribed diameters are better for overall safety because they

help to maintain lower speeds. In high-speed environments, however, the design

of the approach geometry is more critical than in low-speed environments. Larger

inscribed diameters generally allow for the provision of better approach geometry,

which leads to a decrease in vehicle approach speeds. Larger inscribed diameters

also reduce the angle formed between entering and circulating vehicle paths, thereby

reducing the relative speed between these vehicles and leading to reduced entering-circulating crash rates (2). Therefore, roundabouts in high-speed environments

may require diameters that are somewhat larger than those recommended for

low-speed environments. Very large diameters (greater than 60 m [200 ft]), however, should generally not be used because they will have high circulating speeds

and more crashes with greater severity. Exhibit 6-19 provides recommended ranges

of inscribed circle diameters for various site locations.

For a single-lane roundabout,

the minimum inscribed circle

diameter is 30 m (100 ft) to

accommodate a WB-15 (WB-50)

vehicle.

For a double-lane roundabout,

the minimum inscribed circle

diamter is 45 m (150 ft).

Exhibit 6-19. Recommended

inscribed circle diameter ranges.

Mini-Roundabout Single-Unit Truck 13–25m (45–80 ft)

Urban Compact Single-Unit Truck/Bus 25–30m (80–100 ft)

Urban Single Lane WB-15 (WB-50) 30–40m (100–130 ft)

Urban Double Lane WB-15 (WB-50) 45–55m (150–180 ft)

Rural Single Lane WB-20 (WB-67) 35–40m (115–130 ft)

Rural Double Lane WB-20 (WB-67) 55–60m (180–200 ft)

* Assumes 90-degree angles between entries and no more than four legs.

Site Category Typical Design Vehicle

Inscribed Circle

Diameter Range*
Roundabouts: An Informational Guide • 6: Geometric Design 147

6.3.2 Entry width

Entry width is the largest determinant of a roundabout’s capacity. The capacity of

an approach is not dependent merely on the number of entering lanes, but on the

total width of the entry. In other words, the entry capacity increases steadily with

incremental increases to the entry width. Therefore, the basic sizes of entries and

circulatory roadways are generally described in terms of width, not number of

lanes. Entries that are of sufficient width to accommodate multiple traffic streams

(at least 6.0 m [20 ft]) are striped to designate separate lanes. However, the circulatory roadway is usually not striped, even when more than one lane of traffic is

expected to circulate (for more details related to roadway markings, see Chapter 7).

As shown in Exhibit 6-1, entry width is measured from the point where the yield

line intersects the left edge of the traveled-way to the right edge of the traveledway, along a line perpendicular to the right curb line. The width of each entry is

dictated by the needs of the entering traffic stream. It is based on design traffic

volumes and can be determined in terms of the number of entry lanes by using

Chapter 4 of this guide. The circulatory roadway must be at least as wide as the

widest entry and must maintain a constant width throughout.

To maximize the roundabout’s safety, entry widths should be kept to a minimum.

The capacity requirements and performance objectives will dictate that each entry

be a certain width, with a number of entry lanes. In addition, the turning requirements of the design vehicle may require that the entry be wider still. However,

larger entry and circulatory widths increase crash frequency. Therefore, determining the entry width and circulatory roadway width involves a trade-off between

capacity and safety. The design should provide the minimum width necessary for

capacity and accommodation of the design vehicle in order to maintain the highest

level of safety. Typical entry widths for single-lane entrances range from 4.3 to 4.9

m (14 to 16 ft); however, values higher or lower than this range may be required for

site-specific design vehicle and speed requirements for critical vehicle paths.

When the capacity requirements can only be met by increasing the entry width,

this can be done in two ways:

1. By adding a full lane upstream of the roundabout and maintaining parallel

lanes through the entry geometry; or

2. By widening the approach gradually (flaring) through the entry geometry.

Exhibit 6-20 and Exhibit 6-21 illustrate these two widening options.

Entry width is the largest

determinant of a roundabout’s

capacity.

Entry widths should be kept to

a minimum to maximize safety

while achieving capacity and

performance objectives.
148 Federal Highway Administration

As discussed in Chapter 4, flaring is an effective means of increasing capacity

without requiring as much right-of-way as a full lane addition. While increasing the

length of flare increases capacity, it does not increase crash frequency. Consequently, the crash frequency for two approaches with the same entry width will be

essentially the same, whether they have parallel entry lanes or flared entry designs. Entry widths should therefore be minimized and flare lengths maximized to

achieve the desired capacity with minimal effect on crashes. Generally, flare lengths

should be a minimum of 25 m (80 ft) in urban areas and 40 m (130 ft) in rural areas.

However, if right-of-way is constrained, shorter lengths can be used with noticeable effects on capacity (see Chapter 4).

Exhibit 6-21. Approach

widening by entry flaring.

Exhibit 6-20. Approach

widening by adding full lane.

Flare lengths should be

at least 25 m in urban areas and

40 m in rural areas.
Roundabouts: An Informational Guide • 6: Geometric Design 149

In some cases, a roundabout designed to accommodate design year traffic volumes, typically projected 20 years from the present, can result in substantially

wider entries and circulatory roadway than needed in the earlier years of operation.

Because safety will be significantly reduced by the increase in entry width, the

designer may wish to consider a two-phase design solution. In this case, the firstphase design would provide the entry width requirements for near-term traffic volumes with the ability to easily expand the entries and circulatory roadway to accommodate future traffic volumes. The interim solution should be accomplished by

first laying out the ultimate plan, then designing the first phase within the ultimate

curb lines. The interim roundabout is often constructed with the ultimate inscribed

circle diameter, but with a larger central island and splitter islands. At the time

additional capacity is needed, the splitter and central islands can be reduced in size

to provide additional widths at the entries, exits, and circulatory roadway.

6.3.3 Circulatory roadway width

The required width of the circulatory roadway is determined from the width of the

entries and the turning requirements of the design vehicle. In general, it should

always be at least as wide as the maximum entry width (up to 120 percent of the

maximum entry width) and should remain constant throughout the roundabout (3).

6.3.3.1 Single-lane roundabouts

At single-lane roundabouts, the circulatory roadway should just accommodate the

design vehicle. Appropriate vehicle-turning templates or a CAD-based computer

program should be used to determine the swept path of the design vehicle through

each of the turning movements. Usually the left-turn movement is the critical path

for determining circulatory roadway width. In accordance with AASHTO policy, a

minimum clearance of 0.6 m (2 ft) should be provided between the outside edge of

the vehicle’s tire track and the curb line. AASHTO Table III-19 (1994 edition) provides derived widths required for various radii for each standard design vehicle.

In some cases (particularly where the inscribed diameter is small or the design

vehicle is large) the turning requirements of the design vehicle may dictate that the

circulatory roadway be so wide that the amount of deflection necessary to slow

passenger vehicles is compromised. In such cases, the circulatory roadway width

can be reduced and a truck apron, placed behind a mountable curb on the central

island, can be used to accommodate larger vehicles. However, truck aprons generally provide a lower level of operation than standard nonmountable islands. They

are sometimes driven over by four-wheel drive automobiles, may surprise inattentive motorcyclists, and can cause load shifting on trucks. They should, therefore, be

used only when there is no other means of providing adequate deflection while

accommodating the design vehicle.

6.3.3.2 Double-lane roundabouts

At double-lane roundabouts, the circulatory roadway width is usually not governed

by the design vehicle. The width required for one, two, or three vehicles, depending on the number of lanes at the widest entry, to travel simultaneously through

the roundabout should be used to establish the circulatory roadway width. The

Two-phase designs allow for

small initial entry widths that

can be easily expanded in the

future when needed to

accommodate greater traffic

volumes.

Truck aprons generally provide a

lower level of operations, but

may be needed to provide

adequate deflection while still

accommodating the design

vehicle.
150 Federal Highway Administration

combination of vehicle types to be accommodated side-by-side is dependent upon

the specific traffic conditions at each site. If the entering traffic is predominantly

passenger cars and single-unit trucks (AASHTO P and SU vehicles), where semitrailer traffic is infrequent, it may be appropriate to design the width for two passenger vehicles or a passenger car and a single-unit truck side-by-side. If semitrailer traffic is relatively frequent (greater than 10 percent), it may be necessary to

provide sufficient width for the simultaneous passage of a semi-trailer in combination with a P or SU vehicle.

Exhibit 6-22 provides minimum recommended circulatory roadway widths for twolane roundabouts where semi-trailer traffic is relatively infrequent.

6.3.4 Central island

The central island of a roundabout is the raised, nontraversable area encompassed

by the circulatory roadway; this area may also include a traversable apron. The

island is typically landscaped for aesthetic reasons and to enhance driver recognition of the roundabout upon approach. Central islands should always be raised, not

depressed, as depressed islands are difficult for approaching drivers to recognize.

In general, the central island should be circular in shape. A circular-shaped central

island with a constant-radius circulatory roadway helps promote constant speeds

around the central island. Oval or irregular shapes, on the other hand, are more

difficult to drive and can promote higher speeds on the straight sections and reduced speeds on the arcs of the oval. This speed differential may make it harder for

entering vehicles to judge the speed and acceptability of gaps in the circulatory

traffic stream. It can also be deceptive to circulating drivers, leading to more lossof-control crashes. Noncircular central islands have the above disadvantages to a

rapidly increasing degree as they get larger because circulating speeds are higher.

Oval shapes are generally not such a problem if they are relatively small and speeds

are low. Raindrop-shaped islands may be used in areas where certain movements

do not exist, such as interchanges (see Chapter 8), or at locations where certain

turning movements cannot be safely accommodated, such as roundabouts with

one approach on a relatively steep grade.

Exhibit 6-22. Minimum

circulatory lane widths for

 two-lane roundabouts.

45 m (150 ft) 9.8 m (32 ft) 25.4 m (86 ft)

50 m (165 ft) 9.3 m (31 ft) 31.4 m (103 ft)

55 m (180 ft) 9.1 m (30 ft) 36.8 m (120 ft)

60 m (200 ft) 9.1 m (30 ft) 41.8 m (140 ft)

65 m (215 ft) 8.7 m (29 ft) 47.6 m (157 ft)

70 m (230 ft) 8.7 m (29 ft) 52.6 m (172 ft)

* Based on 1994 AASHTO Table III-20, Case III(A) (4). Assumes infrequent semi-trailer use (typically less

than 5 percent of the total traffic). Refer to AASHTO for cases with higher truck percentages.

Inscribed Circle

Diameter

Minimum Circulatory

Lane Width*

Central Island

Diameter
Roundabouts: An Informational Guide • 6: Geometric Design 151

As described in Section 6.2.1, the size of the central island plays a key role in

determining the amount of deflection imposed on the through vehicle’s path. However, its diameter is entirely dependent upon the inscribed circle diameter and the

required circulatory roadway width (see Sections 6.3.1 and 6.3.3, respectively).

Therefore, once the inscribed diameter, circulatory roadway width, and initial entry

geometry have been established, the fastest vehicle path must be drawn though

the layout, as described in Section 6.2.1.3, to determine if the central island size is

adequate. If the fastest path exceeds the design speed, the central island size may

need to be increased, thus increasing the overall inscribed circle diameter. There

may be other methods for increasing deflection without increasing the inscribed

diameter, such as offsetting the approach alignment to the left, reducing the entry

width, or reducing the entry radius. These treatments, however, may preclude the

ability to accommodate the design vehicle.

In cases where right-of-way, topography, or other constraints preclude the ability

to expand the inscribed circle diameter, a mountable apron may be added to the

outer edge of the central island. This provides additional paved area to allow the

over-tracking of large semi-trailer vehicles on the central island without compromising the deflection for smaller vehicles. Exhibit 6-23 shows a typical central island with a traversable apron.

Where aprons are used, they should be designed so that they are traversable by

trucks, but discourage passenger vehicles from using them. They should generally

be 1 to 4 m (3 to 13 ft) wide and have a cross slope of 3 to 4 percent away from the

central island. To discourage use by passenger vehicles, the outer edge of the

apron should be raised a minimum of 30 mm (1.2 in) above the circulatory roadway surface (6). The apron should be constructed of colored and/or textured paving

Exhibit 6-23. Example of central

island with a traversable apron.

Leeds, MD
152 Federal Highway Administration

materials to differentiate it from the circulatory roadway. Care must be taken to

ensure that delivery trucks will not experience load shifting as their rear trailer

wheels track across the apron.

Issues regarding landscaping and other treatments within the central island are

discussed in Chapter 7.

In general, roundabouts in rural environments typically need larger central islands

than urban roundabouts in order to enhance their visibility and to enable the design

of better approach geometry (2).

6.3.5 Entry curves

As shown in Exhibit 6-1, the entry curves are the set of one or more curves along

the right curb (or edge of pavement) of the entry roadway leading into the circulatory roadway. It should not be confused with the entry path curve, defined by the

radius of the fastest vehicular travel path through the entry geometry (R1

 on Exhibit

6-12).

The entry radius is an important factor in determining the operation of a roundabout as it has significant impacts on both capacity and safety. The entry radius, in

conjunction with the entry width, the circulatory roadway width, and the central

island geometry, controls the amount of deflection imposed on a vehicle’s entry

path. Larger entry radii produce faster entry speeds and generally result in higher

crash rates between entering and circulating vehicles. In contrast, the operational

performance of roundabouts benefits from larger entry radii. As described in Chapter 4, British research has found that the capacity of an entry increases as its entry

radius is increased (up to 20 m [65 ft], beyond which entry radius has little effect on

capacity.

The entry curve is designed curvilinearly tangential to the outside edge of the

circulatory roadway. Likewise, the projection of the inside (left) edge of the entry

roadway should be curvilinearly tangential to the central island. Exhibit 6-24 shows

a typical roundabout entrance geometry.

The primary objective in selecting a radius for the entry curve is to achieve the

speed objectives, as described in Section 6.2.1. The entry radius should first produce an appropriate design speed on the fastest vehicular path. Second, it should

desirably result in an entry path radius (R1

) equal to or less than the circulating path

radius (R2

) (see Section 6.2.1.5).
Roundabouts: An Informational Guide • 6: Geometric Design 153

Exhibit 6-24. Single-lane

roundabout entry design.

6.3.5.1 Entry curves at single-lane roundabouts

For single-lane roundabouts, it is relatively simple to achieve the entry speed

objectives. With a single traffic stream entering and circulating, there is no conflict between traffic in adjacent lanes. Thus, the entry radius can be reduced or

increased as necessary to produce the desired entry path radius. Provided sufficient clearance is given for the design vehicle, approaching vehicles will adjust

their path accordingly and negotiate through the entry geometry into the circulatory roadway.

Entry radii at urban single-lane roundabouts typically range from 10 to 30 m (33 to

98 ft). Larger radii may be used, but it is important that the radii not be so large as

to result in excessive entry speeds. At local street roundabouts, entry radii may

be below 10 m (33 ft) if the design vehicle is small.

At rural and suburban locations, consideration should be given to the speed differential between the approaches and entries. If the difference is greater than 20

km/h (12 mph), it is desirable to introduce approach curves or some other speed

reduction measures to reduce the speed of approaching traffic prior to the entry

curvature. Further details on rural roundabout design are provided in Section 6.5.

6.3.5.2 Entry curves at double-lane roundabouts

At double-lane roundabouts, the design of the entry curvature is more complicated. Overly small entry radii can result in conflicts between adjacent traffic

streams. This conflict usually results in poor lane utilization of one or more lanes

and significantly reduces the capacity of the approach. It can also degrade the

safety performance as sideswipe crashes may increase. Techniques and guidelines for avoiding conflicts between adjacent entry lanes at double-lane roundabouts are provided in Section 6.4.
154 Federal Highway Administration

Exhibit 6-25. Single-lane

roundabout exit design.

6.3.6 Exit curves

Exit curves usually have larger radii than entry curves to minimize the likelihood of

congestion at the exits. This, however, is balanced by the need to maintain low

speeds at the pedestrian crossing on exit. The exit curve should produce an exit

path radius (R3

 in Exhibit 6-12) no smaller than the circulating path radius (R2). If the

exit path radius is smaller than the circulating path radius, vehicles will be traveling

too fast to negotiate the exit geometry and may crash into the splitter island or into

oncoming traffic in the adjacent approach lane. Likewise, the exit path radius should

not be significantly greater than the circulating path radius to ensure low speeds at

the downstream pedestrian crossing.

The exit curve is designed to be curvilinearly tangential to the outside edge of the

circulatory roadway. Likewise, the projection of the inside (left) edge of the exit

roadway should be curvilinearly tangential to the central island. Exhibit 6-25 shows

a typical exit layout for a single-lane roundabout.
Roundabouts: An Informational Guide • 6: Geometric Design 155

6.3.6.1 Exit curves at single-lane roundabouts

At single-lane roundabouts in urban environments, exits should be designed to

enforce a curved exit path with a design speed below 40 km/h (25 mph) in order

to maximize safety for pedestrians crossing the exiting traffic stream. Generally,

exit radii should be no less than 15 m (50 ft). However, at locations with pedestrian activity and no large semi-trailer traffic, exit radii may be as low as 10 to 12 m

(33 to 39 ft). This produces a very slow design speed to maximize safety and

comfort for pedestrians. Such low exit radii should only be used in conjunction

with similar or smaller entry radii on urban compact roundabouts with inscribed

circle diameters below 35 m (115 ft).

In rural locations where there are few pedestrians, exit curvature may be designed with large radii, allowing vehicles to exit quickly and accelerate back to

traveling speed. This, however, should not result in a straight path tangential to

the central island because many locations that are rural today become urban in

the future. Therefore, it is recommended that pedestrian activity be considered at

all exits except where separate pedestrian facilities (paths, etc.) or other restrictions eliminate the likelihood of pedestrian activity in the foreseeable future.

6.3.6.2 Exit curves at double-lane roundabouts

As with the entries, the design of the exit curvature at double-lane roundabouts is

more complicated than at single-lane roundabouts. Techniques and guidelines for

avoiding conflicts between adjacent exit lanes at double-lane roundabouts are

provided in Section 6.4.

6.3.7 Pedestrian crossing location and treatments

Pedestrian crossing locations at roundabouts are a balance among pedestrian

convenience, pedestrian safety, and roundabout operations:

• Pedestrian convenience: Pedestrians want crossing locations as close to the

intersection as possible to minimize out-of-direction travel. The further the crossing is from the roundabout, the more likely that pedestrians will choose a shorter

route that may put them in greater danger.

• Pedestrian safety: Both crossing location and crossing distance are important.

Crossing distance should be minimized to reduce exposure of pedestrian-vehicle conflicts. Pedestrian safety may also be compromised at a yield-line crosswalk because driver attention is directed to the left to look for gaps in the

circulating traffic stream. Crosswalks should be located to take advantage of

the splitter island; crosswalks located too far from the yield line require longer

splitter islands. Crossings should also be located at distances away from the

yield line measured in increments of approximate vehicle length to reduce the

chance that vehicles will be queued across the crosswalk.

Pedestrian crossing locations

must balance pedestrian

convenience, pedestrian safety,

and roundabout operations.
156 Federal Highway Administration

• Roundabout operations: Roundabout operations (primarily vehicular) can also

be affected by crosswalk locations, particularly on the exit. A queuing analysis

at the exit crosswalk may determine that a crosswalk location of more than one

vehicle length away may be required to reduce to an acceptable level the risk of

queuing into the circulatory roadway. Pedestrians may be able to distinguish

exiting vehicles from circulating vehicles (both visually and audibly) at crosswalk

locations further away from the roundabout, although this has not been confirmed by research.

With these issues in mind, pedestrian crossings should be designed as follows:

• The pedestrian refuge should be a minimum width of 1.8 m (6 ft) to adequately

provide shelter for persons pushing a stroller or walking a bicycle (see Section

6.2.3).

• At single-lane roundabouts, the pedestrian crossing should be located one vehicle-length (7.5 m [25 ft]) away from the yield line. At double-lane roundabouts,

the pedestrian crossing should be located one, two, or three car lengths (approximately 7.5 m, 15 m, or 22.5 m [25 ft, 50 ft, or 75 ft]) away from the yield line.

• The pedestrian refuge should be designed at street level, rather than elevated

to the height of the splitter island. This eliminates the need for ramps within the

refuge area, which can be cumbersome for wheelchairs.

• Ramps should be provided on each end of the crosswalk to connect the crosswalk to other crosswalks around the roundabout and to the sidewalk network.

• It is recommended that a detectable warning surface, as recommended in the

Americans with Disabilities Act Accessibility Guidelines (ADAAG) §4.29 (Detectable Warnings), be applied to the surface of the refuge within the splitter

island as shown in Exhibit 6-26. Note that the specific provision of the ADAAG

requiring detectable warning surface at locations such as ramps and splitter

islands (defined in the ADAAG as “hazardous vehicle areas”) has been suspended until July 26, 2001 (ADAAG §4.29.5). Where used, a detectable warning

surface shall meet the following requirements (7):

- The detectable warning surface shall consist of raised truncated domes

with a nominal diameter of 23 mm (0.9 in), a nominal height of 5 mm (0.2

in), and a nominal center-to-center spacing of 60 mm (2.35 in).

- The detectable warning surface shall contrast visually with adjoining surfaces, either light-on-dark or dark-on-light. The material used to provide

contrast shall be an integral part of the walking surface.

- The detectable warning surface shall begin at the curb line and extend

into the pedestrian refuge area a distance of 600 mm (24 in). This creates

a minimum 600-mm (24-in) clear space between detectable warning surfaces for a minimum splitter island width of 1.8 m (6 ft) at the pedestrian

crossing. This is a deviation from the requirements of (suspended) ADAAG

§4.29.5, which requires a 915-mm (36-in) surface width. However, this

deviation is necessary to enable visually impaired pedestrians to distinguish the two interfaces with vehicular traffic.

In urban areas, speed tables (flat-top road humps) could be considered for wheelchair users, provided that good geometric design has reduced absolute vehicle

Detectable warning surfaces

should be applied within the

pedestrian refuge.
Roundabouts: An Informational Guide • 6: Geometric Design 157

Exhibit 6-26. Minimum

splitter island dimensions.

Splitter islands perform

multiple functions and should

generally be provided.

speeds to less than 20 km/h (12 mph) near the crossing. Pedestrian crossings

across speed tables must have detectable warning material as described above to

clearly delineate the edge of the street. Speed tables should generally be used

only on streets with approach speeds of 55 km/h (35 mph) or less, as the introduction of a raised speed table in higher speed environments may increase the likelihood of single-vehicle crashes and is not consistent with the speed consistency

philosophy presented in this document.

6.3.8 Splitter islands

Splitter islands (also called separator islands or median islands) should be provided

on all roundabouts, except those with very small diameters at which the splitter

island would obstruct the visibility of the central island. Their purpose is to provide

shelter for pedestrians (including wheelchairs, bicycles, and baby strollers), assist

in controlling speeds, guide traffic into the roundabout, physically separate entering and exiting traffic streams, and deter wrong-way movements. Additionally, splitter

islands can be used as a place for mounting signs (see Chapter 7).

The splitter island envelope is formed by the entry and exit curves on a leg, as

shown previously in Exhibits 6-24 and 6-25. The total length of the island should

generally be at least 15 m (50 ft) to provide sufficient protection for pedestrians and

to alert approaching drivers to the roundabout geometry. Additionally, the splitter

island should extend beyond the end of the exit curve to prevent exiting traffic from

accidentally crossing into the path of approaching traffic.

Exhibit 6-26 shows the minimum dimensions for a splitter island at a singlelane roundabout, including the location of the pedestrian crossing as discussed

in Section 6.3.7.
158 Federal Highway Administration

While Exhibit 6-26 provides minimum dimensions for splitter islands, there are

benefits to providing larger islands. Increasing the splitter island width results in

greater separation between the entering and exiting traffic streams of the same

leg and increases the time for approaching drivers to distinguish between exiting

and circulating vehicles. In this way, larger splitter islands can help reduce confusion for entering motorists. A recent study by the Queensland Department of Main

Roads found that maximizing the width of splitter islands has a significant effect on

minimizing entering/circulating vehicle crash rates (2). However, increasing the width

of the splitter islands generally requires increasing the inscribed circle diameter.

Thus, these safety benefits may be offset by higher construction cost and greater

land impacts.

Standard AASHTO guidelines for island design should be followed for the splitter

island. This includes using larger nose radii at approach corners to maximize island

visibility and offsetting curb lines at the approach ends to create a funneling effect.

The funneling treatment also aids in reducing speeds as vehicles approach the

roundabout. Exhibit 6-27 shows minimum splitter island nose radii and offset dimensions from the entry and exit traveled ways.

Exhibit 6-27. Minimum splitter

island nose radii and offsets.

Larger splitter islands enhance

safety, but require that the

inscribed circle diameter be

increased.
Roundabouts: An Informational Guide • 6: Geometric Design 159

6.3.9 Stopping sight distance

Stopping sight distance is the distance along a roadway required for a driver to

perceive and react to an object in the roadway and to brake to a complete stop

before reaching that object. Stopping sight distance should be provided at every

point within a roundabout and on each entering and exiting approach.

National Cooperative Highway Research Program (NCHRP) Report 400, Determination of Stopping Sight Distances (8), recommends the formula given in Equation

6-2 for determining stopping sight distance (presented in metric units, followed by

a conversion of the equation to U.S. customary units).

(6-2a, metric)

where: d = stopping sight distance, m;

t = perception-brake reaction time, assumed to be 2.5 s;

V = initial speed, km/h; and

a = driver deceleration, assumed to be 3.4 m/s2

.

where: d = stopping sight distance, ft;

t = perception-brake reaction time, assumed to be 2.5 s;

V = initial speed, mph; and

a = driver deceleration, assumed to be 11.2 ft/s2.

Exhibit 6-28 gives recommended stopping sight distances for design, as computed

from the above equations.

d tV V

a

= + ( . )( )( ) . 0 278 0 039

2

d tV V

a

= + ( . )( )( ) . 1 468 1 087

2

Exhibit 6-28. Design values for

stopping sight distances.

10 8.1

20 18.5

30 31.2

40 46.2

50 63.4

60 83.0

70 104.9

80 129.0

90 155.5

100 184.2 *

Speed

(km/h)

Computed

Distance* (m)

Speed

(mph)

Computed

Distance* (ft)

10 46.4

15 77.0

20 112.4

 25 152.7

30 197.8

35 247.8

40 302.7

45 362.

50 427.2

55 496.7

Assumes 2.5 s perception-braking time, 3.4 m/s2 (11.2 ft/s2) driver deceleration
160 Federal Highway Administration

Stopping sight distance should be measured using an assumed height of driver’s

eye of 1,080 mm (3.54 ft) and an assumed height of object of 600 mm (1.97 ft) in

accordance with the recommendations to be adopted in the next AASHTO “Green

Book” (8).

At roundabouts, three critical types of locations should be checked at a minimum:

• Approach sight distance (Exhibit 6-29);

• Sight distance on circulatory roadway (Exhibit 6-30); and

• Sight distance to crosswalk on exit (Exhibit 6-31).

Forward sight distance at entry can also be checked; however, this will typically be

satisfied by providing adequate stopping sight distance on the circulatory roadway

itself.

Exhibit 6-30. Sight distance

on circulatory roadway.

Exhibit 6-29. Approach sight

distance.

At least three critical types of

locations should be checked for

stopping sight distance.
Roundabouts: An Informational Guide • 6: Geometric Design 161

6.3.10 Intersection sight distance

Intersection sight distance is the distance required for a driver without the right of

way to perceive and react to the presence of conflicting vehicles. Intersection sight

distance is achieved through the establishment of adequate sight lines that allow a

driver to see and safely react to potentially conflicting vehicles. At roundabouts,

the only locations requiring evaluation of intersection sight distance are the entries.

Intersection sight distance is traditionally measured through the determination of a

sight triangle. This triangle is bounded by a length of roadway defining a limit away

from the intersection on each of the two conflicting approaches and by a line connecting those two limits. For roundabouts, these “legs” should be assumed to

follow the curvature of the roadway, and thus distances should be measured not

as straight lines but as distances along the vehicular path.

Intersection sight distance should be measured using an assumed height of driver’s

eye of 1,080 mm (3.54 ft) and an assumed height of object of 1,080 mm (3.54 ft) in

accordance with the recommendations to be adopted in the next AASHTO “Green

Book” (4).

Exhibit 6-32 presents a diagram showing the method for determining intersection

sight distance. As can be seen in the exhibit, the sight distance “triangle” has two

conflicting approaches that must be checked independently. The following two

subsections discuss the calculation of the length of each of the approaching sight

limits.

Exhibit 6-31. Sight distance to

crosswalk on exit.

Roundabout entries require

adequate intersection sight

distance.
162 Federal Highway Administration

6.3.10.1 Length of approach leg of sight triangle

The length of the approach leg of the sight triangle should be limited to 15 m (49

ft). British research on sight distance determined that excessive intersection sight

distance results in a higher frequency of crashes. This value, consistent with British and French practice, is intended to require vehicles to slow down prior to

entering the roundabout, which allows them to focus on the pedestrian crossing

prior to entry. If the approach leg of the sight triangle is greater than 15 m (49 ft),

it may be advisable to add landscaping to restrict sight distance to the minimum

requirements.

6.3.10.2 Length of conflicting leg of sight triangle

A vehicle approaching an entry to a roundabout faces conflicting vehicles within

the circulatory roadway. The length of the conflicting leg is calculated using Equation

6-3:

(6-3a, metric)

where: b = length of conflicting leg of sight triangle, m

Vmajor = design speed of conflicting movement, km/h,

discussed below

tc = critical gap for entering the major road, s, equal

to 6.5 s

(6-3b, U.S. customary)

where: b = length of conflicting leg of sight triangle, ft

Vmajor = design speed of conflicting movement, mph,

discussed below

tc = critical gap for entering the major road, s, equal

to 6.5 s

b Vt = 1 468 major c . ( )( )

Exhibit 6-32. Intersection sight

distance

b Vt = 0 278 major c . ( )( )
Roundabouts: An Informational Guide • 6: Geometric Design 163

Two conflicting traffic streams should be checked at each entry:

• Entering stream, comprised of vehicles from the immediate upstream entry.

The speed for this movement can be approximated by taking the average of the

entry path speed (path with radius R1

 from Exhibit 6-12) and the circulating path

speed (path with radius R2

 from Exhibit 6-12).

• Circulating stream, comprised of vehicles that entered the roundabout prior to

the immediate upstream entry. This speed can be approximated by taking the

speed of left turning vehicles (path with radius R4

 from Exhibit 6-12).

The critical gap for entering the major road is based on the amount of time required

for a vehicle to turn right while requiring the conflicting stream vehicle to slow no

less than 70 percent of initial speed. This is based on research on critical gaps at

stop-controlled intersections, adjusted for yield-controlled conditions (9). The critical gap value of 6.5 s given in Equation 6-3 is based on the critical gap required for

passenger cars, which are assumed to be the most critical design vehicle for intersection sight distance. This assumption holds true for single-unit and combination

truck speeds that are at least 10 km/h (6 mph) and 15 to 20 km/h (9 to 12 mph)

slower than passenger cars, respectively.

Exhibit 6-33. Computed

length of conflicting leg of

intersection sight triangle.

In general, it is recommended to provide no more than the minimum required

intersection sight distance on each approach. Excessive intersection sight distance

can lead to higher vehicle speeds that reduce the safety of the intersection for all

road users (vehicles, bicycles, pedestrians). Landscaping can be effective in restricting sight distance to the minimum requirements.

Note that the stopping sight distance on the circulatory roadway (Exhibit 6-30) and

the intersection sight distance to the circulating stream (Exhibit 6-32) imply restrictions on the height of the central island, including landscaping and other objects,

within these zones. In the remaining central area of the central island, higher landscaping may serve to break the forward vista for through vehicles, thereby contributing to speed reduction. However, should errant vehicles encroach on the central

island, Chapter 7 provides recommended maximum grades on the central island to

minimize the probability of the vehicles rolling over, causing serious injury.

Conflicting

Approach Speed

(mph)

Computed

Distance (m)

Computed

Distance (ft)

Conflicting

Approach Speed

(km/h)

Providing more than the

minimum required intersection

sight distance can lead to

higher speeds that reduce

intersection safety.

20 36.1

25 45.2

30 54.2

35 63.2

40 72.3

10 95.4

15 143.0

20 190.1

25 238.6

30 286.3
164 Federal Highway Administration

6.3.11 Vertical considerations

Elements of vertical alignment design for roundabouts include profiles,

superelevation, approach grades, and drainage.

6.3.11.1 Profiles

The vertical design of a roundabout begins with the development of approach roadway and central island profiles. The development of each profile is an iterative process that involves tying the elevations of the approach roadway profiles into a

smooth profile around the central island.

Generally, each approach profile should be designed to the point where the approach baseline intersects with the central island. A profile for the central island is

then developed which passes through these four points (in the case of a fourlegged roundabout). The approach roadway profiles are then readjusted as necessary to meet the central island profile. The shape of the central island profile is

generally in the form of a sine curve. Examples of how the profile is developed can

be found in Exhibits 6-34, 6-35, and 6-36, which consist of a sample plan, profiles

on each approach, and a profile along the central island, respectively. Note that the

four points where the approach roadway baseline intersects the central island

baseline are identified on the central island profile.

Exhibit 6-34. Sample plan

view.
Roundabouts: An Informational Guide • 6: Geometric Design 165

Exhibit 6-35. Sample

approach profile.

Exhibit 6-36. Sample central

island profile.
166 Federal Highway Administration

6.3.11.2 Superelevation

As a general practice, a cross slope of 2 percent away from the central island

should be used for the circulatory roadway. This technique of sloping outward is

recommended for four main reasons:

• It promotes safety by raising the elevation of the central island and improving its

visibility;

• It promotes lower circulating speeds;

• It minimizes breaks in the cross slopes of the entrance and exit lanes; and

• It helps drain surface water to the outside of the roundabout (2, 6).

The outward cross slope design means vehicles making through and left-turn movements must negotiate the roundabout at negative superelevation. Excessive negative superelevation can result in an increase in single-vehicle crashes and loss-ofload incidents for trucks, particularly if speeds are high. However, in the intersection environment, drivers will generally expect to travel at slower speeds and will

accept the higher side force caused by reasonable adverse superelevation (10).

Exhibit 6-37 provides a typical section across the circulatory roadway of a roundabout without a truck apron. Exhibit 6-38 provides a typical section for a roundabout with a truck apron. Where truck aprons are used, the slope of the apron

should be 3 to 4 percent; greater slopes may increase the likelihood of loss-of-load

incidents.

Exhibit 6-37. Typical

circulatory roadway section.

Exhibit 6-38. Typical section

with a truck apron.

Negative superelevation (- 2%)

should generally be used for the

circulatory roadway.
Roundabouts: An Informational Guide • 6: Geometric Design 167

6.3.11.3 Locating roundabouts on grades

It is generally not desirable to locate roundabouts in locations where grades

through the intersection are greater than four percent. The installation of roundabouts on roadways with grades lower than three percent is generally not problematic (6). At locations where a constant grade must be maintained through the

intersection, the circulatory roadway may be constructed on a constant-slope

plane. This means, for instance, that the cross slope may vary from +3 percent

on the high side of the roundabout (sloped toward the central island) to -3 percent on the low side (sloped outward). Note that central island cross slopes will

pass through level at a minimum of two locations for roundabouts constructed

on a constant grade.

Care must be taken when designing roundabouts on steep grades. On approach

roadways with grades steeper than -4 percent, it is more difficult for entering

drivers to slow or stop on the approach. At roundabouts on crest vertical curves

with steep approaches, a driver’s sight lines will be compromised, and the roundabout may violate driver expectancy. However, under the same conditions, other

types of at-grade intersections often will not provide better solutions. Therefore,

the roundabout should not necessarily be eliminated from consideration at such

a location. Rather, the intersection should be relocated or the vertical profile modified, if possible.

6.3.11.4 Drainage

With the circulatory roadway sloping away from the central island, inlets will

generally be placed on the outer curbline of the roundabout. However, inlets may

be required along the central island for a roundabout designed on a constant

grade through an intersection. As with any intersection, care should be taken to

ensure that low points and inlets are not placed in crosswalks. If the central

island is large enough, the designer may consider placing inlets in the central

island.

6.3.12 Bicycle provisions

With regard to bicycle treatments, the designer should strive to provide bicyclists the choice of proceeding through the roundabout as either a vehicle or a

pedestrian. In general, bicyclists are better served by treating them as vehicles.

However, the best design provides both options to allow cyclists of varying degrees of skill to choose their more comfortable method of navigating the roundabout.

To accommodate bicyclists traveling as vehicles, bike lanes should be terminated

in advance of the roundabout to encourage cyclists to mix with vehicle traffic.

Under this treatment, it is recommended that bike lanes end 30 m (100 ft) upstream of the yield line to allow for merging with vehicles (11). This method is

most successful at smaller roundabouts with speeds below 30 km/h (20 mph),

where bicycle speeds can more closely match vehicle speeds.

To accommodate bicyclists who prefer not to use the circulatory roadway, a widened sidewalk or a shared bicycle/pedestrian path may be provided physically

separated from the circulatory roadway (not as a bike lane within the circulatory

Avoid locating roundabouts

in areas where grades through

the intersection are greater

than 4%.

Terminate bicycle lanes prior to

a roundabout.
168 Federal Highway Administration

Exhibit 6-39. Possible

provisions for bicycles.

6.3.13 Sidewalk treatments

Where possible, sidewalks should be set back from the edge of the circulatory

roadway in order to discourage pedestrians from crossing to the central island,

particularly when an apron is present or a monument on the central island. Equally

important, the design should help pedestrians with visual impairments to recognize that they should not attempt to cross streets from corner to corner but at

designated crossing points. To achieve these goals, the sidewalk should be designed so that pedestrians will be able to clearly find the intended path to the

crosswalks. A recommended set back distance of 1.5 m (5 ft) (minimum 0.6 m [2

ft]) should be used, and the area between the sidewalk and curb can be planted

with low shrubs or grass (see Chapter 7). Exhibit 6-40 shows this technique.

roadway). Ramps or other suitable connections can then be provided between this

sidewalk or path and the bike lanes, shoulders, or road surface on the approaching

and departing roadways. The designer should exercise care in locating and designing the bicycle ramps so that they are not misconstrued by pedestrians as an unmarked pedestrian crossing. Nor should the exits from the roadway onto a shared

path allow cyclists to enter the shared path at excessive speeds. Exhibit 6-39 illustrates a possible design of this treatment. The reader is encouraged to refer to the

AASHTO Guide for Development of Bicycle Facilities (12) for a more detailed discussion of the design requirements for bicycle and shared-use path design.

Set back sidewalks 1.5 m (5 ft)

from the circulatory roadway

where possible.

Ramps leading to a shared

pathway can be used to

accommodate bicyclists

traveling as pedestrians.
Roundabouts: An Informational Guide • 6: Geometric Design 169

Exhibit 6-40. Sidewalk

treatments.

6.3.14 Parking considerations and bus stop locations

Parking or stopping in the circulatory roadway is not conducive to proper roundabout operations and should be prohibited. Parking on entries and exits should

also be set back as far as possible so as not to hinder roundabout operations or to

impair the visibility of pedestrians. AASHTO recommends that parking should

end at least 6.1 m (20 ft) from the crosswalk of an intersection (4). Curb extensions or “bulb-outs” can be used to clearly mark the limit of permitted parking

and reduce the width of the entries and exits.

For safety and operational reasons, bus stops should be located as far away from

entries and exits as possible, and never in the circulatory roadway.

• Near-side stops: If a bus stop is to be provided on the near side of a roundabout, it should be located far enough away from the splitter island so that a

vehicle overtaking a stationary bus is in no danger of being forced into the

splitter island, especially if the bus starts to pull away from the stop. If an

approach has only one lane and capacity is not an issue on that entry, the bus

stop could be located at the pedestrian crossing in the lane of traffic. This is

not recommended for entries with more than one lane, because vehicles in

the lane next to the bus may not see pedestrians.

• Far-side stops: Bus stops on the far side of a roundabout should be constructed

with pull-outs to minimize queuing into the roundabout. These stops should

be located beyond the pedestrian crossing to improve visibility of pedestrians

to other exiting vehicles.
170 Federal Highway Administration

Exhibit 6-41. Example of

right-turn bypass lane.

6.3.15 Right-turn bypass lanes

In general, right-turn bypass lanes (or right-turn slip lanes) should be avoided, especially in urban areas with bicycle and pedestrian activity. The entries and exits of

bypass lanes can increase conflicts with bicyclists. The generally higher speeds of

bypass lanes and the lower expectation of drivers to stop increases the risk of

collisions with pedestrians. However, in locations with minimal pedestrian and bicycle activity, right-turn bypass lanes can be used to improve capacity where there

is heavy right turning traffic.

The provision of a right-turn bypass lane allows right-turning traffic to bypass the

roundabout, providing additional capacity for the through and left-turn movements

at the approach. They are most beneficial when the demand of an approach exceeds its capacity and a significant proportion of the traffic is turning right. However, it is important to consider the reversal of traffic patterns during the opposite

peak time period. In some cases, the use of a right-turn bypass lane can avoid the

need to build an additional entry lane and thus a larger roundabout. To determine if

a right-turn bypass lane should be used, the capacity and delay calculations in

Chapter 4 should be performed. Right-turn bypass lanes can also be used in locations where the geometry for right turns is too tight to allow trucks to turn within

the roundabout.

Right-turn bypass lanes can be

used in locations with minimal

pedestrian and bicycle activity

to improve capacity when heavy

right-turning traffic exists.

Exhibit 6-41 shows an example of a right-turn bypass lane.
Roundabouts: An Informational Guide • 6: Geometric Design 171

There are two design options for right-turn bypass lanes. The first option, shown in

Exhibit 6-42, is to carry the bypass lane parallel to the adjacent exit roadway, and

then merge it into the main exit lane. Under this option, the bypass lane should be

carried alongside the main roadway for a sufficient distance to allow vehicles in the

bypass lane and vehicles exiting the roundabout to accelerate to comparable speeds.

The bypass lane is then merged at a taper rate according to AASHTO guidelines for

the appropriate design speed. The second design option for a right-turn bypass

lane, shown in Exhibit 6-43, is to provide a yield-controlled entrance onto the adjacent exit roadway. The first option provides better operational performance than

the second does. However, the second option generally requires less construction

and right-of-way than the first.

The option of providing yield control on a bypass lane is generally better for both

bicyclists and pedestrians and is recommended as the preferred option in urban

areas where pedestrians and bicyclists are prevalent. Acceleration lanes can be

problematic for bicyclists because they end up being to the left of accelerating

motor vehicles. In addition, yield control at the end of a bypass lane tends to slow

motorists down, whereas an acceleration lane at the end of a bypass lane tends to

promote higher speeds.

The radius of the right-turn bypass lane should not be significantly larger than the

radius of the fastest entry path provided at the roundabout. This will ensure vehicle

speeds on the bypass lane are similar to speeds through the roundabout, resulting

in safe merging of the two roadways. Providing a small radius also provides greater

safety for pedestrians who must cross the right-turn slip lane.

Exhibit 6-42. Configuration

of right-turn bypass lane with

acceleration lane.

Right-turn bypass lanes can

merge back into the main exit

roadway or provide a yieldcontrolled entrance onto the

main exit roadway.
172 Federal Highway Administration

6.4 Double-Lane Roundabouts

While the fundamental principles described above apply to double-lane roundabouts

as well as single-lane roundabouts, designing the geometry of double-lane roundabouts is more complicated. Because multiple traffic streams may enter, circulate

through, and exit the roundabout side-by-side, consideration must be given to how

these adjacent traffic streams interact with each other. Vehicles in adjacent entry

lanes must be able to negotiate the roundabout geometry without competing for

the same space. Otherwise, operational and/or safety deficiencies can occur.

6.4.1 The natural vehicle path

As discussed in Section 6.2.1, the fastest path through the roundabout is drawn to

ensure the geometry imposes sufficient curvature to achieve a safe design speed.

This path is drawn assuming the roundabout is vacant of all other traffic and the

vehicle cuts across adjacent travel lanes, ignoring all lane markings. In addition to

evaluating the fastest path, at double-lane roundabouts the designer must also

evaluate the natural vehicle paths. This is the path an approaching vehicle will naturally take, assuming there is traffic in all approach lanes, through the roundabout

geometry.

Exhibit 6-43. Configuration of

right-turn bypass with yield at

exit leg.
Roundabouts: An Informational Guide • 6: Geometric Design 173

As two traffic streams approach the roundabout in adjacent lanes, they will be

forced to stay in their lanes up to the yield line. At the yield point, vehicles will

continue along their natural trajectory into the circulatory roadway, then curve around

the central island, and curve again into the opposite exit roadway. The speed and

orientation of the vehicle at the yield line determines its natural path. If the natural

path of one lane interferes or overlaps with the natural path of the adjacent lane,

the roundabout will not operate as safely or efficiently as possible.

The key principle in drawing the natural path is to remember that drivers cannot

change the direction of their vehicle instantaneously. Neither can they change their

speed instantaneously. This means that the natural path does not have sudden

changes in curvature; it has transitions between tangents and curves and between

consecutive reversing curves. Secondly, it means that consecutive curves should

be of similar radius. If a second curve has a significantly smaller radius than the

first curve, the driver will be traveling too fast to negotiate the turn and may lose

control of the vehicle. If the radius of one curve is drawn significantly smaller than

the radius of the previous curve, the path should be adjusted.

To identify the natural path of a given design, it may be advisable to sketch the

natural paths over the geometric layout, rather than use a computer drafting program

or manual drafting equipment. In sketching the path, the designer will naturally draw

transitions between consecutive curves and tangents, similar to the way a driver

would negotiate an automobile. Freehand sketching also enables the designer to feel

how changes in one curve affect the radius and orientation of the next curve. In

general, the sketch technique allows the designer to quickly obtain a smooth, natural

path through the geometry that may be more difficult to obtain using a computer.

Exhibit 6-44 illustrates a sketched natural path of a vehicle through a typical doublelane roundabout.

Exhibit 6-44. Sketched natural

paths through a double-lane

roundabout.
174 Federal Highway Administration

Exhibit 6-45. Path overlap at a

double-lane roundabout.

6.4.2 Vehicle path overlap

Vehicle path overlap occurs when the natural path through the roundabout of one

traffic stream overlaps the path of another. This can happen to varying degrees. It can

reduce capacity, as vehicles will avoid using one or more of the entry lanes. It can

also create safety problems, as the potential for sideswipe and single-vehicle crashes

is increased. The most common type of path overlap is where vehicles in the left lane

on entry are cut off by vehicles in the right lane, as shown in Exhibit 6-45.

6.4.3 Design method to avoid path overlap

Achieving a reasonably low design speed at a double-lane roundabout while avoiding vehicle path overlap can be difficult because of conflicting interaction between

the various geometric parameters. Providing small entry radii can produce low entry speeds, but often leads to path overlap on the entry, as vehicles will cut across

lanes to avoid running into the central island. Likewise, providing small exit radii

can aid in keeping circulating speeds low, but may result in path overlap at the

exits.

6.4.3.1 Entry curves

At double-lane entries, the designer needs to balance the need to control entry

speed with the need to minimize path overlap. This can be done a variety of ways

that will vary significantly depending on site-specific conditions, and it is thus inappropriate to specify a single method for designing double-lane roundabouts. Regardless of the specific design method employed, the designer should maintain

the overall design principles of speed control and speed consistency presented in

Section 6.2.

One method to avoid path overlap on entry is to start with an inner entry curve that

is curvilinearly tangential to the central island and then draw parallel alignments to

determine the position of the outside edge of each entry lane. These curves can

range from 30 to 60 m (100 to 200 ft) in urban environments and 40 to 80 m (130 to

260 ft) in rural environments. These curves should extend approximately 30 m (100
Roundabouts: An Informational Guide • 6: Geometric Design 175

Exhibit 6-46. One method of

entry design to avoid path

overlap at double-lane

roundabouts.

Another method to reduce entry speeds and avoid path overlap is to use a smallradius (generally 15 to 30 m [50 to 100 ft]) curve approximately 10 to 15 m (30 to 50

ft) upstream of the yield line. A second, larger-radius curve (or even a tangent) is

then fitted between the first curve and the edge of the circulatory roadway. In this

way, vehicles will still be slowed by the small-radius approach curve, and they will

be directed along a path that is tangential to the central island at the time they

reach the yield line. Exhibit 6-47 demonstrates this alternate method of design.

ft) to provide clear indication of the curvature to the driver. The designer should

check the critical vehicle paths to ensure that speeds are sufficiently low and consistent between vehicle streams. The designer should also ensure that the portion

of the splitter island in front of the crosswalk meets AASHTO recommendations

for minimum size. Exhibit 6-46 demonstrates this method of design.

Exhibit 6-47. Alternate

method of entry design to avoid

path overlap at double-lane

roundabouts.
176 Federal Highway Administration

As in the case of single-lane roundabouts, it is a primary objective to ensure that

the entry path radius along the fastest path is not substantially larger than the

circulating path radius. Referring to Exhibit 6-12, it is desirable for R1

 to be less than

or approximately equal to R2

. At double-lane roundabouts, however, R1 should not

be excessively small. If R1

 is too small, vehicle path overlap may result, reducing

the operational efficiency and increasing potential for crashes. Values for R1

 in the

range of 40 to 70 m (130 to 230 ft) are generally preferable. This results in a design

speed of 35 to 45 km/h (22 to 28 mph).

The entry path radius, R1 , is controlled by the offset between the right curb line on

the entry roadway and the curb line of the central island (on the driver’s left). If the

initial layout produces an entry path radius above the preferred design speed, one

way to reduce it is to gradually shift the approach to the left to increase the offset;

however, this may increased adjacent exit speeds. Another method to reduce the

entry path radius is to move the initial, small-radius entry curve closer to the circulatory roadway. This will decrease the length of the second, larger-radius curve and

increase the deflection for entering traffic. However, care must be taken to ensure

this adjustment does not produce overlapping natural paths.

6.4.3.2 Exit curves

To avoid path overlap on the exit, it is important that the exit radius at a double-lane

roundabout not be too small. At single-lane roundabouts, it is acceptable to use a

minimal exit radius in order to control exit speeds and maximize pedestrian safety.

However, the same is not necessarily true at double-lane roundabouts. If the exit

radius is too small, traffic on the inside of the circulatory roadway will tend to exit

into the outside exit lane on a more comfortable turning radius.

At double-lane roundabouts in urban environments, the principle for maximizing

pedestrian safety is to reduce vehicle speeds prior to the yield and maintain similar

(or slightly lower) speeds within the circulatory roadway. At the exit points, traffic

will still be traveling slowly, as there is insufficient distance to accelerate significantly. If the entry and circulating path radii (R1

 and R2 , as shown on Exhibit 6-12)

are each 50 m (165 ft), exit speeds will generally be below 40 km/h (25 mph) regardless of the exit radius.

To achieve exit speeds slower than 40 km/h (25 mph), as is often desirable in environments with significant pedestrian activity, it may be necessary to tighten the

exit radius. This may improve safety for pedestrians at the possible expense of

increased vehicle-vehicle collisions.

6.5 Rural Roundabouts

Roundabouts located on rural roads often have special design considerations because approach speeds are higher than urban or local streets and drivers generally

do not expect to encounter speed interruptions. The primary safety concern in rural

locations is to make drivers aware of the roundabout with ample distance to comfortably decelerate to the appropriate speed. This section provides design guidelines for providing additional speed-reduction measures on rural roundabout approaches.
Roundabouts: An Informational Guide • 6: Geometric Design 177

6.5.1 Visibility

Perhaps the most important element affecting safety at rural intersections is the

visibility of the intersection itself. Roundabouts are no different from stop-controlled

or signalized intersections in this respect except for the presence of curbing along

roadways that are typically not curbed. Therefore, although the number and severity of multiple-vehicle collisions at roundabouts may decrease (as discussed previously), the number of single-vehicle crashes may increase. This potential can be

minimized with attention to proper visibility of the roundabout and its approaches.

Where possible, the geometric alignment of approach roadways should be constructed to maximize the visibility of the central island and the general shape of the

roundabout. Where adequate visibility cannot be provided solely through geometric alignment, additional treatments (signing, pavement markings, advanced warning beacons, etc.) should be considered (see Chapter 7). Note that many of these

treatments are similar to those that would be applied to rural stop-controlled or

signalized intersections.

6.5.2 Curbing

On an open rural highway, changes in the roadway’s cross-section can be an effective means to help approaching drivers recognize the need to reduce their speed.

Rural highways typically have no outside curbs with wide paved or gravel shoulders. Narrow shoulder widths and curbs on the outside edges of pavement, on the

other hand, generally give drivers a sense they are entering a more urbanized setting, causing them to naturally slow down. Thus, consideration should be given to

reducing shoulder widths and introducing curbs when installing a roundabout on

an open rural highway.

Curbs help to improve delineation and to prevent “corner cutting,” which helps to

ensure low speeds. In this way, curbs help to confine vehicles to the intended

design path. The designer should carefully consider all likely design vehicles, including farm equipment, when setting curb locations. Little research has been performed to date regarding the length of curbing required in advance of a rural roundabout. In general, it may be desirable to extend the curbing from the approach for

at least the length of the required deceleration distance to the roundabout.

6.5.3 Splitter islands

Another effective cross-section treatment to reduce approach speeds is to use

longer splitter islands on the approaches (10). Splitter islands should generally be

extended upstream of the yield bar to the point at which entering drivers are expected to begin decelerating comfortably. A minimum length of 60 m (200 ft) is

recommended (10). Exhibit 6-48 provides a diagram of such a splitter island design.

The length of the splitter island may differ depending upon the approach speed.

The AASHTO recommendations for required braking distance with an alert driver

should be applied to determine the ideal splitter island length for rural roundabout

approaches.

A further speed-reduction technique is the use of landscaping on the extended

splitter island and roadside to create a “tunnel” effect. If such a technique is used,

the stopping and intersection sight distance requirements (sections 6.3.9 and 6.3.10)

will dictate the maximum extent of such landscaping.

Roundabout visibility is a key

design element at rural

locations.

Curbs should be provided at all

rural roundabouts.

Extended splitter islands are

recommended at rural

locations.
178 Federal Highway Administration

Exhibit 6-48. Extended splitter

island treatment.

6.5.4 Approach curves

Roundabouts on high-speed roads (speeds of 80 km/h [50 mph] or higher), despite

extra signing efforts, may not be expected by approaching drivers, resulting in erratic behavior and an increase in single-vehicle crashes. Good design encourages

drivers to slow down before reaching the roundabout, and this can be most effectively achieved through a combination of geometric design and other design treatments (see Chapter 7). Where approach speeds are high, speed consistency on

the approach needs to be addressed to avoid forcing all of the reduction in speed to

be completed through the curvature at the roundabout.

The radius of an approach curve (and subsequent vehicular speeds) has a direct

impact on the frequency of crashes at a roundabout. A study in Queensland, Australia, has shown that decreasing the radius of an approach curve generally decreases the approaching rear-end vehicle crash rate and the entering-circulating

and exiting-circulating vehicle crash rates (see Chapter 5). On the other hand, decreasing the radius of an approach curve may increase the single-vehicle crash rate

on the curve, particularly when the required side-friction for the vehicle to maintain

its path is too high. This may encourage drivers to cut across lanes and increase

sideswipe crash rates on the approach curve (2).

One method to achieve speed reduction that reduces crashes at the roundabout

while minimizing single-vehicle crashes is the use of successive curves on approaches. The study in Queensland, Australia, found that by limiting the change in

85th-percentile speed on successive geometric elements to 20 km/h (12 mph), the

crash rate was reduced. It was found that the use of successive reverse curves

prior to the roundabout approach curve reduced the single-vehicle crash rate and

the sideswipe crash rate on the approach. It is recommended that approach speeds

immediately prior to the entry curves of the roundabout be limited to 60 km/h (37

mph) to minimize high-speed rear-end and entering-circulating vehicle crashes.
Roundabouts: An Informational Guide • 6: Geometric Design 179

Exhibit 6-49. Use of

successive curves on high

speed approaches.

Equations 6-4 and 6-5 can be used to estimate the operating speed of two-lane

rural roads as a function of degree of curvature. Equation 6-6 can be used similarly

for four-lane rural roads (13).

A series of progressively sharper

curves on a high-speed

roundabout approach helps

slow traffic to an appropriate

entry speed.

Two-lane rural roads:

V DD 85 = − ≥° 103 66 1 95 3 . ., (6-4)

V D 85 = <° 97 9 3 . , (6-5)

where: V85 = 85th-percentile speed, km/h (1 km/h = 0.621 mph); and

D = degree of curvature, degrees = 1746.38 / R

R = radius of curve, m

Four-lane rural roads:

V D 85 = − 103 66 1 95 . . (6-6)

where: V85 = 85th-percentile speed, km/h (1 km/h = 0.621 mph); and

D = degree of curvature, degrees = 1746.38 / R

R = radius of curve, m

6.6 Mini-Roundabouts

As discussed in Chapter 1, a mini-roundabout is an intersection design alternative

that can be used in place of stop control or signalization at physically constrained

intersections to help improve safety problems and excessive delays at minor approaches. Mini-roundabouts are not traffic calming devices but rather are a form of

roundabout intersection. Exhibit 6-50 presents an example of a mini-roundabout.

Mini-roundabouts are not

recommended where approach

speeds are greater than 50 km/h

(30 mph), nor in locations

with high U-turning volumes.

Exhibit 6-49 shows a typical rural roundabout design with a succession of three

curves prior to the yield line. As shown in the exhibit, these approach curves should

be successively smaller radii in order to minimize the reduction in design speed

between successive curves. The aforementioned Queensland study found that

shifting the approaching roadway laterally by 7 m (23 ft) usually enables adequate

curvature to be obtained while keeping the curve lengths to a minimum. If the

lateral shift is too small, drivers are more likely to cut into the adjacent lane (2).
180 Federal Highway Administration

Exhibit 6-50. Example of a

mini-roundabout.

Mini-roundabouts should only be considered in areas where all approaching roadways have an 85th-percentile speed of less than 50 km/h (30 mph). In addition,

mini-roundabouts are not recommended in locations in which high U-turn traffic is

expected, such as at the ends of street segments with access restrictions. Miniroundabouts are not well suited for high volumes of trucks, as trucks will occupy

most of the intersection when turning.

The design of the central island of a mini-roundabout is defined primarily by the

requirement to achieve speed reduction for passenger cars. As discussed previously in Section 6.2, speed reduction for entering vehicles and speed consistency

with circulating vehicles are important. Therefore, the location and size of the central island are dictated by the inside of the swept paths of passenger cars that is

needed to achieve a maximum recommended entry speed of 25 km/h (15 mph).

The central island of a mini-roundabout is typically a minimum of 4 m (13 ft) in

diameter and is fully mountable by large trucks and buses. Composed of asphalt,

concrete, or other paving material, the central island should be domed at a height

of 25 to 30 mm per 1 m diameter (0.3 to 0.36 in per 1 ft diameter), with a maximum

height of 125 mm (5 in) (14). Although fully mountable and relatively small, it is

essential that the central island be clear and conspicuous (14, 15). Chapter 7 provides a sample signing and striping planing plan for mini-roundabout.

The outer swept path of passenger cars and large vehicles is typically used to

define the location of the yield line and boundary of each splitter island with the

circulatory roadway. Given the small size of a mini-roundabout, the outer swept

path of large vehicles may not be coincident with the inscribed circle of the roundabout, which is defined by the outer curbs. Therefore, the splitter islands and yield

line may extend into the inscribed circle for some approach geometries. On the

other hand, for very small mini-roundabouts, such as the one shown in Exhibit 6-

50, all turning trucks will pass directly over the central island while not encroaching

on the circulating roadway to the left which may have opposing traffic. In these

cases, the yield line and splitter island should be set coincident with the inscribed

The central island of a

mini-roundabout should be

clear and conspicuous.
Roundabouts: An Informational Guide • 6: Geometric Design 181

6.7 References

1. Department of Transport of Northrhine-Westfalia, Germany. Empfehlungen zum

Einsatz und zur Gestaltung von Mini-Kreisverkehrsplaetzen (Guidelines for the

Use and Design of Mini-Roundabouts). Dusseldorf, Germany, 1999.

2. Queensland Department of Main Roads (QDMR). Relationships between Roundabout Geometry and Accident Rates. Queensland, Australia: Infrastructure Design of the Technology Division of QDMR, April 1998.

3. Department of Transport (United Kingdom). Geometric Design of Roundabouts.

TD 16/93. September 1993.

4. American Association of State Highway and Transportation Officials (AASHTO). A

Policy on Geometric Design of Highways and Streets. Washington, D.C.: AASHTO,

1994.

5. Pein, W.E. Trail Intersection Design Guidelines. Prepared for State Bicycle/Pedestrian Program, State Safety Office, Florida Department of Transportation. Highway Safety Research Center, University of North Carolina, September 1996.

6. Service d’Etudes Techniques des Routes et Autoroutes (SETRA—Center for Technical Studies of Roads and Highways). Aménagement des Carrefours Interurbains

sur les Routes Principales (Design of Rural Intersections on Major Roads). Ministry of Transport and Housing, December 1998.

7. Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities (ADAAG). 36 CFR Part 1191. As amended through January 1998.

8. Fambro, D.B., et al. NCHRP Report 400: Determination of Stopping Sight Distances. National Cooperative Highway Research Program, Transportation Research

Board, National Research Council. Washington, D.C.: National Academy Press,

1997.

9. Harwood, D.W., et al. NCHRP Report 383: Intersection Sight Distances. National

Cooperative Highway Research Program, Transportation Research Board, National

Research Council. Washington, D.C.: National Academy Press, 1996.

10. Austroads. Guide to Traffic Engineering Practice, Part 6—Roundabouts. Sydney,

Australia: Austroads, 1993.

11. Florida Department of Transportation. Florida Roundabout Guide. Florida Department of Transportation, March 1996.

12. American Association of State Highway and Transportation Officials (AASHTO).

Guide for Development of Bicycle Facilities. Washington, D.C.: AASHTO, 1991.

13. Krammes, R., et al. Horizontal Alignment Design Consistency for Rural Two-Lane

Highways. Publication No. FHWA-RD-94-034. Washington, D.C.: Federal Highway Administration, January 1995.

14. Sawers, C. Mini-roundabouts: Getting them right!. Canterbury, Kent, United Kingdom: Euro-Marketing Communications, 1996.

15. Brilon, W., and L. Bondzio. Untersuchung von Mini-Kreisverkehrsplaetzen (Investigation of Mini-Roundabouts). Ruhr-University Bochum, Germany, 1999.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 183

7.1 Signing 185

7.1.1 Relationship with the Manual on Uniform Traffic Control

Devices 185

7.1.2 Regulatory signs 186

7.1.3 Warning signs 188

7.1.4 Guide signs 191

7.1.5 Urban signing considerations 192

7.1.6 Rural and suburban signing considerations 194

7.1.7 Mini-roundabout signing considerations 195

7.2 Pavement Markings 197

7.2.1 Relationship with the Manual on Uniform Traffic Control

Devices 197

7.2.2 Approach and entry pavement markings 197

7.2.3 Circulatory roadway pavement markings 200

7.2.4 Mini-roundabout pavement markings 201

7.3 Illumination 202

7.3.1 Need for illumination 202

7.3.2 Standards and recommended practices 203

7.3.3 General recommendations 204

7.3.4 Clear zone requirements 205

7.4 Work Zone Traffic Control 205

7.4.1 Pavement markings 205

7.4.2 Signing 205

7.4.3 Lighting 206

Traffic Design 7

and Landscaping
184 Federal Highway Administration

Exhibit 7-1. YIELD sign (R1-2). 186

Exhibit 7-2. ONE WAY sign (R6-1R). 186

Exhibit 7-3. KEEP RIGHT sign (R4-7). 186

Exhibit 7-4. Lane-use control signing for roundabouts with double-lane

entries. 188

Exhibit 7-5. Lane-use control signing for roundabouts with heavy

turning traffic. 188

Exhibit 7-6. Circular Intersection sign (W2-6). 189

Exhibit 7-7. Advisory speed plate (W13-1). 189

Exhibit 7-8. Roundabout Ahead sign. 189

Exhibit 7-9. YIELD AHEAD sign (W3-2a). 189

Exhibit 7-10. Large Arrow sign (W1-6). 190

Exhibit 7-11. Chevron plate (W1-8a). 190

Exhibit 7-12. Pedestrian Crossing sign (W11-2a). 190

Exhibit 7-13. Examples of advance destination guide signs. 191

Exhibit 7-14. Exit guide sign (D1-1). 192

Exhibit 7-15. Sample signing plan for an urban roundabout. 193

Exhibit 7-16. Sample signing plan for a rural roundabout. 194

Exhibit 7-17. Examples of speed reduction treatments. 195

Exhibit 7-18. Sample signing plan for a mini-roundabout. 196

Exhibit 7-19. Examples of yield lines. 198

Exhibit 7-20. Approach pavement markings. 199

Exhibit 7-21. Sample pavement marking plan for a mini-roundabout. 201

Exhibit 7-22. Illumination of a roundabout. 202

Exhibit 7-23. Recommended street illumination levels. 204

Exhibit 7-24. Landscaping of the central island. 208

7.4.4 Construction staging 206

7.4.5 Public education 207

7.5 Landscaping 207

7.5.1 Advantages 207

7.5.2 Central island landscaping 208

7.5.3 Splitter island and approach landscaping 208

7.5.4 Maintenance 209

7.6 References 209
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 185

This chapter presents guidelines on the design of traffic elements, illumination,

and landscaping associated with roundabouts. The design of these elements is

critical in achieving the desired operational and safety features of a roundabout, as

well as the desired visibility and aesthetics. This chapter is divided into the following sections:

• Signing;

• Pavement Markings;

• Illumination;

• Work Zone Traffic Control; and

• Landscaping.

7.1 Signing

The overall concept for roundabout signing is similar to general intersection signing. Proper regulatory control, advance warning, and directional guidance are required to avoid driver expectancy related problems. Signs should be located where

they have maximum visibility for road users but a minimal likelihood of even momentarily obscuring pedestrians as well as motorcyclists and bicyclists, who are

the most vulnerable of all roundabout users. Signing needs are different for urban

and rural applications and for different categories of roundabouts.

7.1.1 Relationship with the Manual on Uniform Traffic Control Devices

The Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD)

(1) and Standard Highway Signs (2), as well as local applicable standards, govern

the design and placement of signs. To the extent possible, this guide has been

prepared in accordance with the 1988 edition of the MUTCD. However, roundabouts present a number of new signing issues that are not addressed in the 1988

edition. For this reason, a number of new signs or uses for existing signs have

been introduced that are under consideration for inclusion in the next edition of the

MUTCD. Until such signs or uses are formally adopted, these recommendations

should be considered provisional and are subject to MUTCD Section 1A-6, “Manual

Changes, Interpretations and Authority to Experiment.”

The following signs and applications recommended below are subject to these

conditions:

• Use of YIELD signs on more than one approach to an intersection (Section 7.1.2.1);

• Long chevron plate (Section 7.1.2.2);

• Roundabout Ahead sign (Section 7.1.3.1);

• Advance diagrammatic guide signs (Section 7.1.4.1); and

• Exit guide signs (Section 7.1.4.2).

Signing, striping, illumination,

and landscaping are the critical

finishing touches for an

effectively functioning

roundabout.

Chapter 7 Traffic Design and Landscaping
186 Federal Highway Administration

7.1.2 Regulatory signs

A number of regulatory signs are appropriate for roundabouts and are described

below.

7.1.2.1 YIELD sign

A YIELD sign (R1-2), shown in Exhibit 7-1, is required at the entrance to the roundabout. For single-lane approaches, one YIELD sign placed on the right side is sufficient, although a second YIELD sign mounted in the splitter island on the left side

of the approach may be used. For approaches with more than one lane, the designer should place YIELD signs on both the left and right sides of the approach.

This practice is consistent with the recommendations of the MUTCD on the location of STOP and YIELD signs on single-lane and multilane approaches (MUTCD,

§2B-9). To prevent circulating vehicles from yielding unnecessarily, the face of the

yield sign should not be visible from the circulatory roadway. YIELD signs may also

be used at the entrance to crosswalks on both the entry and exit legs of an approach. However, the designer should not use both YIELD signs and Pedestrian

Crossing signs (see Section 7.1.3.5) to mark a pedestrian crossing, as the yield

signs at the roundabout entrance may be obscured.

YIELD signs are required on all

approaches.

7.1.2.3 KEEP RIGHT sign

KEEP RIGHT signs (R4-7 or text variations R4-7a and R4-7b) should be used at the

nose of all nonmountable splitter islands. This sign is shown in Exhibit 7-3.

For small splitter islands, a Type 1 object marker may be substituted for the KEEP

RIGHT sign. This may reduce sign clutter and improve the visibility of the YIELD

sign.

Exhibit 7-1. YIELD sign (R1-2).

Exhibit 7-2. ONE WAY sign

(R6-1R).

Exhibit 7-3. KEEP RIGHT sign

(R4-7).

ONE WAY signs establish the

direction of traffic flow within

the roundabout.

7.1.2.2 ONE WAY sign

ONE WAY signs (R6-1R) may be used in the central island opposite the entrances.

An example is shown in Exhibit 7-2. The ONE WAY sign may be supplemented with

chevron signs to emphasize the direction of travel within the circulatory roadway

(see Section 7.1.3.4).

At roundabouts with one-way streets on one or more approaches, the use of a

regulatory ONE WAY sign may be confusing. In these cases, a Large Arrow warning sign (see Section 7.1.3.3) may be used.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 187

7.1.2.4 Lane-use control signs

For roundabouts with multiple entry lanes, it can often be confusing for unfamiliar

drivers to know which lanes to use for the various left, through, and right movements. There is no international consensus on the effectiveness of lane-use signs

and/or pavement markings.

The designation of lanes on entry to a roundabout is directly related to a number of

factors:

• Traffic volume balance. Roundabouts with especially heavy left- or right-turning

traffic may require more than one lane to handle the expected demand (see

Chapter 4).

• Exit lane requirements. In general, the number of exit lanes provided should be

the minimum required to handle the expected exit volume. This may not correspond with the number of entry lanes on the opposite side of the roundabout

that would use the exit as through vehicles (see Chapter 4).

• The rules of the road. Drivers have a reasonable expectation that multiple through

lanes entering a roundabout will have an equal number of receiving lanes on

exit on the far side of the roundabout (see Chapter 2).

Lane-use control signs are generally not required where the number of receiving

lanes for through vehicles on exit matches the number of entry lanes, as shown in

Exhibit 7-4. Lane-use control signs should be used only for the following conditions:

• Where only a single exit lane is provided to receive two lanes of vehicles making through movements, lane-use designations should be made to indicate that

an entry lane drops as a turning movement (see Exhibit 7-4). This does not include cases where an approach is flared from one to two lanes at the roundabout.

• Where left- or right-turning traffic demand dictates the need for more than one

left-turn lane or more than one right-turn lane for capacity reasons (see Exhibit 7-5).

The use of a left-turn-only lane designation as shown in the exhibits may be initially

confusing to drivers. This type of designation has worked successfully in other

countries, and there is no evidence to suggest that it will not work in the United

States. However, given the general unfamiliarity of roundabouts to drivers in the

United States at this time, it is recommended that double-lane roundabouts be

designed to avoid the use of lane-use control signs wherever possible, at least until

drivers become more accustomed to driving roundabouts.

Lane-use control signs are

generally not recommended.
188 Federal Highway Administration

Exhibit 7-4. Lane-use control

signing for roundabouts with

double-lane entries.

Exhibit 7-5. Lane-use control

signing for roundabouts with

heavy turning traffic.

7.1.3 Warning signs

A number of warning signs are appropriate for roundabouts and are described below. The amount of warning a motorist needs is related to the intersection setting

and the vehicular speeds on approach roadways. The specific placement of warning signs is governed by the applicable sections of the MUTCD.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 189

7.1.3.1 Circular Intersection sign

A Circular Intersection sign (W2-6) may be installed on each approach in advance of

the roundabout. This sign, given in Exhibit 7-6, is proposed as part of the next

edition of the MUTCD. When used, it is recommended that this sign be modified to

reflect the number and alignment of approaches.

Exhibit 7-6. Circular

Intersection sign (W2-6).

It is also recommended that an advisory speed plate (W13-1) be used with this

sign, as shown in Exhibit 7-7. The speed given on the advisory speed plate should

be no higher than the design speed of the circulatory roadway, as determined in

Chapter 6.

Exhibit 7-7. Advisory speed

plate (W13-1).

An alternative to the Circular Intersection sign, called a Roundabout Ahead sign, has

been proposed and is shown in Exhibit 7-8. The rationale for this sign is given in

Appendix C. At a minimum it is recommended that the Roundabout Ahead sign be

used in place of the Circular Intersection sign at mini-roundabouts (see Section 7.1.7).

Exhibit 7-8. Roundabout

Ahead sign.

7.1.3.2 YIELD AHEAD sign

A YIELD AHEAD sign (W3-2 or W3-2a) should be used on all approaches to a roundabout in advance of the yield sign. These signs provide drivers with advance warning that a YIELD sign is approaching. The preferred symbolic form of this sign is

shown in Exhibit 7-9.

Exhibit 7-9. YIELD AHEAD sign

(W3-2a).

YIELD AHEAD signs warn drivers

of the upcoming YIELD sign.
190 Federal Highway Administration

7.1.3.3 Large Arrow sign

A Large Arrow sign with a single arrow pointing to the right (W1-6) should be used

in the central island opposite the entrances, unless a regulatory ONE-WAY sign

has been used. The Large Arrow sign is shown in Exhibit 7-10.

7.1.3.4 Chevron Plate

The Large Arrow may be supplemented or replaced by a long chevron board (W1-

8a, as proposed in the next edition of the MUTCD) to emphasize the direction of

travel within the circulatory roadway.

Exhibit 7-11. Chevron plate

(W1-8a).

7.1.3.5 Pedestrian Crossing

Pedestrian Crossing signs (W11-2a) may be used at pedestrian crossings within a

roundabout at both entries and exits. Pedestrian Crossing signs should be used at

all pedestrian crossings at double-lane entries, double-lane exits, and right-turn

bypass lanes. This sign is shown in Exhibit 7-12.

The use of Pedestrian Crossing signs is dependent on the specific laws of the

governing state. If the crosswalk at a roundabout is not considered to be part of

the intersection and is instead considered a marked midblock crossing, Pedestrian

Crossing signs are required. Where installed, Pedestrian Crossing signs should be

located in such a way to not obstruct view of the YIELD sign.

Exhibit 7-12. Pedestrian

Crossing sign (W11-2a).

Chevron plates can be especially

useful for nighttime visibility for

sites without illumination.

Exhibit 7-10. Large Arrow sign

(W1-6).
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 191

7.1.4 Guide signs

Guide signs are important in providing drivers with proper navigational information. This is especially true at roundabouts where out-of-direction travel may disorient unfamiliar drivers. A number of guide signs are appropriate for roundabouts

and are described below.

7.1.4.1 Advance destination guide signs

Advance destination guide signs should be used in all rural locations and in urban/

suburban areas where appropriate. The sign should be either a destination sign

using text (D1-3) or using diagrams. Examples of both are shown in Exhibit 7-13.

Diagrammatic signs are preferred because they reinforce the form and shape of

the approaching intersection and make it clear to the driver how they are expected

to navigate the intersection. Advance destination guide signs are not necessary at

local street roundabouts or in urban settings where the majority of traffic tends to

be familiar with the site.

Exhibit 7-13. Examples of

advance destination guide

signs.

Diagrammatic Style (Preferred)

Lothian, MD

Leeds, MD Taneytown, MD

Long Beach, CA

The circular shape in a

diagrammatic sign provides an

important visual cue to all users

of the roundabout.
192 Federal Highway Administration

Exhibit 7-14. Exit guide sign

(D1-1).

7.1.4.2 Exit guide signs

Exit guide signs (D1-1) are recommended to designate the destinations of each

exit from the roundabout. These signs are conventional intersection direction signs

or directional route marker assemblies and can be placed either on the right-hand

side of the roundabout exit or in the splitter island. An example is shown in Exhibit

7-14.

7.1.4.3 Route confirmation signs

For roundabouts involving the intersection of one or more numbered routes, route

confirmation assemblies should be installed directly after the roundabout exit. These

provide drivers with reassurance that they have selected the correct exit at the

roundabout. These assemblies should be located no more than 30 m (100 ft) beyond the intersection in urban areas and 60 m (200 ft) beyond the intersection in

rural areas.

7.1.5 Urban signing considerations

The amount of signing required at individual locations is largely based on engineering judgment. However, in practice, the designer can usually use fewer and smaller

signs in urban settings than in rural settings. This is true because drivers are generally traveling at lower vehicular speeds and have higher levels of familiarity at urban

intersections. Therefore, in many urban settings the advance destination guide signs

can be eliminated. However, some indication of street names should be included

in the form of exit guide signs or standard street name signs. Another consideration in urban settings is the use of minimum amounts of signing to avoid sign

clutter. A sample signing plan for an urban application is shown in Exhibit 7-15.

Exit guide signs reduce the

potential for disorientation.

The designer needs to balance the

need for adequate signing

with the tendency to use

too many signs.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 193

Exhibit 7-15. Sample signing

plan for an urban roundabout.
194 Federal Highway Administration

Exhibit 7-16. Sample signing

plan for a rural roundabout.

7.1.6 Rural and suburban signing considerations

Rural and suburban conditions are characterized by higher approach speeds. Route

guidance tends to be focused more on destinations and numbered routes rather

than street names. A sample signing plan for a rural application is shown in Exhibit

7-16.

Rural signing needs to be more

conspicuous than urban signing

due to higher approach speeds.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 195

In cases where high speeds are expected (in excess of 80 km/h [50 mph]) and the

normal signage and geometric features are not expected to produce the desired

reduction in vehicle speeds, the following measures may also be considered (examples of some of these treatments are given in Exhibit 7-17):

• Large advance warning signs;

• Addition of hazard identification beacons to approach signing;

• Use of rumble strips in advance of the roundabout;

• Pavement marking across pavement; and

• Use of speed warning signs. These can be triggered by speeds exceeding an

acceptable threshold.

These speed reduction treatments

can apply to all intersection

types, not just roundabouts.

Exhibit 7-17. Examples of

speed reduction treatments.

Warning beacons. Leeds, MD Rumble strips. Cearfoss, MD

Speed warning signs. Leeds, MD

7.1.7 Mini-roundabout signing considerations

Due to their small size and unique features, mini-roundabouts require a somewhat

different signing treatment than the larger urban roundabouts. The principal differences in signing at mini-roundabouts as compared to other urban roundabouts are

the following:

• The central island is fully mountable. Therefore, no ONE WAY signs, Large Arrow signs, or chevrons can be located there. It is recommended that the direction of circulation be positively indicated through the use of pavement markings, as discussed in Section 7.2.4.

• The splitter islands are either painted or are fully mountable. Therefore, KEEP

RIGHT signs are not appropriate for mini-roundabouts.
196 Federal Highway Administration

• Typically, advance directional guide signs and exit guide signs are unnecessary,

given the size of the mini-roundabout and the nature of the approach roadways

(generally low-speed local streets). However, standard street name signs (D3)

should be used.

• The Roundabout Ahead warning sign discussed in Section 7.1.3.1 should be

used on each approach in advance of the YIELD sign. The Circular Intersection

warning gives no indication of the direction of circulation required at the miniroundabout.

Exhibit 7-18 gives a sample signing plan for a mini-roundabout.

Exhibit 7-18. Sample signing

plan for a mini-roundabout.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 197

7.2 Pavement Markings

Typical pavement markings for roundabouts consist of delineating the entries and

the circulatory roadway.

7.2.1 Relationship with the Manual on Uniform Traffic Control Devices

As with signing, the MUTCD (1) and applicable local standards govern the design

and placement of pavement markings. Roundabouts present a number of new

pavement marking issues that are not addressed in the 1988 edition of the MUTCD.

For this reason, a number of new pavement markings or uses for existing pavement markings have been introduced that are under consideration for inclusion in

the next edition of the MUTCD. Until such pavement markings or uses are formally

adopted, these recommendations should be considered provisional and are subject to MUTCD Section 1A-6, “Manual Changes, Interpretations and Authority to

Experiment.”

The following pavement markings and applications recommended below are subject to these conditions:

• YIELD lines (Section (7.2.2.1); and

• Symbolic YIELD legend (Section 7.2.2.2).

7.2.2 Approach and entry pavement markings

Approach and entry pavement markings consist of yield lines, pavement word and

symbol markings, and channelization markings. In addition, multilane approaches

require special attention to pavement markings. The following sections discuss

these in more detail.

7.2.2.1 Yield lines

Yield lines should be used to demarcate the entry approach from the circulatory

roadway. Yield lines should be located along the inscribed circle at all roundabouts

except mini-roundabouts (see Section 7.2.4). No yield lines should be placed to

demarcate the exit from the circulatory roadway.

The MUTCD currently provides no standard for yield lines. The recommended yield

line pavement marking is a broken line treatment consisting of 400-mm (16-in)

wide stripes with 1-m (3-ft) segments and 1-m (3-ft) gaps. This type of yield line is

the simplest to install.

Alternatively, several European countries use a yield line marking consisting of a

series of white triangles (known as “shark’s teeth”). These markings tend to be

more visible to approaching drivers. Exhibit 7-19 presents examples of broken line

and “shark’s teeth” yield line applications. The “shark’s teeth” ahead of the broken

line has been recommended for adoption in the next edition of the MUTCD.

Yield lines provide a visual separation

between the approach and the

circulatory roadway.

“Shark’s teeth” provide more visual

“punch” but require a new template

for installation.
198 Federal Highway Administration

Exhibit 7-19. Examples of

yield lines.

Broken line. Leeds, MD

7.2.2.2 Pavement word and symbol markings

In some cases, the designer may want to consider pavement word or symbol

markings to supplement the signing and yield line marking. This typically consists

of the word YIELD painted on the entrance to the roundabout immediately prior to

the yield line. These markings should conform to the standards given in the appropriate section of the MUTCD (§3B-20).

Alternatively, some European countries paint a symbolic yield sign upstream of the

yield line. This treatment has the advantage of being symbolic; however, such a

treatment has not seen widespread use in the United States to date.

7.2.2.3 Lane-use control markings

If lane-use control signing has been used to designate specific lane use on an

approach with more than one lane, it is recommended that corresponding arrow

legends be used within each lane. See Section 7.1.2.4 for more discussion of the

use of lane-use controls.

7.2.2.4 Approach markings

Typically, pavement markings are provided around raised splitter islands and rightturn bypass islands to enhance driver recognition of the changing roadway.

Channelization markings shall be yellow when to the left of the traffic stream and

white when to the right of the traffic stream. For a roundabout splitter island, pavement markings shall be yellow adjacent to the entry and exit and white adjacent to

the circulatory roadway. Exhibit 7-20 presents a recommended pavement marking

plan for the channelization on a typical single-lane approach to a roundabout. Optionally, edge stripes may end at the points of the splitter islands, allowing the

curbs themselves to provide edge delineation.

Raised pavement markers are generally recommended for supplementing pavement

markings. These have the benefit of additional visibility at night and in inclement

weather. However, they increase maintenance costs and can be troublesome in

areas requiring frequent snow removal. In addition, raised pavement markers should

not be used in the path of travel of bicycles.

Pavement word markings are

less effective in rainy or

especially snowy climates.

Raised pavement markers are

useful supplements to

pavement markings.

“Shark’s teeth.” Lothian, MD
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 199

Exhibit 7-20. Approach

pavement markings.

For small splitter islands (in area less than 7 m2 [75 ft2), the island may consist of

pavement markings only. However, where possible, curbed splitter islands should

be used.

7.2.2.5 Pedestrian crosswalk markings

Pedestrian crosswalk markings should generally be installed at all pedestrian crossing locations within roundabouts in urban locations. Because the crosswalk at a

roundabout is located away from the yield line, it is important to channelize pedestrians to the appropriate crossing location. These markings should not be construed

as a safety device, as data from other countries suggest that the presence of

markings has no appreciable effect on pedestrian safety. Rather, markings provide

guidance for pedestrians in navigating a roundabout and provide a visual cue to

drivers of where pedestrians may be within the roadway. The use of crosswalk

markings in this manner is consistent with published recommendations (3). Marked

crosswalks are generally not needed at locations where the crosswalk is distinguished from the roadway by visually contrasting pavement colors and textures.

A crosswalk marking using a series of lines parallel to the flow of traffic (known as

a “zebra crosswalk”) is recommended. These lines should be approximately 0.3 m

to 0.6 m (12 in to 24 in) wide, spaced 0.3 m to 1.0 m (12 in to 36 in) apart, and span

the width of the crosswalk (similar to the recommendations in MUTCD §3B-18).

Crosswalk markings should be installed across both the entrance and exit of each

leg and across any right turn bypass lanes. The crosswalk should be aligned with

Zebra crosswalks provide an

important visual cue for drivers

and pedestrians.
200 Federal Highway Administration

the ramps and pedestrian refuge in the splitter island and have markings that are

generally perpendicular to the flow of vehicular traffic.

The zebra crosswalk has a number of advantages over the traditional transverse

crosswalk marking in roundabout applications:

• Because the crosswalk at a roundabout is set back from the yield line, the zebra

crosswalk provides a higher degree of visibility.

• The zebra crosswalk is distinct from traditional transverse crosswalk markings

typically used at signalized intersections, thus alerting both drivers and pedestrians that this intersection is different from a signalized intersection.

• The zebra crosswalk is also less likely to be confused with the yield line than a

transverse crosswalk.

• Although the initial cost is somewhat higher, the zebra crossing may require

less maintenance due to the ability to space the markings to avoid vehicle tire

tracks.

In rural locations where pedestrian activity is expected to be minimal, pedestrian

crosswalk markings are optional. Pedestrian crosswalk markings should not be

used at roundabouts without illumination (see Section 7.3 for an identification of

these cases) because the headlights of vehicles may not be sufficient to illuminate

a pedestrian in time to avoid a collision (4). Regardless of whether the crosswalk is

marked, all roundabouts with any reasonable possibility of pedestrian activity should

have geometric features to accommodate pedestrians as described in Chapter 6.

In addition to pavement markings, flashing warning lights mounted in the pavement and activated by a pedestrian push button or other method may be considered. These are not part of the current MUTCD and thus must be treated as an

experimental traffic control device (see Section 7.2.1).

7.2.2.6 Bike lane markings

Bicycle striping treatments should be used when an existing (or proposed) bike

lane is part of the roadway facility. Exhibit 7-20 shows a recommended treatment

for bike lanes on an approach to a roundabout.

7.2.3 Circulatory roadway pavement markings

In general, lane lines should not be striped within the circulatory roadway, regardless of the width of the circulatory roadway. Circulatory lane lines can be misleading in that they may provide drivers a false sense of security.

In addition, bike lane markings within the circulatory roadway are not recommended.

The additional width of a bike lane within the circulatory roadway increases vehicular speed and increases the probability of motor vehicle-cyclist crashes. Bicyclists

should circulate with other vehicles, travel through the roundabout as a pedestrian

on the sidewalk, or use a separate shared-use pedestrian and bicycle facility where

provided.

Circulatory pavement markings

are generally not recommended.

Bike lanes within the

roundabout are

 not recommended.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 201

7.2.4 Mini-roundabout pavement markings

Mini-roundabouts require pavement marking treatments that are somewhat different from other urban roundabouts. The following pavement marking treatments

are recommended for mini-roundabouts.

• Pavement marking arrows should be provided in the circulatory roadway in front

of each entry to indicate the direction of circulation. As noted in the discussion

of signing treatments (Section 7.1.7), no signs can be placed in the fully mountable central island.

• At a minimum, the edges of the mountable central island and splitter islands

should be painted to improve their visibility.

A sample pavement marking plan for a mini-roundabout is given in Exhibit 7-21.

Exhibit 7-21. Sample pavement

marking plan for

a mini-roundabout.
202 Federal Highway Administration

7.3.1 Need for illumination

The need for illumination varies somewhat based on the location in which the roundabout is located.

7.3.1.1 Urban conditions

In urban settings, illumination should be provided for the following reasons:

• Most if not all approaches are typically illuminated.

• Illumination is necessary to improve the visibility of pedestrians and bicyclists.

7.3.1.2 Suburban conditions

For roundabouts in suburban settings, illumination is recommended. For safety

reasons, illumination is necessary when:

• One or more approaches are illuminated.

• An illuminated area in the vicinity can distract the driver’s view.

• Heavy nighttime traffic is anticipated.

7.3 Illumination

For a roundabout to operate satisfactorily, a driver must be able to enter the roundabout, move through the circulating traffic, and separate from the circulating stream

in a safe and efficient manner. To accomplish this, a driver must be able to perceive

the general layout and operation of the intersection in time to make the appropriate

maneuvers. Adequate lighting should therefore be provided at all roundabouts.

Exhibit 7-22 shows an example of an illuminated roundabout at night.

Exhibit 7-22. Illumination of

a roundabout.

Loveland, CO
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 203

Continuity of illumination must be provided between illuminated areas and the

roundabout itself (5). An unlit roundabout with one or more illuminated approaches

is dangerous. This is because a driver approaching on an unlit approach will be

attracted to the illuminated area(s) and may not see the roundabout.

7.3.1.3 Rural conditions

For rural roundabouts, illumination is recommended but not mandatory. If there is

no power supply in the vicinity of the intersection, the provision of illumination can

be costly. When lighting is not provided, the intersection should be well signed and

marked so that it can be correctly perceived by day and night. The use of reflective

pavement markers and retroreflective signs (including chevrons supplementing the ONE-WAY signs) should be used when lighting cannot be installed in a

cost-effective manner.

Where illumination can be provided, any raised channelization or curbing should be

illuminated. In general, a gradual illumination transition zone of approximately 80 m

(260 ft) should be provided beyond the final trajectory changes at each exit (5). This

helps drivers adapt their vision from the illuminated environment of the roundabout back into the dark environment of the exiting roadway, which takes approximately 1 to 2 seconds. In addition, no short-distance dark areas should be allowed

between two consecutive illuminated areas (5).

7.3.2 Standards and recommended practices

The following standards and recommended practices should be consulted in completing the lighting plan:

• AASHTO, An Information Guide for Roadway Lighting (6). This is the basic guide

for highway lighting. It includes information on warranting conditions and design criteria.

• AASHTO, Standard Specifications for Structural Supports for Highway Signs,

Luminaires and Traffic Signals (7). This specification contains the strength requirements of the poles and bracket arms for various wind loads, as well as the

frangibility requirements. All luminaire supports, poles, and bracket arms must

comply with these specifications.

• IES RP-8: The American National Standard Practice for Roadway Lighting (8).

This Recommended Practice, published by the Illuminating Engineering Society, provides standards for average-maintained illuminance, luminance, and small

target visibility, as well as uniformity of lighting. Recommended illumination

levels for streets with various classifications and in various areas are given in

Exhibit 7-23.
204 Federal Highway Administration

Arterial Commercial 17 lx (1.7 fc) 3 to 1

Intermediate 13 lx (1.3 fc)

Residential 9 lx (0.9 fc)

Collector Commercial 12 lx (1.2 fc) 4 to 1

Intermediate 9 lx (0.9 fc)

Residential 6 lx (0.6 fc)

Local Commercial 9 lx (0.9 fc) 6 to 1

Intermediate 7 lx (0.7 fc)

Residential 4 lx (0.4 fc)

Exhibit 7-23. Recommended

street illumination levels.

Definitions:

Commercial A business area of a municipality where ordinarily there are many pedestrians during

night hours. This definition applies to densely developed business areas outside, as well

as within, the central part of a municipality. The area contains land use which attracts a

relatively heavy volume of nighttime vehicular and/or pedestrian traffic on a frequent

basis.

Intermediate Those areas of a municipality often with moderately heavy nighttime pedestrian activity

such as in blocks having libraries, community recreation centers, large apartment buildings, industrial buildings, or neighborhood retail stores.

Residential A residential development, or a mixture of residential and small commercial establishments, with few pedestrians at night.

Note: Values in table assume typical asphalt roadway surface (pavement classification R2 or R3). Consult

the IES document for other pavement surfaces.

Source: Illuminating Engineering Society RP-8 (8)

Street

Classification

Area

Classification

Average

Maintained

Illuminance

Values

Illuminance

Uniformity Ratio

(Average to

Minimum)

7.3.3 General recommendations

The primary goal of illumination is to ensure perception of the approach and mutual

visibility among the various categories of users. To achieve this, the following features are recommended:

• The overall illumination of the roundabout should be approximately equal to the

sum of the illumination levels of the intersecting roadways. If the approaching

roadways have been designed to the illumination levels given in Exhibit 7-23,

this may result in illumination levels at the roundabout ranging from 9 lx (0.8 fc)

for roundabouts at the intersection of local streets in residential areas to 36 lx

(3.4 fc) for roundabouts at the intersection of arterials in commercial areas.

Local illumination standards should also be considered when establishing the

illumination at the roundabout to ensure that the lighting is consistent.

• Good illumination should be provided on the approach nose of the splitter islands, at all conflict areas where traffic is entering the circulating stream, and at

all places where the traffic streams separate to exit the roundabout.

• It is preferable to light the roundabout from the outside in towards the center.

This improves the visibility of the central island and the visibility of circulating

vehicles to vehicles approaching to the roundabout. Ground-level lighting within

the central island that shines upwards towards objects in the central island can

improve their visibility.

Lighting from the central island

causes vehicles to be backlit and

thus less visible.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 205

• Special consideration should be given to lighting pedestrian crossing and bicycle merging areas.

7.3.4 Clear zone requirements

As discussed in Chapter 5, the proportion of single-vehicle crashes at roundabouts

is high compared to other intersection types. This is because roundabouts consist

of a number of relatively small-radii horizontal curves for each traveled path through

the roundabout. Drivers travel on these curves with quite high values of side friction, particularly at roundabouts in higher speed areas. Single-vehicle crashes, which

predominantly involve out-of-control vehicles, increase with an increased amount

of side friction.

Because of the relatively high number of out-of-control vehicles, it is desirable to

have adequate amounts of clear zone where there are no roadside hazards on each

side of the roadway. Lighting supports and other poles should not be placed within

small splitter islands or on the right-hand perimeter just downstream of an exit

point. Lighting poles should be avoided in central islands when the island diameter

is less than 20 m (65 ft).

The reader should refer to the AASHTO Roadside Design Guide for a more detailed

discussion of clear zone requirements (9).

7.4 Work Zone Traffic Control

During the construction of a roundabout it is essential that the intended travel path

be clearly identified. This may be accomplished through pavement markings, signing, delineation, channelizing devices, and guidance from police and/or construction personnel, depending on the size and complexity of the roundabout. Care

should be taken to minimize the channelizing devices so that the motorist, bicyclist, and pedestrian has a clear indication of the required travel path. Each installation should be evaluated separately, as a definitive guideline for the installation of

roundabouts is beyond the scope of this guide. Refer to Part 6 of the MUTCD for

requirements regarding work zone traffic control.

7.4.1 Pavement markings

The pavement markings used in work zones should be the same layout and dimension as those used for the final installation. Because of the confusion of a work

area and the change in traffic patterns, additional pavement markings may be used

to clearly show the intended direction of travel. In some cases when pavement

markings cannot be placed, channelizing devices should be used to establish the

travel path.

7.4.2 Signing

The signing in work zones should consist of all necessary signing for the efficient

movement of traffic through the work area, preconstruction signing advising the pubConstruction signing for a

roundabout should follow the

MUTCD standard.
206 Federal Highway Administration

lic of the planned construction, and any regulatory and warning signs necessary for

the movement of traffic outside of the immediate work area. The permanent roundabout signing should be installed where practicable during the first construction

stage so that it is available when the roundabout is operable. Permanent signing

that cannot be installed initially should be placed on temporary supports in the

proposed location until permanent installation can be completed.

7.4.3 Lighting

Permanent lighting, as described in Section 7.3, should be used to light the work

area. If lighting will not be used, pavement markings, as described in Section 7.2,

should be used.

7.4.4 Construction staging

As is the case with any construction project, before any work can begin, all traffic

control devices should be installed as indicated in the traffic control plan or recommended typical details. This traffic control shall remain in place as long as it applies

and then be removed when the message no longer applies to the condition.

Prior to work that would change the traffic patterns to that of a roundabout, certain

peripheral items may be completed. This would include permanent signing (covered), lighting, and some pavement markings. These items, if installed prior to the

construction of the central island and splitter islands, would expedite the opening

of the roundabout and provide additional safety during construction.

When work has commenced on the installation of the roundabout, it is desirable

that it be completed as soon as possible to minimize the time the public is faced

with an unfinished layout or where the traffic priority may not be obvious. If possible, all work, including the installation of splitter islands and striping, should be

done before the roundabout is open to traffic.

If it is necessary to leave a roundabout in an uncompleted state overnight, the

splitter islands should be constructed before the central island. Any portion of the

roundabout that is not completed should be marked, delineated, and signed in

such a way as to clearly outline the intended travel path. Pavement markings that

do not conform to the intended travel path should be removed.

It is highly desirable to detour traffic for construction of a roundabout. This will

significantly reduce the construction time and cost and will increase the safety of

the construction personnel. If it is not possible to detour all approaches, detour as

many approaches as possible and stage the remainder of the construction as follows:

1. Install and cover proposed signing.

2. Construct outside widening if applicable.

3. Reconstruct approaches if applicable.

Construction staging should be

considered during the siting of

the roundabout, especially if it

must be built under traffic.
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 207

Public education during

construction is as important as

the public education effort

during the planning process.

4. Construct splitter islands and delineate the central island. At this point the signs

should be uncovered and the intersection should operate as a roundabout.

5. Finish construction of the central island.

7.4.5 Public education

It is important to educate the public whenever there is a change in traffic patterns.

It is especially important for a roundabout because a roundabout will be new to

most motorists. The techniques discussed in Chapter 2 can be applied during the

construction period. The following are some specific suggestions to help alleviate

initial driver confusion.

• Hold public meetings prior to construction;

• Prepare news releases/handouts detailing what the motorist can expect before, during, and after construction;

• Install variable message signs before and during construction;

• Use Travelers Advisory Radio immediately prior to and during construction to

disseminate information on “How to drive,” etc.; and

• Install signing during and after construction that warns of changed traffic patterns.

7.5 Landscaping

This section provides an overview of the use of landscaping in the design of a

roundabout.

7.5.1 Advantages

Landscaping in the central island, in splitter islands (where appropriate), and along

the approaches can benefit both public safety and community enhancement.

The landscaping of the roundabout and approaches should:

• Make the central island more conspicuous;

• Improve the aesthetics of the area while complementing surrounding

streetscapes as much as possible;

• Minimize introducing hazards to the intersection, such as trees, poles, walls,

guide rail, statues, or large rocks;

• Avoid obscuring the form of the roundabout or the signing to the driver;

• Maintain adequate sight distances, as discussed in Chapter 6;

• Clearly indicate to the driver that they cannot pass straight through the intersection;

• Discourage pedestrian traffic through the central island; and

• Help blind and visually impaired pedestrians locate sidewalks and crosswalks.

Landscaping is one of the

distinguishing features that

gives roundabouts an aesthetic

advantage over traditional

intersections.
208 Federal Highway Administration

7.5.2 Central island landscaping

The central island landscaping can enhance the safety of the intersection by making the intersection a focal point and by lowering speeds. Plant material should be

selected so that sight distance (discussed in Chapter 6) is maintained, including

consideration of future maintenance requirements to ensure adequate sight distance for the life of the project. Large, fixed landscaping (trees, rocks, etc.) should

be avoided in areas vulnerable to vehicle runoff. In northern areas, the salt tolerance of any plant material should be considered, as well as snow storage and

removal practices. In addition, landscaping that requires watering may increase

the likelihood of wet and potentially slippery pavement. Exhibit 7-24 shows the

recommended placement of landscaping within the central island.

The slope of the central island should not exceed 6:1 per the requirements of the

AASHTO Roadside Design Guide (9).

Where truck aprons are used in conjunction with a streetscape project, the pavement should be consistent with other streetscape elements. However, the material used for the apron should be different than the material used for the sidewalks

so that pedestrians are not encouraged to cross the circulatory roadway. Street

furniture that may attract pedestrian traffic to the central island, such as benches or

monuments with small text, must be avoided. If fountains or monuments are being considered for the central island, they must be designed in a way that will

enable proper viewing from the perimeter of the roundabout. In addition, they must

be located and designed to minimize the possibility of impact from an errant vehicle.

Avoid items in the central island

that might tempt people to take

a closer look.

Exhibit 7-24. Landscaping of

the central island.

7.5.3 Splitter island and approach landscaping

In general, unless the splitter islands are very large or long, they should not contain

trees, planters, or light poles. Care must be taken with the landscaping to avoid

obstructing sight distance, as the splitter islands are usually located within the

critical sight triangles (see Chapter 6).
Roundabouts: An Informational Guide • 7: Traffic Design and Landscaping 209

Landscaping on the approaches to the roundabout can enhance safety by making

the intersection a focal point and by reducing the perception of a high-speed through

traffic movement. Plant material in the splitter islands (where appropriate) and on

the right and left side of the approaches can help to create a funneling effect and

induce a decrease in speeds approaching the roundabout. Landscaping in the corner radii will help to channelize pedestrians to the crosswalk areas and discourage

pedestrians from crossing to the central island.

7.5.4 Maintenance

A realistic maintenance program should be considered in the design of the landscape features of a roundabout. It may be unrealistic to expect a typical highway

agency to maintain a complex planting plan. Formal agreements may be struck

with local civic groups and garden clubs for maintenance where possible. Liability

issues should be considered in writing these agreements. Where there is no interest in maintaining the proposed enhancements, the landscape design should consist of simple plant materials or hardscape items that require little or no maintenance.

7.6 References

1. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. Washington, D.C.: FHWA, 1988.

2. Federal Highway Administration (FHWA). Standard Highway Signs. Washington, D.C.: FHWA, 1979.

3. Smith, S.A., and R.L. Knoblauch. “Guidelines for the Installation of Crosswalk

Markings.” In Transportation Research Record 1141. Transportation Research

Board, National Research Council, Washington, D.C., 1987.

4. Herms, B.F. “Some Visual Aspects of Pedestrian Crosswalks.” In Proceedings,

22nd California Street and Highway Conference, Institute of Transportation and

Traffic Engineering, University of California, Los Angeles, January 1970.

5. Centre d’Etudes sur les Réseaux les Transports, l’Urbanisme et les constructions publiques (CERTU). L’Éclairage des Carrefours à Sens Giratoire (The Illumination of Roundabout Intersections). Lyon, France: CERTU, 1991.

6. American Association of State Highway and Transportation Officials (AASHTO).

An Information Guide for Roadway Lighting. Washington, D.C.: AASHTO, 1985.

7. American Association of State Highway and Transportation Officials (AASHTO).

Standard Specifications for Structural Supports for Highway Signs, Luminaires

and Traffic Signals. Washington, D.C.: AASHTO, 1994.

8. Illuminating Engineering Society (IES). American National Standard Practice for

Roadway Lighting. Standard RP-8. December 1982.

9. American Association of State Highway and Transportation Officials (AASHTO).

Roadside Design Guide. Washington, D.C.: AASHTO, 1989.

Ensure that whatever

landscaping is installed, it will

be maintained.
8 System Considerations

8.1 Traffic Signals at Roundabouts 213

8.1.1 Metered entrance 214

8.1.2 Nearby vehicular and pedestrian signals 214

8.1.3 Full signalization of the circulatory roadway 215

8.2 At-Grade Rail Crossings 215

8.3 Closely Spaced Roundabouts 217

8.4 Roundabout Interchanges 219

8.4.1 Two-bridge roundabout interchange 219

8.4.2 One-bridge roundabout interchange 220

8.4.3 Analysis of roundabout interchanges 222

8.4.4 Geometric design parameters 223

8.5 Roundabouts in an Arterial Network 223

8.5.1 Platooned arrivals on roundabout approaches 224

8.5.2 Roundabout departure pattern 224

8.5.3 Wide nodes and narrow roads 225

8.6 Microscopic Simulation 227

8.6.1 How to use simulation 227

8.6.2 Examples of simulation models 228

8.7 References 229
212 Federal Highway Administration

Exhibit 8-1. Rail crossing treatments at roundabouts. 216

Exhibit 8-2. Methods for accommodating a rail crossing

 adjacent to a roundabout. 217

Exhibit 8-3. Example of closely spaced offset T-intersections

with roundabouts. 218

Exhibit 8-4. Through bypass lanes at staggered T-intersections. 218

Exhibit 8-5. Two-bridge roundabout interchange. 219

Exhibit 8-6. Examples of two-bridge roundabout interchanges. 220

Exhibit 8-7. Examples of one-bridge roundabout interchanges

 with circular central islands. 221

Exhibit 8-8. One-bridge roundabout interchange with

raindrop-shaped central islands. 222

Exhibit 8-9. Roundabouts in an arterial network. 223

Exhibit 8-10. Wide nodes and narrow roads. 226

Exhibit 8-11. Summary of simulation models for roundabout

 analysis. 228
Roundabouts: An Informational Guide • 8: System Considerations 213

Roundabouts have been considered as isolated intersections in most other international roundabout guides and publications. However, roundabouts may need to

fit into a network of intersections, with the traffic control functions of a roundabout

supporting the function of nearby intersections and vice versa. The purpose of this

chapter is to provide some guidance on potentially difficult, but not uncommon,

circumstances or constraints.

Many countries whose initial design and driver experience was with isolated roundabouts have since extended their application to transportation system design and

operation. This chapter addresses the appropriate use of roundabouts in a roadway

network context and the benefits obtained. Since the design of each roundabout

should generally follow the principles of isolated roundabout design, the discussion is at a conceptual and operational level and generally complements the planning of isolated roundabouts discussed in Chapter 3. In many cases, site-specific

issues will determine the appropriate roundabout design elements.

To establish some fundamental understanding for subsequent discussion, three

design issues at an isolated roundabout are presented. First, this chapter will describe the requirements and effects of signal control of one or more legs of a

roundabout, as well as the entire roundabout. It is noted that fully signalized roundabouts are not desirable. Next, modified designs that incorporate at-grade rail crossings are discussed. It is noted that intersections with rail lines passing through

them or near them are not desirable. However, these situations do occur and would

then need to be analyzed.

Building upon this understanding, the next sections address design and performance of two closely spaced roundabouts and the specific application to roundabout interchanges. This is followed by issues pertaining to the use of roundabouts

on an arterial or network that may include or replace coordinated signalized intersections. Finally, the role of microscopic simulation models in assisting with analysis of these system effects is reviewed.

8.1 Traffic Signals at Roundabouts

Although yield control of entries is the default at roundabouts, when necessary,

traffic circles and roundabouts have been signalized by metering one or more entries, or signalizing the circulatory roadway at each entry. Roundabouts should never

be planned for metering or signalization. However, unexpected demand may dictate the need after installation. Each of these will be discussed in turn. In the first

case, entrance metering can be implemented at the entrance or some distance

upstream.

This chapter considers

roundabouts as they relate to

other elements of the

transportation system,

including other intersections.

Chapter 8 System Considerations
214 Federal Highway Administration

8.1.1 Metered entrance

Roundabouts operate effectively only when there are sufficient longer and acceptable gaps between vehicles in the circulatory lanes. If there is a heavy movement

of circulating drivers, then entering drivers at the next downstream entry may not

be able to enter. This situation occurs most commonly during the peak periods, and

the performance of the roundabout can be greatly improved with entrance metering.

The concept of entrance metering at roundabouts is similar to ramp metering on

freeways. A convenient sign is a changeable one that reads “Stop on red signal”

and shows the usual yield sign for a roundabout otherwise. The sign would also

include a yellow and red signal above the sign. The operation of the sign would be

to show drivers the roundabout sign, display the yellow light and the sign “Stop on

red signal,” and finally display the red light and the same text sign. This would

cause entering vehicles to stop and allow the vehicles at the downstream entrance

to proceed. A queue length detector on the downstream entrance may be used to

indicate to the signal controller when the metering should be activated and deactivated. Once on the circulatory roadway, vehicles are not stopped from leaving the

roundabout.

8.1.2 Nearby vehicular and pedestrian signals

Another method of metering is the use, with appropriate timing, of a nearby upstream signalized intersection or a signalized pedestrian crossing on the subject

approach road. Unlike pure entry metering, such controls may stop vehicles from

entering and leaving the roundabout, so expected queue lengths on the roundabout exits between the metering signal and the circulatory roadway should be

compared with the proposed queuing space.

Because of additional objectives and constraints, metering by upstream signals is

generally not as effective as direct entrance metering. However, a signalized pedestrian crossing may be desirable on its own merits. More than one entrance can

be metered, and the analyst needs to identify operational states and evaluate each

one separately to provide a weighted aggregate performance measure.

When disabled pedestrians and/or school children are present at a high-volume

site, a pedestrian-actuated traffic signal could be placed 20 to 50 m (65 to 165 ft)

from the yield line. This longer distance than at an unsignalized crossing may be

required because the vehicle queues downstream of the roundabout exit will be

longer. The trade-offs for any increased distance requirement are increased walking distances and higher exiting vehicle speeds. An analysis of signal timing will be

needed to minimize queuing of vehicles into the roundabouts.

Roundabouts should not be

planned for metering or

signalization unless

 unexpected demand dictates

this need after installation.

Nearby intersections or

pedestrian crossing signals can

also meter traffic, but not as

effectively as direct

 entrance metering.
Roundabouts: An Informational Guide • 8: System Considerations 215

Full signalization of the

circulatory roadway requires

careful coordination and

vehicle progression.

8.1.3 Full signalization of the circulatory roadway

Full signalization that includes control of circulating traffic at junctions with major

entrances is possible at large-diameter multilane traffic circles or rotaries that have

adequate storage space on the circulatory roadway. The double-lane roundabout

dimensions resulting from the design criteria recommended in this guide may preclude such possibilities. As stated previously, full signalization should in any case

only be considered as a retrofit alternative resulting from unanticipated traffic demands. Other feasible alternatives should also be considered, such as flaring critical approaches, along with the associated widening of the circulatory roadway;

converting a large-diameter rotary to a more compact modern roundabout form; or

converting to a conventional signalized intersection. This guide recommends that

signalizing roundabouts to improve capacity be considered only when it is the most

cost-effective solution.

Traffic signals at fully signalized rotaries should be timed carefully to prevent queuing on the circulatory roadway by ensuring adequate traffic progression of circulating traffic and especially critical movements. Introducing continuous or part-time

signals on the circulatory roadway requires careful design of geometry, signs, lane

markings, and signal timing settings, and literature on this specific topic should be

consulted (1, 2).

8.2 At-Grade Rail Crossings

Locating any intersection near an at-grade railroad crossing is generally discouraged. However, roundabouts are sometimes used near railroad-highway at-grade

crossings. Rail transit, including stations, have successfully been incorporated into

the medians of approach roadways to a roundabout, with the tracks passing through

the central island. In such situations, the roundabout either operates partially during train passage, or is completely closed to allow the guided vehicles or trains to

pass through. The treatment of at-grade rail crossings should follow primarily the

recommendations of the Manual on Uniform Traffic Control Devices (MUTCD) (3).

Another relevant reference is the FHWA Railroad-Highway Grade Crossing Handbook (4).

There are essentially two ways in which rails can interact with a roundabout, as

shown in Exhibit 8-1:

• Through the center; or

• Across one leg in close proximity to the roundabout.
216 Federal Highway Administration

In either case, traffic must not be forced to stop on the tracks. A new intersection

should not be designed with railroad tracks passing through the center of it. However, on occasions, the rail line passes through an existing intersection area. The

traffic engineer might be faced with a decision whether to change the intersection

type to a roundabout or to grade-separate the crossing.

A gated rail crossing through the center of a roundabout can be accommodated in

two ways. The first method is to prevent all vehicular traffic from entering the roundabout. The second method is to prevent traffic from crossing the tracks while still

allowing some movements to occur. This latter method will have lower delays and

queues, but it may be more confusing and less safe.

A gated rail crossing adjacent to a roundabout can be accommodated in two ways,

as shown in Exhibit 8-2:

• Method A: Closure only at rail crossing. This method prohibits vehicles from

crossing the rails but still allows vehicles to enter and leave the circulatory roadway. This method allows for many of the movements through the roundabout to

continue to run free, if a queue does not build to the point of impeding circulation within the roundabout. A queuing analysis should be performed using the

expected volume crossing the rails and the expected duration of rail crossing to

determine the likelihood that this blockage will occur. In general, this method

works better than Method B if there is sufficient separation between the roundabout and the rail crossing. If blockage is anticipated, the designer should choose

Method B.

• Method B: Closure at rail crossing and at most entries to the roundabout. This

method closes all entries to the roundabout except for the entry nearest the rail

crossing. This allows any vehicles in the roundabout to clear prior to the arrival

of the train. In addition, a gate needs to be provided on the approach to the rail

crossing exiting the roundabout to protect against possible U-turns in the roundabout. This causes increased queuing on all approaches but is generally safer

than Method A when there is insufficient storage capacity between the roundabout and rail crossing.

Closing only the leg with the

rail crossing may work if

queues are not anticipated to

back into the

 circulatory roadway.

Exhibit 8-1. Rail crossing

treatments at roundabouts.
Roundabouts: An Informational Guide • 8: System Considerations 217

(b) Closure at rail crossing and at most entries to roundabout

(a) Closure only at rail crossing

8.3 Closely Spaced Roundabouts

It is sometimes desirable to consider the operation of two or more roundabouts in

close proximity to each other. In these cases, the expected queue lengths at each

roundabout become important. Exhibit 8-3 presents an example of closely spaced

T-intersections. The designer should compute the 95th-percentile queues for each

approach to check that sufficient queuing space is provided for vehicles between

the roundabouts. If there is insufficient space, then drivers will occasionally queue

into the upstream roundabout and may cause it to lock.

Exhibit 8-2. Methods for

accommodating a rail crossing

adjacent to a roundabout.
218 Federal Highway Administration

France (5)

Closely spaced roundabouts may improve safety by “calming” the traffic on the

major road. Drivers may be reluctant to accelerate to the expected speed on the

arterial if they are also required to slow again for the next close roundabout. This

may benefit nearby residents.

When roundabouts are used at offset T-intersections, there is an opportunity to

bypass one through lane direction on the major road at each roundabout. Exhibit

8-4 presents sketches of through bypass lanes for the two basic types of offset

T-intersection configurations. In both cases, through traffic in each direction needs

to negotiate only one roundabout, and capacity is therefore typically improved. The

weaving section should be analyzed both for capacity and for safety through an

evaluation of the relative speeds of the weaving vehicles.

Exhibit 8-4. Through

bypass lanes at staggered

T-intersections.

Exhibit 8-3. Example of

closely spaced offset

T-intersection with

roundabouts.

Closely spaced roundabouts

may have a traffic calming

effect on the major road.

Option A (roundabout precedes

bypass) is preferred.
Roundabouts: An Informational Guide • 8: System Considerations 219

Of the two arrangements shown in Exhibit 8-4, Option A (roundabout precedes

bypass) is preferred. The roundabout offers a visual cue to drivers to slow in Arrangement A and encourages slower (and therefore safer) driving through the two

roundabouts. If Option B (bypass precedes roundabout) is used, the merges and

diverges could occur at higher speeds. It may be appropriate in this case to omit

the bypass lane and pass all through traffic through both roundabouts. Another

advantage of Option A is that there would be less queuing of traffic on the road

space between the roundabouts.

Note that when conventional T-intersections are used, Option A is less preferable

than Option B due to the need to provide interior storage space for left turns in

Option A. Therefore, roundabouts may be a satisfactory solution for cases like

Option A.

8.4 Roundabout Interchanges

Freeway ramp junctions with arterial roads are potential candidates for roundabout

intersection treatment. This is especially so if the subject interchange typically has

a high proportion of left-turn flows from the off-ramps and to the on-ramps during

certain peak periods, combined with limited queue storage space on the bridge

crossing, off-ramps, or arterial approaches. In such circumstances, roundabouts

operating within their capacity are particularly amenable to solving these problems

when compared with other forms of intersection control.

8.4.1 Two-bridge roundabout interchange

There are two basic types of roundabout interchanges. The first is a large diameter

roundabout centered over or under a freeway. The ramps connect directly into the

roundabout, as do the legs from the crossroad. This is shown in Exhibit 8-5.

Exhibit 8-5. Two-bridge

roundabout interchange.

Source: Based on (6)

The freeway may go either

over or under the circulatory

roadway.
220 Federal Highway Administration

This type of interchange requires two bridges. If the roundabout is above the freeway as shown in Exhibit 8-5, then the bridges may be curved. Alternatively, if the

freeway goes over the roundabout then up to four bridges may be required. The

number of bridges will depend on the optimum span of the type of structure compared with the inscribed diameter of the roundabout island and on whether the

one bridge is used for both freeway directions or whether there is one bridge for

each direction. The road cross-section will also influence the design decision. Exhibit 8-6 shows an example from the United Kingdom. The designer should decide

if the expected speeds of vehicles at larger roundabouts are acceptable.

Exhibit 8-6. Examples of

 two-bridge roundabout

interchanges.

A50/Heron Cross, United Kingdom (mirrored to show right-hand-side driving)

8.4.2 One-bridge roundabout interchange

The second basic type uses a roundabout at each side of the freeway and is a

specific application of closely spaced roundabouts discussed in the previous section. A bridge is used for the crossroad over the freeway or for a freeway to cross

over the minor road. Again, two bridges may be used when the freeway crosses

over the minor road.

This interchange form has been used successfully in some cases to defer the need

to widen bridges. Unlike signalized ramps that may require exclusive left-turn lanes

across the bridge and extra queue storage, this type of roundabout interchange

exhibits very little queuing between the intersections since these movements are

almost unopposed. Therefore, the approach lanes across the bridge can be minimized.

The actual roundabouts can have two different shapes or configurations. The first

configuration is a conventional one with circular central islands. This type of configuration is recommended when it is desirable to allow U-turns at each roundabout or to provide access to legs other than the cross street and ramps. Examples

from the United Kingdom and France are shown in Exhibit 8-7.

One-bridge roundabout

interchanges have been

successfully used to defer the

need for bridge widening.
Roundabouts: An Informational Guide • 8: System Considerations 221

Exhibit 8-7 (continued).

Examples of one-bridge

roundabout interchanges with

circular central islands.

France

Exhibit 8-7. Examples of

one-bridge roundabout

interchanges with circular

central islands.

United Kingdom
222 Federal Highway Administration

8.4.3 Analysis of roundabout interchanges

The traffic performance evaluation of the roundabout interchange is the same as

for a single conventional roundabout. The maximum entry capacity is dependent

on the circulatory flow and the geometry of the roundabouts. The evaluation process is included in Chapter 4.

The benefits and costs associated with this type of interchange also follow those

for a single roundabout. A potential benefit of roundabout interchanges is that the

queue length on the off-ramps may be less than at a signalized intersection. In

almost all cases, if the roundabout would operate below capacity, the performance

of the on-ramp is likely to be better than if the interchange is signalized. The headway between vehicles leaving the roundabout along the on-ramp is more random

than when signalized intersections are used. This more random ramp traffic allows

for smoother merging behavior on the freeway and a slightly higher performance

at the freeway merge area compared with platooned ramp traffic from a signalized

intersection.

The second configuration uses raindrop-shaped central islands that preclude some

turns at the roundabout. This configuration is best used when ramps (and not frontage roads) intersect at the roundabout. A raindrop central island can be considered

to be a circular shape blocked at one end. In this configuration, a driver wanting to

make a U-turn has to drive around both raindrop-shaped central islands. This configuration has an additional advantage in that it makes wrong-way turns into the

off-ramps more difficult. On the other hand, drivers do not have to yield when

approaching from the connecting roadway between the two roundabouts. If the

roundabout is designed poorly, drivers may be traveling faster than they should to

negotiate the next roundabout safely. The designer should analyze relative speeds

to evaluate this alternative. On balance, if the length of the connecting road is

short, this design may offer safety advantages. Exhibit 8-8 provides an example of

this type of interchange configuration.

Interstate 70/Avon Road, Avon, CO

Raindrop central islands make

wrong-way movements more

difficult, but require navigating

two roundabouts to

make a U-turn.

Roundabouts produce more

random headways on ramps

than signalized intersections,

resulting in smoother merging

behavior on the freeway.

Exhibit 8-8. One-bridge

roundabout interchange with

raindrop-shaped

 central islands.
Roundabouts: An Informational Guide • 8: System Considerations 223

The traffic at any entry is the same for both configurations. The entry capacity is

the same and the circulating flow is the same for the large single roundabout (Exhibit 8-6) and for the second configuration of the two teardrop roundabout system

(Exhibit 8-8). Note that the raindrop form may be considered and analyzed as a

single large roundabout as in the circular roundabout interchange, but with a

“pinched” waistline across or under one bridge rather than two. The relative performance of these systems will only be affected by the geometry of the roundabouts

and islands. The system with the two circular roundabouts will have a slightly different performance depending upon the number of U-turns.

8.4.4 Geometric design parameters

The design parameters are not restrained by any requirement here. They are only

constrained by the physical space available to the designer and the configuration

selected. The raindrop form can be useful if grades are a design issue since they

remove a potential cross-slope constraint on the missing circulatory road segments.

If there are more roads intersecting with the interchange than the single cross

road, then two independent circular roundabouts are likely to be the best solution.

8.5 Roundabouts in an Arterial Network

In order to understand how roundabouts operate within a roadway system, it is

important to understand their fundamental arrival and departure characteristics

and how they may interact with other intersections. Exhibit 8-9 gives an example

of a series of roundabouts along an arterial street.

Exhibit 8-9. Roundabouts in

an arterial network.

Avon Road, Avon, CO

The Avon Road network

consists of five roundabouts

(all pictured)—two at the

interchange ramp terminals

and three along the arterial

south of the freeway.
224 Federal Highway Administration

8.5.1 Platooned arrivals on roundabout approaches

The performance of a roundabout is affected by its proximity to signalized intersections. If a signalized intersection is very close to the roundabout, it causes vehicles

to enter the roundabout in closely spaced platoons; more importantly, it results in

regular periods when no vehicles enter. These latter periods provide an excellent

opportunity for traffic on the next downstream entry to enter. Since the critical gap

is larger than the follow-up time, a roundabout becomes more efficient when the

vehicles are handled as packets of vehicles rather than as isolated vehicles.

When the signalized intersection is some distance from the roundabout, then the

vehicles’ arrival patterns have fewer closely spaced platoons. Platoons tend to disperse as they move down the road. The performance of a roundabout will be reduced under these circumstances when compared with a close upstream signal. If

arrival speeds are moderate, then few longer gaps allow more drivers to enter a

roundabout than a larger number of shorter gaps. If arrival speeds are low, then

there are more opportunities for priority-sharing (where entering and circulating

vehicles alternate) and priority-reversals (where the circulating vehicles tend to

yield to entering vehicles) between entering and circulating traffic streams, and the

influence of platoon dispersal is not as marked.

8.5.2 Roundabout departure pattern

Traffic leaving a roundabout tends to be more random than if another type of intersection control were used. A roundabout may therefore affect the performance of

Signalized intersections close

to roundabouts produce gaps

in traffic that can be used by

minor street traffic to enter the

major street.

Even one circulating vehicle in

a roundabout will result in a

platoon breaking down.

other unsignalized intersections or driveways more than if the intersection was

signalized. However, as this traffic travels further along the road downstream of

the roundabout, the faster vehicles catch up to the slower vehicles and the proportion of platooning increases.

In the case of a well-defined platoon from an upstream signalized intersection

arriving at a downstream unsignalized intersection just after a well-defined platoon

arrives from the other direction, it may be difficult for the minor street drivers at

this unsignalized intersection to enter the link. If, on the other hand, one of these

signalized intersections were to be replaced by a roundabout, then the effect of the

random traffic from the roundabout might be relatively advantageous. Under these

conditions, more dispersed platoons (or random) traffic could assist drivers entering along the link at the unsignalized intersection.

If a roundabout is used in a network of coordinated signalized intersections, then it

may be difficult to maintain the closely packed platoons required. If a tightly packed

platoon approached a roundabout, it could proceed through the roundabout as long

as there was no circulating traffic or traffic upstream from the left. Only one circulating vehicle would result in the platoon breaking down. Hence, the use of roundabouts in a coordinated signalized network needs to be evaluated carefully. One

possibility for operating roundabouts within a signal network is to signalize the

major approaches of the roundabout and coordinate them with adjacent upstream

and downstream signalized intersections.
Roundabouts: An Informational Guide • 8: System Considerations 225

Another circumstance in which a roundabout may be advantageous is as an alternative to signal control at a critical signalized intersection within a coordinated network. Such intersections are the bottlenecks and usually determine the required

cycle length, or are placed at a signal system boundary to operate in isolated actuated mode to minimize their effect on the rest of the surrounding system. If a

roundabout can be designed to operate within its capacity, it may allow a lowering

of the system cycle length with resultant benefits to delays and queues at other

intersections.

Because roundabouts accommodate U-turns more easily than do signals, they may

also be useful as an access management tool. Left-turn exits from driveways onto

an arterial which may currently experience long delays and require two-stage leftturn movements could be replaced with a simpler right turn, followed by a U-turn at

the next roundabout.

8.5.3 Wide nodes and narrow roads

The ultimate manifestation of roundabouts in a system context is to use them in

lieu of signalized intersections. Some European cities such as Nantes, France, and

some Australian cities have implemented such a policy. It is generally recognized

that intersections (or nodes), not road segments (or links), are typically the bottlenecks in urban roadway networks. A focus on maximizing intersection capacity

rather than widening streets may therefore be appropriate. Efficient, signalized

intersections, however, usually require that exclusive turn lanes be provided, with

sufficient storage to avoid queue spillback into through lanes and adjacent intersections. In contrast, roundabouts may require more right-of-way at the nodes, but

this may be offset by not requiring as many basic lanes on the approaches, relative

to signalized arterials. This concept is demonstrated in Exhibit 8-10.

Analysis tools, such as those provided in Chapter 4, should be used to evaluate the

arterial or network. These may be supplemented by appropriate use of microscopic

simulation models as discussed next. Supplemental techniques to increase the

capacity of critical approaches may be considered if necessary, such as bypass

lanes, flaring of approaches and tapering of exits, and signalization of some roundabout approaches.

Roundabouts as an access

management tool.

Roundabouts may require

more right-of-way at

intersections, but may also

allow fewer lanes (and less

right-of-way) between

intersections.
226 Federal Highway Administration

Exhibit 8-10. Wide nodes and

narrow roads.
Roundabouts: An Informational Guide • 8: System Considerations 227

8.6 Microscopic Simulation

Microscopic simulation of traffic has become a valuable aid in assessing the system

performance of traffic flows on networks, as recognized by the Highway Capacity

Manual 2000 (7). Analysis of many of the treatments discussed in this chapter may

benefit from the use of appropriate simulation models used in conjunction with analytic models of isolated roundabouts discussed in Chapter 4. These effects include

more realistic modeling of arrival and departure profiles, time-varying traffic patterns,

measurement of delay, spatial extent and interaction of queues, fuel consumption,

emissions, and noise. However, the user must carefully select the appropriate models and calibrate the model for a particular use, either against field data, or other

validated analytic models. It would also be advisable to check with others to see if

there have been any problems associated with the use of the model.

8.6.1 How to use simulation

Microscopic simulation models are numerous and new ones are being developed,

while existing models are upgraded frequently. Each model may have particular

strengths and weaknesses. Therefore, when selecting a model, analysts should consider the following:

• Should a simulation model be used, or is an isolated analytic roundabout model

sufficient?

• What are the model input requirements, are they sufficient, and how can they be

provided or estimated?

• What outputs does the model provide in animated, graphical, or tabular form?

• What special features of the model are pertinent to the problem being addressed?

• Does the user manual for the simulation model specifically address modeling a

roundabout?

• How sensitive is the model to various geometric parameters?

• Is there literature on the validation of this model for evaluating roundabouts?

• Is there sufficient information available on the microscopic processes being used

by the model such as car following, gap acceptance, lane changing, or steering?

(The availability of animation can assist in exposing model logic.)

• Are relevant past project examples available?

When a simulation model is used, the analyst is advised to use the results to make

relative comparisons of the differences between results from changing conditions,

and not to conclude that the absolute values found from the model are equivalent to

field results. It is also advisable to perform a sensitivity analysis by changing selected

parameters over a range and comparing the results. If a particular parameter is found
228 Federal Highway Administration

to affect the outcomes significantly, then more attention should be paid to accurate representation and calibration of this parameter. Finally, the analyst should

check differences in results from using different random number seeds. If the differences are large, then the simulation time should be increased substantially.

8.6.2 Examples of simulation models

Five commercially available microscopic simulation models are CORSIM, Integration, Simtraffic, Paramics, and VISSIM. The first three are North American models;

Paramics is from Scotland, and VISSIM is from Germany. The following sections

present a brief overview of each model. Since software packages (and simulation

models in particular) are in constant development, the user is encouraged to consult the most current information available on each model.

Simulation results are best

used for relative comparisons,

rather than relying on absolute

values produced by the model.

Exhibit 8-11. Summary of

simulation models for

roundabout analysis.

Name Scope Notes (1999 versions)

CORSIM Urban streets, freeways FHWA has been investigating modifications that may be required for CORSIM to

adequately model controls such as stop and yield control at roundabouts through

gap acceptance logic. In this research, roundabouts have been coded as a circle

of four yield-controlled T-intersections. The effect of upstream signals on each

approach and their relative offsets has also been reported (8).

Integration Urban streets, freeways Integration has documented gap acceptance logic for permitted movements at

signal-, yield-, and stop-controlled intersections. As with CORSIM, Integration

requires coding a roundabout simply as a series of short links and nodes with

yield control on the entrances.

Simtraffic Urban streets Simtraffic is a simulation model closely tied to the signal timing software package

Synchro. Simtraffic has the capability to model unsignalized intersections and

thus may be suitable for modeling roundabouts. However, no publications to date

have demonstrated the accuracy of Simtraffic in modeling roundabout operations.

Paramics Urban streets, freeways Paramics has been used in the United Kingdom and internationally for a wide

range of simulation projects. It has been specifically compared with ARCADY in

evaluating roundabouts (9). The model has a coding feature to automatically code

a roundabout intersection at a generic node, which may then be edited. The

model has been used in the United Kingdom for a number of actual roundabout

evaluations. The model specifically employs a steering logic on the circulatory

roadway to track a vehicle from an entry vector to a target exit vector (10).

VISSIM Urban streets, transit networks VISSIM is widely used in Germany for modeling urban road and transit networks,

including roundabouts. Roundabout examples are provided with the software,

including explicit modeling of transit and pedestrians. Modeling a roundabout

requires detailed coding of link connectors, control, and gap acceptance

parameters (11).
Roundabouts: An Informational Guide • 8: System Considerations 229

8.7 References

1. Brown, M. TRL State of the Art Review—The Design of Roundabouts. London:

HMSO, 1995.

2. Hallworth, M.S. “Signalling Roundabouts.” In Traffic Engineering + Control, Vol.

33, No. 6, June 1992.

3. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control

Devices. Washington, D.C.: FHWA, 1988.

4. Federal Highway Administration. Railroad-Highway Grade Crossing Handbook,

2nd edition. Report number FHWA-TS-86-215, September 1986.

5. Centre D’Etudes sur les Réseaux, les Transports, l’Urbanisme, et les Constructions Publiques (CERTU) (Center for Studies on Transportation Networks, Urban

Planning, and Public Works). Carrefours Urbains (Urban Intersections) Guide.

Lyon, France: CERTU, January 1999.

6. Department of Transport (United Kingdom). Geometric Design of Roundabouts.

TD 16/93. September 1993.

7. Transportation Research Board. Highway Capacity Manual. Special Report 209.

Washington, D.C.: Transportation Research Board, National Research Council,

July 1999 (draft).

8. Courage, K.G. “Roundabout Modeling in CORSIM.” Presented at the Third International Symposium on Intersections without Traffic Signals, Portland, Oregon,

U.S.A., 1997.

9. Paramics, Ltd. “Comparison of Arcady and Paramics for Roundabout Flows.”

Version 0.3. August 23, 1996.

10.Duncan, G. “Paramics Technical Report: Car-Following, Lane-Changing and Junction Modelling.” Edinburgh, Scotland: Quadstone, Ltd., 1997.

11. Innovative Transportation Concepts, LLC. VISSIM—User Manual. Program Version 2.32–2.36. November 10, 1997.
Roundabouts: An Informational Guide • Glossary 231

Glossary

85th-percentile speed—a speed value obtained from a set of field-measured speeds where

only 15 percent of the observed speeds are greater (source: HCM 2000).

AADT—see average annual daily traffic.

AASHO—American Association of State Highway Officials. Predecessor to AASHTO.

AASHTO—American Association of State Highway and Transportation Officials.

accessible—describes a site, building, facility, or portion thereof that complies with the Americans with Disabilities Act Accessibility Guidelines (source: ADAAG).

accessible route—a continuous, unobstructed path connecting all accessible elements and

spaces of a building or facility. Exterior accessible routes may include parking access aisles,

curb ramps, crosswalks at vehicular ways, walks, ramps, and lifts (source: ADAAG).

accident—see crash.

ADA—Americans with Disabilities Act.

ADAAG—Americans with Disabilities Act Accessibility Guidelines.

all-way stop control—all approaches at the intersections have stop signs where all drivers

must come to a complete stop. The decision to proceed is based in part on the rules of the

road, which suggest that the driver on the right has the right-of-way, and also on the traffic

conditions of the other approaches (source: HCM 2000).

angle, entry—see entry angle.

approach—the portion of a roadway leading into a roundabout.

approach capacity—the capacity provided at the yield line during a specified period of time.

approach curvature—a series of progressively sharper curves used on an approach to slow

traffic to a safe speed prior to reaching the yield line.

approach road half-width—term used in the United Kingdom regression models. The approach half width is measured at a point in the approach upstream from any entry flare, from

the median line or median curb to the nearside curb along a line perpendicular to the curb. See

also approach width. (source: UK Geometric Design of Roundabouts)

approach speed—the posted or 85th-percentile speed on an approach prior to any geometric

or signing treatments designed to slow speeds.

approach width—the width of the roadway used by approaching traffic upstream of any

changes in width associated with the roundabout. The approach width is typically no more

than half the total roadway width.

apron—the mountable portion of the central island adjacent to the circulatory roadway. Used

in smaller roundabouts to accommodate the wheel tracking of large vehicles.

average annual daily traffic—the total volume passing a point or segment of a highway

facility in both directions for one year divided by the number of days in the year (source: HCM

2000).

average effective flare length—term used in the United Kingdom regression models. Defined by a geometric construct and is approximately equivalent to the length of flare that can

be effectively used by vehicles. (source: UK Geometric Design of Roundabouts)

AWSC—see all-way stop control.

back of queue—the distance between the yield line of a roundabout and the farthest reach of

an upstream queue, expressed as a number of vehicles. The vehicles previously stopped at

the front of the queue may be moving (adapted from HCM 2000).

A

B
232 Federal Highway Administration

benefit-cost analysis—a method of economic evaluation that uses the benefit-cost ratio as

the measure of effectiveness.

benefit-cost ratio—the difference in benefits between an alternative and the no-build scenario, divided by the difference in costs between the alternative and the no-build scenario.

See also incremental benefit-cost ratio.

bulb-out—see curb extension.

capacity—the maximum sustainable flow rate at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a specified

time period under a given roadway, geometric, traffic, environmental, and control conditions.

Usually expressed as vehicles per hour, passenger cars per hour, or persons per hour (source:

HCM 2000).

capacity, approach—see approach capacity.

capacity, roundabout—see roundabout capacity.

capital recovery factor—a factor that converts a present value cost into an annualized cost

over a period of n years using an assumed discount rate of i percent.

central island—the raised area in the center of a roundabout around which traffic circulates.

CFR—Code of Federal Regulations.

channelization—the separation or regulation of conflicting traffic movements into definite

paths of travel by traffic islands or pavement marking to facilitate the safe and orderly movements of both vehicles and pedestrians (source: 1994 AASHTO Green Book).

circle, inscribed—see inscribed circle.

circular intersection—an intersection that vehicles traverse by circulating around a central

island.

circulating flow—see circulating volume.

circulating path radius—the minimum radius on the fastest through path around the central

island.

circulating traffic—vehicles located on the circulatory roadway.

circulating volume—the total volume in a given period of time on the circulatory roadway

immediately prior to an entrance.

circulatory roadway—the curved path used by vehicles to travel in a counterclockwise fashion around the central island.

circulatory roadway width—the width between the outer edge of the circulatory roadway

and the central island, not including the width of any apron.

circulating speed—the speed vehicles travel at while on the circulatory roadway.

community enhancement roundabout—a roundabout used for aesthetic or community

enhancement reasons, rather than as a solution to traffic problems. When used, often located

in commercial and civic districts.

conflict point—a location where the paths of two vehicles, or a vehicle and a bicycle or

pedestrian, merge, diverge, cross, or queue behind each other.

conflict, crossing—see crossing conflict.

conflict, diverge—see diverge conflict.

conflict, merge—see merge conflict.

conflict, queuing—see queuing conflict.

conflicting flows—the two paths that merge, diverge, cross, or queue behind each other at

a conflict point.

control delay—delay experienced by vehicles at an intersection due to movements at slower

speeds and stops on approaches as vehicles move up in the queue.

C
Roundabouts: An Informational Guide • Glossary 233

crash—a collision between a vehicle and another vehicle, a pedestrian, a bicycle, or a fixed

object.

crash frequency—the average number of crashes at a location per period of time.

crash rate—the number of crashes at a location or on a roadway segment, divided by the

number of vehicles entering the location or by the length of the segment.

CRF—see capital recovery factor.

crossing conflict—the intersection of two traffic streams, including pedestrians. Crossing

conflicts are the most severe type of conflict.

curb extension—the construction of curbing such that the width of a street is reduced. Often

used to provide space for parking or a bus stop or to reduce pedestrian crossing distances.

curb ramp—a short ramp cutting through a curb or built up to it (source: ADAAG).

curvature, approach—see approach curvature.

D factor—the proportion of the two-way traffic assigned to the peak direction.

deflection—the change in trajectory of a vehicle imposed by geometric features of the roadway.

degree of saturation—see volume-to-capacity ratio.

delay—additional travel time experienced by a driver, passenger, or pedestrian beyond what

would reasonably be desired for a given trip.

delay, control—see control delay.

delay, geometric—see geometric delay.

demand flow—the number of vehicles or persons that would like to use a roadway facility

during a specified period of time.

departure width—the width of the roadway used by departing traffic downstream of any

changes in width associated with the roundabout. The departure width is typically no more

than half the total roadway width.

design user—any user (motorized or nonmotorized) that can be reasonably be anticipated to

use a facility.

design vehicle—the largest vehicle that can reasonably be anticipated to use a facility.

detectable warning surface—a standardized surface feature built in or applied to walking

surfaces or other elements to warn visually impaired people of hazards on a circulation path

(source: ADAAG).

diameter, inscribed circle—see inscribed circle diameter.

distance, set-back—see set-back distance.

diverge conflict—the separation of two traffic streams, typically the least severe of all conflicts.

double-lane roundabout—a roundabout that has at least one entry with two lanes, and a

circulatory roadway that can accommodate more than one vehicle traveling side-by-side.

downstream—the direction toward which traffic is flowing (source: HCM 2000).

entering traffic—vehicles located on a roundabout entrance.

entering volume—the total volume in a given period of time on an entrance to a roundabout.

entry angle—term used in the United Kingdom regression models. It serves as a geometric

proxy for the conflict angle between entering and circulating streams and is determined through

a geometric construct. (source: UK Geometric Design of Roundabouts)

entry flare—the widening of an approach to multiple lanes to provide additional capacity at

the yield line and storage.

entry flow—see entering volume.

D

E
234 Federal Highway Administration

entry path curvature—term used in the United Kingdom to describe a measure of the amount

of entry deflection to the right imposed on vehicles at the entry to a roundabout. (source: UK

Geometric Design of Roundabouts)

entry path radius—the minimum radius on the fastest through path prior to the yield line.

entry radius—the minimum radius of curvature of the outside curb at the entry.

entry speed—the speed a vehicle is traveling at as it crosses the yield line.

entry width—the width of the entry where it meets the inscribed circle, measured perpendicularly from the right edge of the entry to the intersection point of the left edge line and the

inscribed circle.

entry, perpendicular—see perpendicular entry.

exit path radius—the minimum radius on the fastest through path into the exit.

exit radius—the minimum radius of curvature of the outside curb at the exit.

exit width—the width of the exit where it meets the inscribed circle, measured perpendicularly from the right edge of the exit to the intersection point of the left edge line and the

inscribed circle.

exiting traffic—vehicles departing a roundabout by a particular exit.

extended splitter island—see splitter island, extended.

FHWA—Federal Highway Administration.

flare—see entry flare.

flare, entry—see entry flare.

flow, circulating—see circulating volume.

flow, demand—see demand flow.

flow, entry—see entry volume.

flows, conflicting—see conflicting flows.

geometric delay—the delay caused by the alignment of the lane or the path taken by the

vehicle on a roadway or through an intersection.

geometric design—a term used in this document to describe the design of horizontal and

vertical alignment and cross-sectional elements of a roadway.

give way—term used in the United Kingdom and Australia for yield.

“give way” rule—rule adopted in the United Kingdom in November 1966 which required that

all vehicles entering a roundabout give way, or yield, to circulating vehicles.

HCM—Highway Capacity Manual.

IES—Illuminating Engineers Society.

incremental benefit-cost ratio—the difference in benefits between two alternatives, divided

by the difference in costs between the two alternatives. See also benefit-cost ratio.

inscribed circle—the circle forming the outer edge of the circulatory roadway.

inscribed circle diameter—the basic parameter used to define the size of a roundabout,

measured between the outer edges of the circulatory roadway. It is the diameter of the largest circle that can be inscribed within the outline of the intersection.

interchange—a grade-separated junction of two roadways, where movement from one roadway to the other is provided for.

F

G

H

I
Roundabouts: An Informational Guide • Glossary 235

intersection—an at-grade junction of two or more roadways.

intersection sight distance—the distance required for a driver without the right-of-way to

perceive and react to the presence of conflicting vehicles.

island, central—see central island.

island, median—see splitter island.

island, separator—see splitter island.

island, splitter—see splitter island.

ITE—Institute of Transportation Engineers.

K factor—the proportion of the AADT assigned to the design hour.

left-turn path radius—the minimum radius on the fastest path of the conflicting left-turn

movement.

level of service—a qualitative measure describing operational conditions within a traffic stream,

generally described in terms of service measures such as speed and travel time, freedom to

maneuver, traffic interruptions, comfort, and convenience.

line, yield—see yield line.

locking—stoppage of traffic on the circulatory roadway caused by queuing backing into the

roundabout from one of the exits, resulting in traffic being unable to enter or circulate.

LOS—see level of service.

maximum service volume—the maximum hourly rate at which vehicles, bicycles, or persons can be reasonably expected to traverse a point or uniform section of a roadway during

an hour under specific assumed conditions while maintaining a designated level of service.

(source: HCM 2000)

measures of effectiveness—a quantitative parameter whose value is an indicator of the

performance of a transportation facility or service from the perspective of the users of the

facility or service.

median island—see splitter island.

merge conflict—the joining of two traffic streams.

mini-roundabout—small roundabouts used in low-speed urban environments. The central

island is fully mountable, and the splitter islands are either painted or mountable.

model, crash prediction—see crash prediction model.

modern roundabout—a term used to distinguish newer circular intersections conforming to

the characteristics of roundabouts from older-style rotaries and traffic circles.

m.o.e.—see measures of effectiveness.

mountable—used to describe geometric features that can be driven upon by vehicles without damage, but not intended to be in the normal path of traffic.

multilane roundabout—a roundabout that has at least one entry with two or more lanes,

and a circulatory roadway that can accommodate more than one vehicle traveling side-byside.

MUTCD—Manual on Uniform Traffic Control Devices.

neighborhood traffic circle—a circular intersection constructed at the intersection of two

local streets for traffic calming and/or aesthetic purposes. They are generally not channelized,

may be uncontrolled or stop-controlled, and may allow left turns to occur left (clockwise) of

the central island.

K

L

M

N
236 Federal Highway Administration

nonconforming traffic circle—see traffic circle.

nontraversable—see raised.

O&M costs—operations and maintenance costs.

peak hour factor—the hourly volume during the maximum-volume hour of the day divided by

the peak 15-minute flow rate within the peak hour; a measure of traffic demand fluctuation

within the peak hour.

pedestrian refuge—an at-grade opening within a median island that allows pedestrians to

safely wait for an acceptable gap in traffic.

perpendicular entry—an entry angle of 70 degrees or more.

PHF—see peak hour factor.

platoon—a group of vehicles or pedestrians traveling together as a group, either voluntarily

or involuntarily because of signal control, geometrics, or other factors.

point, conflict—see conflict point.

priority—the assignment of right-of-way to a particular traffic stream or movement.

progression, signal—see signal progression.

queue—a line of vehicles, bicycles, or persons waiting to be served by the system in which

the flow rate from the front of the queue determines the average speed within the queue.

Slowly moving vehicles or persons joining the rear of the queue are usually considered a part

of the queue. The internal queue dynamics may involve a series of starts and stops. (source:

HCM 2000)

queuing conflict—a conflict that arises within a traffic stream between a lead vehicle and a

following vehicle, when the lead vehicle must come to a stop.

radius, circulating path—see circulating path radius.

radius, entry—see entry radius.

radius, entry path—see entry path radius.

radius, exit—see exit radius.

radius, exit path—see exit path radius.

radius, left-turn path—see left-turn path radius.

radius, right-turn path—see right-turn path radius.

raised—used to describe geometric features with a sharp elevation change that are not intended to be driven upon by vehicles at any time.

ramp, wheelchair—see wheelchair ramp.

refuge, pedestrian—see pedestrian refuge.

right-of-way—(1) an intersection user that has priority over other users. (2) Land owned by a

public agency for transportation uses.

right-turn bypass lane—a lane provided adjacent to, but separated from, the circulatory

roadway, that allows right-turning movements to bypass the roundabout. Also known as a

right-turn slip lane.

right-turn path radius—the minimum radius on the fastest path of a right-turning vehicle.

right-turn slip lane—see right-turn bypass lane.

roadway, circulatory—see circulatory roadway.

O

P

Q

R
Roundabouts: An Informational Guide • Glossary 237

rotary—a term used particularly in the Eastern U.S. to describe an older-style circular intersection that does not have one or more of the characteristics of a roundabout. They often have

large diameters, often in excess of 100 m (300 ft), allowing high travel speeds on the circulatory roadway. Also known as a traffic circle.

roundabout—a circular intersection with yield control of all entering traffic, channelized approaches, counter-clockwise circulation, and appropriate geometric curvature to ensure that

travel speeds on the circulatory roadway are typically less than 50 km/h (30 mph).

roundabout capacity—the maximum number of entering vehicles that can be reasonably

expected to be served by a roundabout during a specified period of time.

roundabout, community enhancement—see community enhancement roundabout.

roundabout, modern—see modern roundabout.

roundabout, multilane—see multilane roundabout.

roundabout, rural double-lane—see rural double-lane roundabout.

roundabout, rural single-lane—see rural single-lane roundabout.

roundabout, single lane—see single-lane roundabout.

roundabout, urban compact—see urban compact roundabout.

roundabout, urban single-lane—see urban single-lane roundabout.

rural double-lane roundabout—a roundabout located in a rural area that has at least one

entry with two lanes, and a circulatory roadway that can accommodate more than one vehicle

traveling side-by-side. They incorporate approach curvature to slow entering traffic to a safe

speed.

rural single-lane roundabout—a roundabout located in a rural area that has single lanes on

all entries and one circulatory lane. This form typically has larger diameters and more tangential exits than urban forms.

separator island—see median island.

service volume—the hourly rate at which vehicles, bicycles, or persons can be reasonably

expected to traverse a point or uniform section of a roadway during an hour under specific

assumed conditions. See also maximum service volume. (Adapted from HCM 2000)

set-back distance—the distance between the edge of the circulatory roadway and the sidewalk.

sharpness of flare—a measure of the rate at which extra width is developed in the entry

flare. (source: UK Geometric Design of Roundabouts)

sight distance, intersection—see intersection sight distance.

sight distance, stopping—see stopping sight distance.

sight triangle—an area required to be free of obstructions to enable visibility between conflicting movements.

signal progression—the use of coordinated traffic signals along a roadway in order to minimize stops and delay to through traffic on the major road.

single-lane roundabout—a roundabout that has single lanes on all entries and one circulatory lane.

speed table—an extended, flat-top road hump sometimes used at pedestrian crossings to

slow traffic and to provide a better visual indication of the crosswalk location.

speed, approach—see approach speed.

speed, circulating—see circulating speed.

speed, entry—see entry speed.

S
238 Federal Highway Administration

splitter island—a raised or painted area on an approach used to separate entering from exiting traffic, deflect and slow entering traffic, and provide storage space for pedestrians crossing that intersection approach in two stages. Also known as a median island or a separator

island.

splitter island, extended—a raised splitter island that begins some distance upstream of the

pedestrian crossing to separate entering and exiting traffic. A design feature of rural roundabouts.

stopping sight distance—the distance along a roadway required for a driver to perceive and

react to an object in the roadway and to brake to a complete stop before reaching that object.

traffic calming—geometric treatments used to slow traffic speeds or to discourage the use

of a roadway by nonlocal traffic.

traffic circle—a circular intersection that does not have one or more of the characteristics of

a roundabout. Also known as a rotary.

traffic circle, neighborhood—see neighborhood traffic circle.

traffic circle, nonconforming—see traffic circle.

traffic design—a term used in this document to describe the design of traffic control devices,

including signing, pavement markings, and construction traffic control.

traffic, circulating—see circulating traffic.

traffic, entering—see entering traffic.

truck apron—see apron.

two-stage crossing—a process in which pedestrians cross a roadway by crossing one direction of traffic at a time, waiting in a pedestrian refuge between the two traffic streams if

necessary before completing the crossing.

two-way stop-control—stop signs are present on the approach(es) of the minor street. Drivers on the minor street or drivers turning left from the major street wait for a gap in the major

street traffic in order to complete a maneuver.

TWSC—see two-way stop control.

U-turn—a turning movement at an intersection in which a vehicle departs the intersection

using the same roadway it used to enter the intersection.

upstream—the direction from which traffic is flowing (source: HCM 2000).

urban compact roundabout—a small roundabout with a raised central island and splitter

islands, with perpendicular approaches that require vehicles to make a distinct right turn into

the circulatory roadway.

urban double-lane roundabout—an urban roundabout with at least one entry with two

lanes, and a circulatory roadway that can accommodate more than one vehicle traveling sideby-side. They have similar speed characteristics as urban single-lane roundabouts.

urban single-lane roundabout—a roundabout with single lane entries on all legs and one

circulatory lane. Entries are less perpendicular than the urban compact roundabout, allowing

somewhat higher speeds with higher capacities.

UVC—Uniform Vehicle Code.

vehicle, design—see design vehicle.

volume, circulating—see circulating volume.

volume, entering—see entering volume.

volume, service—see service volume.

volume-to-capacity ratio—the ratio of flow rate to capacity for a transportation facility.

V

U

T
Roundabouts: An Informational Guide • Glossary 239

wheelchair ramp—see curb ramp.

width, approach—see approach width.

width, circulatory roadway—see circulatory roadway width.

width, departure—see departure width.

width, entry—see entry width.

width, exit—see exit width.

yield—an intersection control in which controlled traffic must stop only if higher priority traffic

is present.

yield line—a pavement marking used to mark the point of entry from an approach into the

circulatory roadway and generally marked along the inscribed circle. If necessary, entering

traffic must yield to circulating traffic before crossing this line into the circulatory roadway.

zebra crossing—a crossing marked by transverse white stripes where vehicles are required

to yield to pedestrians.

W

Y

Z
240 Federal Highway Administration

Bibliography

5.1 U.S. References

American Association of State Highway Officials (AASHO). A Policy on Design of Urban Highways and Arterial Streets. Washington, D.C.: AASHO, 1973.

American Association of State Highway and Transportation Officials (AASHTO). Guide for Development of Bicycle Facilities. Washington, D.C.: AASHTO, 1991.

—. An Information Guide for Roadway Lighting. Washington, D.C.: AASHTO, 1985.

—. A Manual on User Benefit Analysis of Highway and Bus Transit Improvements. Washington, D.C.: AASHTO, 1977.

—. A Policy on Geometric Design of Highways and Streets. Washington, D.C.: AASHTO, 1994.

—. Roadside Design Guide. Washington, D.C.: AASHTO, 1989.

—. Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic

Signals. Washington, D.C.: AASHTO, 1994.

Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities (ADAAG).

36 CFR Part 1191. As amended through January 1998.

Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. Washington, D.C.: FHWA, 1988.

—. Older Driver Highway Design Handbook. Publication No. FHWA-RD-97-135. Washington,

D.C.: FHWA, January 1998.

—. Standard Highway Signs. Washington, D.C.:, FHWA, 1979.

—. Railroad-Highway Grade Crossing Handbook, 2nd edition. Report number FHWA-TS-86-

215, September 1986.

Illuminating Engineering Society (IES). American National Standard Practice for Roadway Lighting. Standard RP-8. December 1982.

Institute of Transportation Engineers. Manual of Transportation Engineering Studies (H.D.

Robertson, J.E. Hummer, and D.C. Nelson, ed.). Englewood Cliffs, N.J.: Prentice-Hall, 1994.

—. Transportation Planning Handbook (J. Edwards, Jr., ed.). Englewood Cliffs, New Jersey:

Prentice Hall, 1992.

National Committee on Uniform Traffic Laws and Ordinances (NCUTLO). Uniform Vehicle Code

and Model Traffic Ordinance. Evanston, Illinois: NCUTLO, 1992.

Pein, W.E. Trail Intersection Design Guidelines. Prepared for State Bicycle/Pedestrian Program,

State Safety Office, Florida Department of Transportation. Highway Safety Research Center,

University of North Carolina. September 1996.
Roundabouts: An Informational Guide • Bibliography 241

Transportation Research Board. Highway Capacity Manual. Special Report 209. Washington,

D.C.: Transportation Research Board, National Research Council, 1994.

—. Highway Capacity Manual. Special Report 209. Washington, D.C.: Transportation Research

Board, National Research Council, July 1999 (draft).

5.2 Roundabout Design Guides

5.2.1 United States

Florida Department of Transportation. Florida Roundabout Guide. Florida Department of Transportation, March 1996

Maryland Department of Transportation. Roundabout Design Guidelines. State of Maryland

Department of Transportation, State Highway Administration, 1995.

Ourston & Doctors, Inc. Roundabout Design Guidelines. 1995.

5.2.2 Australia/New Zealand

Australia/New Zealand Standard. Road lighting. Part 1.3: Vehicular traffic (Category V) lighting—Guide to design, installation, operation and maintenance. Report no. AS/NZS 1158.1.3:1997.

Published jointly by Homebush, New South Wales (Australia): Standards Australia and

Wellington (New Zealand): Standards New Zealand. 1997.

Austroads. Guide to Traffic Engineering Practice, Part 6—Roundabouts. Sydney, Australia:

Austroads, 1993.

Queensland Department of Main Roads (QDMR), Road Planning and Design Guidelines (Draft),

Chapter 12, Section 12.4—Roundabouts. Brisbane, Australia: QDMR, 1999.

Queensland Department of Main Roads (QDMR). Relationships between Roundabout Geometry and Accident Rates. Queensland, Australia: Infrastructure Design of the Technology Division of QDMR, April 1998.

Roads and Traffic Authority (RTA), New South Wales (Australia). Roundabouts—Geometric

Design Method. January 1997.

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5.2.3 Germany

Small Roundabouts: Recommendations for Application and Design, Nordrhein-Westfalen Department of City Development and Traffic (MSV), prepared by Werner Brilon, Ruhr-University

Bochum; translated from German by Daniel J. Parrish, 1993.

Haller, et al. Merkblatt für die Anlage von kleinen Kreisverkehrsplätzen (Guideline for the Construction of Small Roundabouts). Cologne, Germany: Forschungsgesellschaft für Straßen- und

Verkehrswesene. V., August 1998.

Department of Transport of Northrhine-Westfalia, Germany. Empfehlungen zum Einsatz und

zur Gestaltung von Mini-Kreisverkehrsplaetzen (Guidelines for the Use and Design of MiniRoundabouts). Dusseldorf, Germany, 1999.
242 Federal Highway Administration

5.2.4 The Netherlands

C.R.O.W. Eenheid in rotondes (Uniformity in roundabouts). Publication 126. Ede, The Netherlands: C.R.O.W., March 1998.

Centrum voor Regelgeving en Onderzoek in de Grond-, Water- en Wegenbouw en de

Verkeerstechniek (C.R.O.W). Rotondes (Roundabouts). Ede, The Netherlands: C.R.O.W. December 1993.

5.2.5 United Kingdom

Department of Transport (United Kingdom). Geometric Design of Roundabouts. TD 16/93.

September 1993.

Sawers, C. Mini-roundabouts: Getting them right!. Canterbury, Kent, United Kingdom: EuroMarketing Communications, 1996

5.2.6 France

Service d’Etudes Techniques des Routes et Autoroutes (SETRA—Center for Technical Studies of

Roads and Highways). Aménagement des Carrefours Interurbains sur les Routes Principales (Design of Rural Intersections on Major Roads). Ministry of Transport and Housing, December 1998.

Centre d’Etudes sur les Réseaux, les Transports, l’Urbanisme et les constructions publiques

(CERTU). L’Éclairage des Carrefours à Sens Giratoire (The Illumination of Roundabout Intersections). Lyon, France: CERTU, 1991.

Centre d’Etudes sur les Réseaux, les Transports, l’Urbanisme, et les Constructions Publiques

(CERTU) (Center for Studies on Transportation Networks, Urban Planning, and Public Works).

Carrefours Urbains (Urban Intersections) Guide. Lyon, France: CERTU, January 1999.

5.2.7 Spain

Ministerio de Fomento. Recomendaciones sobre glorietas (Recommendations on roundabouts).

Ministerio de Fomento, Dirección General de Carreteras, 1996.

5.3 Books and Reports

5.3.1 United States

Bauer, K.M., and D.W. Harwood. Statistical Models of At-Grade Intersection Crashes. Report

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Glauz, W.D., and D.J. Migletz. NCHRP Report 219: Application of Traffic Conflict Analysis at

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Harwood, D.W., et al. NCHRP Report 383: Intersection Sight Distances. National Cooperative

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5.3.2 United Kingdom

Brown, M. TRL State of the Art Review—The Design of Roundabouts. London: HMSO, 1995.

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1120. Crowthorne, England: Transport and Road Research Laboratory, 1984.

5.3.3 Australia

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Troutbeck, R.J. Evaluating the Performance of a Roundabout. SR 45. Australian Road Research Board, August 1989.

VicRoads. Victorian Traffic Handbook, Fourth Edition. Melbourne, Australia: Roads Corporation, 1998.
244 Federal Highway Administration

5.3.4 Germany

Brilon, W., B. Stuwe, and O. Drews. Sicherheit und Leistungsfähigkeit von Kreisverkehrsplätzen

(Safety and Capacity of Roundabouts). Research Report. Ruhr-University Bochum, 1993.

Empfehlungen zum Einsatz und zur Gestaltung kleiner Kresverkehrsplatze, Freistaat Sahsen

Einsatz-Und Gestaltung von Kreisverkehrsplatzen an Bundersstrassen Ausserhalb Bebauter

Gebiete, Brilon, W. and Bondzio, L., Ruhr-Universitat Bochum (Juni 1995).

5.3.5 France

Acts du Seminaire: “Giratoires 92” SETRA, CETUR.

5.3.6 The Netherlands

Schoon, C.C., and J. van Minnen. Accidents on Roundabouts: II. Second study into the road

hazard presented by roundabouts, particularly with regard to cyclists and moped riders. R-93-

16. The Netherlands: SWOV Institute for Road Safety Research, 1993.

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Brilon, W., and L. Bondzio, White Paper: “Summary of International Statistics on Roundabout

Safety” (unpublished), July 1998.

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Brilon, Werner, and Birgit Stuwe, “Capacity and Safety of Roundabouts in West Germany,”

Proceedings, 15th ARRB Conference, Vol. 15, Part 5, 1990, pp. 275–281.

Brilon, Werner, Michael Grossmann, and Birgit Stuwe, “Toward a New German Guideline for Capacity of Unsignalized Intersections,” Transportation Research Record 1320, 1991, pp.168–174.

Brude, U., and J. Larsson, The Safety of Cyclists at Roundabouts—A Comparison Between

Swedish, Danish and Dutch Results. Swedish National Road and Transport Research Institute

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1484 (1995) pp. 16–23.

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Centre d’Etude des Transports Urbains (CETUR), “Safety of Roundabouts in Urban and Suburban Areas,” Paris, 1992.

Chang, Stanley H., “Overcoming Unbalanced Flow Problems at a Roundabouts by Use of

Part-Time Metering Signals,” Master’s Thesis, Monash University, January 1994.
246 Federal Highway Administration

Chin, H.C., “SIMRO: A Model to Simulate Traffic at Roundabouts,” Traffic Engineering & Controls.

Chung, Edward, “Comparison of Roundabout Capacity and Delay Estimates from Analytical

and Simulation Models,” Proceedings, 16th ARRB Conference, Vol. 16, Part 5, 1992.

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CROW. Sign Up for the Bike: Design Manual for a Cycle-Friendly Infrastructure. The Netherlands: Center for Research and Standardization in Civil Engineering (CROW), 1993.

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the ITE District 6 Conference on Roundabouts, Loveland, Colorado, October 1998.

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Oregon Bicycle and Pedestrian Plan, Oregon Department of Transportation, 1995.

Department of Transport, “Determination of Size of Roundabouts at Major/Minor Junctions,”

Departmental Advice Note TA 23/81, 1981.

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Transport Planning and Control (J.D. Criffiths, ed.), 1992, pp. 227–286.

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25B, Bi. 2/3, 1991, pp. 89–102.

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Board, National Research Council, 1996.

Flannery, A., L. Elefteriadou, P. Koza, and J. McFadden, “Safety, delay and capacity of singlelane roundabouts in the United States.” In Transportation Research Record 1646. Washington,

D.C.: Transportation Research Board, National Research Council, 1998, pp. 63–70.

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Flannery, Aimee, and Tapan K. Datta, “Operational Performance Measures of American Roundabouts,” 1996 ITE Compendium of Technical Papers, 1996.

Gambard, J.M., “Safety and Design of Unsignalized Intersections in France,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp.48–61.

Guichet, B., “Roundabouts In France: Development, Safety, Design, and Capacity.” In Proceedings of the Third International Symposium on Intersections Without Traffic Signals (M.

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Hagring, Ola, “The Use of the Cowan M3 Distribution for Modeling Roundabout Flow,” Traffic

Engineering & Control, May 1996, pp. 328–332.
Roundabouts: An Informational Guide • Bibliography 247

Hakkert, A.S., D. Mahalel, and S.A. Asante, “A Comparative Study of Roundabout Capacity

Procedures,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991,

pp. 93–106.

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Transportation Research B, Vol. 25B, 4, pp. 163–174, 1991.

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Hoglund, Paul G., “Case Study. Performance Effects of Changing a Traffic Signal Intersection

to Roundabout,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.),

1991, pp. 141–158.

Horman, C.B., “Design and Analysis of Roundabouts,” Proceedings, 7th ARRB Conference,

Vol. 7, Part 4, 1974, pp. 58–82.

Hughes, B.P., “So You Think You Understand Gap Acceptance!,” Australian Road Research

Board, 19(3), 1989, pp. 195–204.

Innovative Transportation Concepts, LLC, VISSIM—User Manual. Program Version 2.32–2.36.

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pp. 42–45.

Jessen, G.D., Ein Richtlinienvorschlag für die Behandlung der Leistungsfähigkeit von

Knotenpunkten ohne Signalregelung (A guideline suggested for capacity calculations for

unsignalized intersections). Strassenverkehrstechnik, Nr. 7/8, 1968.

Jones, S.E., “Signalling Roundabouts 2. Controlling the Revolution,” Traffic Engineering & Control, Vol. 33, No. 11, November 1992, pp. 606–613.

Kimber, R.M., “Gap-Acceptance and Empiricism in Capacity Prediction,” Transportation Science, Vol. 23, No. 2, 1989.

Kimber, R.M., “The Design of Unsignalized Intersections in the UK,” Intersections Without

Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 20–34.

Lalani, N., “The impact on accidents of the introduction of mini, small and large roundabouts

at major/minor priority junctions.” Traffic Engineering + Control, December 1975.

Lamm, R., and E. M. Choueiri, “Recommendations for Evaluating Horizontal Design Consistency Based on Investigations in the State of New York.” In Transportation Research Record

1122. Washington, D.C.: Transportation Research Board, National Research Council, 1987.

Layfield, R.E., and G. Maycock, “Pedal-Cyclists at Roundabouts,” Traffic Engineering + Control, June 1986, pp. 343–349.
248 Federal Highway Administration

List, George, Siew Leong, Yursi Embong, Azizan Naim, and Jennifer Conley, “Case Study Investigation of Traffic Circle Capacity,” Transportation Research Record 1457, 1994, pp.118–126.

Little, J.D.C., A Proof of the Queueing Formula L = W • λ. Operations Research 9, S. 383–387,

1961.

McDonald, M.; and D.J. Armitage, “The capacity of roundabouts,” Traffic Engineering & Control, Vol. 19, October 1978, pp. 447–450.

Minnen, J. van, “Roundabouts,” Institute for Road Safety Research SWOV, The Netherlands,

1986.

Myers, Edward J., “Modern Roundabouts for Maryland,” ITE Journal, October 1994, pp. 18–22.

Niederhauser, M.E., B.A. Collins, E.J. Myers, “The Use of Roundabouts: Comparison with

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of Transportation Engineers, August 1997.

Ourston, Leif, “British Interchanges, Intersections, and Traffic Control Devices,” Westernite,

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Ourston, Leif, “Wide Nodes and Narrow Roads,” paper presented to the Transportation Research Board 72nd Annual Meeting, January 10–14, 1993.

Ourston, Leif, and Joe G. Bared, “Roundabouts: A Direct Way to Safer Highways,” Public

Roads, Autumn 1995, pp. 41–49.

Ourston, Leif, and Gregory A. Hall, “Modern Roundabout Interchanges come to America,”

1996 ITE Compendium of Technical Papers, 1996.

Paramics, Ltd., “Comparison of Arcady and Paramics for Roundabout Flows,” Version 0.3,

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Redington, T., “The Modern Roundabout Arrives in Vermont,” AASJTP Quarterly Magazine,

Vol. 75, No. 1, 1995, pp. 11–12.

Schoon, C., and J. van Minnen, “The Safety of Roundabouts in The Netherlands,” Traffic Engineering & Control, Vol. 35, No. 3, 1994, pp. 142–148.

Seim, K., “Use, Design and Safety of Small Roundabouts in Norway,” In Intersections Without

Traffic Signals II, Springer-Verlag, Germany (W. Brilon, ed.), 1991, pp. 270–281.

Service d’Etudes Techniques des Routes et Autoroutes (SETRA), Carrefours Giratoires: Evolution des Characteristiques Geometriques, Ministère de l’Equipement, du Logement, de

l’Aménagement du Territoire et des Transports, Documentation Technique 44, SETRA, August

1997, and 60, SETRA, May 1988.

SETRA/CETE de l’Ouest, “Safety Concerns on Roundabouts,” 1998.
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Simon, Michael J., “Roundabouts in Switzerland,” Intersection without Traffic Signals II, SpringerVerlag (Werner Brilon, ed.), 1991, pp. 41–52.

Smith, Mark J., “Improved Signing for Traffic Circles,” New Jersey Department of Transportation, FHWA/NJ-91-003 91-003-7350, 1990.

Smith, S.A., and R. L. Knoblauch, “Guidelines for the Installation of Crosswalk Markings.” In

Transportation Research Record 1141, Transportation Research Board, National Research Council, Washington, D.C., 1987.

Stuwe, Birgit, “Capacity and Safety of Roundabouts—German Results,” Intersection Without

Traffic Signals II, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 1–12.

Tan, Jian-an, “A Microscopic Simulation Model of Roundabout Entry Operations,” Intersection

without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 159–176.

Taylor, Marie C., “UK Techniques for the Prediction of Capacities, Queues, and Delays At

Intersections Without Traffic Signals,” Intersection without Traffic Signals I, Springer-Verlag

(Werner Brilon, ed.), 1991, pp. 274–288.

Technical Research Centre of Finland (VTT), “Traffic effects of a roundabout,” Nordic Road &

Transport Research, No. 1, 1993, pp. 9–11.

Todd, K., “A history of roundabouts in Britain,” Transportation Quarterly, Vol. 45, No. 1, January

1991.

Troutbeck, R.J., “Changes to Analysis and Design of Roundabouts Initiated in the Austroads

Guide” Proceedings, 16th ARRB Conference, Vol. 16, Part 5, 1992.

Troutbeck, R.J., “Capacity and Delays at Roundabouts—A Literature Review,” Australian Road

Research Board, 14(4), 1984, pp. 205–216.

Troutbeck, R.J., “Capacity and Design of Traffic Circles in Australia,” Transportation Research

Record 1398, 1993, pp. 68–74.

Troutbeck, R.J., “Current and Future Australian Practices for the Design of Unsignalized Intersections,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp.

1–19.

Troutbeck, R.J., “Does Gap Acceptance Theory Adequately Predict the Capacity of a Roundabout,” Proceedings, 12th ARRB Conference, Vol. 12, Part 4, 1985, pp. 62–75.

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Vol. 15, Part 5, 1990.

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Unknown source, “Roundabouts—Implications of U.S. Implementation.”
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Van Minnen, J., “Safety of Bicyclists on Roundabouts Deserves Special Attention,” SWOV

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Wong, S.C., “On the Reserve Capacities of Priority Junctions and Roundabouts,” Transportation Research Board, Vol. 30, No. 6, 1996, pp. 441–453.

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5.5 Video Cassettes

Ourston & Doctors, Inc. “I-70/Vail Road.”

—. “Nonconforming Traffic Circle Becomes Modern Roundabout.”

—. “Snow at Roundabouts.”

Maryland DOT. “Modern Roundabouts.”
Roundabouts: An Informational Guide • Appendix A: Operational Analysis Formulas 251

This appendix presents the assumptions used to develop the graphs and charts in

the operational analysis presented in Chapter 4.

A.1 Single-Lane Roundabout

A.1.1 Equations

(A-1)

where: Qe

= entry capacity, pce/h

Qc= circulating flow, pce/h

k

r =− − − − 



 

 1 0 00347 30 0 978  1 . ( ). . φ 0 05 (A-2)

F x = 303 2 (A-3)

f tx c D = + 0 210 1 0 2 2 . ( .) (A-4)

t D D = +

+  −



 





1 0 5

1 60

10

.

exp (A-5)

x v

e v

S 2 1 2

= + −

+

(A-6)

S e v

l = −

′

1 6.( )

(A-7)

where: e = entry width, m

v = approach half width, m

l’ = effective flare length, m

S = sharpness of flare, m/m

D = inscribed circle diameter, m

φ = entry angle, degrees

r = entry radius, m

Q kF fQ fQ F

fQ F

e c c cc

c c

=− ≤

= >

( ),

0,

Appendix A Operational Analysis Formulas
252 Federal Highway Administration

A.1.2 Parameter assumptions

D = 40 m

re = 20 m

φ = 30 degrees

v = 4 m

e = 4 m

l’ = 40 m

S e v

l = −

′ = − = 16 164 4

40

0 .( ) .( )

t D D = +

+  −



 





1 = 0 5

1 60

10

1 4404 .

exp

.

x v

e v

S 2 1 2

4 4 4

1 20

= + 4 −

+

= + −

+ = ( )

F x === 303 303 4 1212 2 ( )

f tx c D = += 0 210 1 0 2 0 5447 2 . ( .).

k

r =− − − − 



 

 1 0 00347 30 0 978  = 1 . ( ). . φ 0 05 1

A.1.3 Final equation

Q Q e c = − 1212 0 5447 . (A-8)

A.2 Double-Lane Roundabout

A.2.1 Equations

See Section A.1.1.

A.2.2 Parameter assumptions

D = 55 m

re = 20 m

φ = 30 degrees

v = 8 m

e = 8 m

l’ = 40 m

For design purposes, when e = v

then l’ is effectively zero.

However, setting l’ = 0 results in

S being undefined. Therefore a

non-zero value of l’ has been

selected. When e = v, any

 non-zero value of l’ results in

S = 0 and x2= v.

For design purposes, when e = v

then l’ is effectively zero.

However, setting l’ = 0 results in

S being undefined. Therefore a

non-zero value of l’ has been

selected. When e = v, any

non-zero value of l’ results in

S = 0 and x2= v.
Roundabouts: An Informational Guide • Appendix A: Operational Analysis Formulas 253

S e v

l = −

′ = − = 16 168 8

40

0 .( ) .( )

t D D = +

+  −



 





1 = 0 5

1 60

10

1 3112 .

exp

.

x v

e v

S 2 1 2

8 8 8

1 20

= + 8 −

+

= + −

+ = ( )

F x === 303 303 8 2424 2 ( )

f tx c D = += 0 210 1 0 2 0 7159 2 . ( .).

k

r =− − − − 



 

 1 0 00347 30 0 978  = 1 . ( ). . φ 0 05 1

A.2.3 Final equation

Q Q e c = − 2424 0 7159 . (A-9)

A.3 Urban Compact Roundabout

The capacity curve for the urban compact roundabout is based on the capacity

curves developed for roundabouts in Germany with single-lane entries and a singlelane circulatory roadway. This equation, developed by Brilon, Wu, and Bondzio is as

follows:

Q Q e c = − 1218 0 74 . (A-10)

where: Qe= entry capacity, pce/h

Qc= circulating flow, pce/h
254 Federal Highway Administration

A.4 Short Lanes

The effect of short lanes (flare) on capacity has been documented by Wu (3). Page

321 of Wu’s paper states that for a right flared approach,

k

xx x

f right

L T

n

R

nF right F right nF right ,

( ) , , ,

=

+ + + + +

1

1 1 1 (A-11)

Dropping some subscripts,

k

x x LT

n

R

n n =

+ + + +

1

1 1 1 ( ) () (A-12)

Noting that the capacities of each lane are the same and that the flows are the

same (that is, the entries are constantly fed with vehicles), this gives:

k

x n = +

1

21 (A-13)

with xLT = xR. Capacity qmax is then

q kq max = i (A-14)

where qi

 is flow in lane i and q1=q2

q q

x max n = +

2

21 (A-15)

qmax2 is the capacity of a two-lane roundabout, the capacity of each entry lane is

qmax2/2 and this is equal to the flow, q, divided by the degree of saturation, x.

q

q

max n

max 2 = + 21 (A-16)

The results of Equation A-16 can be compared with the results from the British

equations. The TRL equations are listed above. The results are listed for four circulating flow conditions: 500 veh/h,1000 veh/h, 1500 veh/h, and 2000 veh/h.
Roundabouts: An Informational Guide • Appendix A: Operational Analysis Formulas 255

Qc = 500 veh/h Qc=1000 veh/h Qc=1500 veh/h Qc=2000 veh/h

n TRL Wu TRL Wu TRL Wu TRL Wu

0 940 940 668 668 395 395 123 123

1 1447 1461 1151 1208 855 955 559 702

2 1636 1640 1321 1356 1006 1072 691 787

3 1737 1737 1411 1436 1086 1135 761 834

4 1799 1799 1468 1487 1136 1175 805 864

5 1841 1841 1506 1522 1170 1203 835 884

6 1872 1871 1534 1547 1195 1223 857 899

7 1896 1895 1555 1566 1214 1238 873 910

8 1914 1913 1571 1581 1229 1250 886 919

9 1929 1928 1585 1594 1240 1260 896 926

10 1941 1940 1596 1604 1250 1268 905 932

11 1951 1950 1605 1612 1258 1274 912 936

12 1960 1959 1612 1619 1265 1280 918 941

13 1967 1966 1619 1626 1271 1285 923 944

14 1974 1973 1625 1631 1276 1289 928 947

15 1979 1978 1630 1636 1281 1293 931 950

16 1984 1983 1635 1640 1285 1296 935 952

17 1989 1988 1639 1644 1288 1299 938 955

18 1993 1992 1642 1647 1292 1302 941 957

19 1996 1996 1645 1650 1294 1304 943 958

20 2000 1999 1648 1653 1297 1306 946 960

2066 2066 1708 1708 1350 1350 992 992

Exhibit A-2. Graphical

comparison of TRL and Wu

short-lane methodologies.

Exhibit A-1. Tabular

comparison of TRL and Wu

short-lane methodologies.
256 Federal Highway Administration

A.5 References

1. Kimber, R.M. The traffic capacity of roundabouts. TRRL Laboratory Report LR

942. Crowthorne, England: Transport and Road Research Laboratory, 1980.

2. Brilon , W., N. Wu, and L. Bondzio. “Unsignalized Intersections in Germany – A

State of the Art 1997.” In Proceedings of the Third International Symposium on

Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.

3. Wu, N. “Capacity of shared/short lanes at unsignalized intersections.” In

Proceedings of the Third International Symposium on Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.
Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs 257

The purpose of this Appendix is to provide examples for each of the six roundabout

categories. Exhibit B-1 lists typical inscribed circle diameter ranges for each roundabout category. Note that the flared-entry roundabout uses the same range of

inscribed circle diameters as the double-lane roundabouts. Note that the dimensions of roundabouts may vary considerably within each category, depending on

site-specific characteristics, including number of legs, approach angles, design

vehicle requirements, and so on. Refer to Chapter 6 for more discussion of specific

dimensions.

Mini-roundabout 13–25 m (45–80 ft)

Urban compact 25–30 m (80–100 ft)

Urban single lane 30–40 m (100–130 ft)

Urban double lane 45–55 m (150–180 ft)

Rural single lane 35–40 m (115–130 ft)

Rural double lane 55–60 m (180–200 ft)

Inscribed Circle

Site Category Diameter Range

The following pages show examples for each of the roundabout categories:

• Exhibit B-2: Typical mini-roundabout.

• Exhibit B-3: Typical urban compact roundabout.

• Exhibit B-4: Typical urban single-lane roundabout.

• Exhibit B-5: Typical urban double-lane roundabout.

• Exhibit B-6: Typical flared-entry roundabout.

• Exhibit B-7: Typical rural single-lane roundabout.

• Exhibit B-8: Typical rural double-lane roundabout.

Exhibit B-1. Typical inscribed

circle diameter ranges by

roundabout category.

Appendix B Example Roundabout Designs
258 Federal Highway Administration

Exhibit B-2. Example of a

typical mini-roundabout.
Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs 259

Exhibit B-3. Example of a

typical urban compact

roundabout.
260 Federal Highway Administration

Exhibit B-4. Example of a

typical single-lane roundabout.
Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs 261

Exhibit B-5. Example of a

typical urban double-lane

roundabout.
262 Federal Highway Administration

Exhibit B-6. Example of a

typical flared-entry roundabout.
Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs 263

Exhibit B-7. Example of a

typical rural single-lane

roundabout.
264 Federal Highway Administration

Exhibit B-8. Example of a

typical rural double-lane

roundabout.
Roundabouts: An Informational Guide • Appendix C: MUTCD Recommendations 265

The purpose of this Appendix is to provide the rationale behind recommended

deviations from the current (1988 edition) or proposed (2000 edition) Manual on

Uniform Traffic Control Devices (MUTCD). The following devices are discussed:

• YIELD Sign

• Roundabout Ahead Sign

C.1 Yield Sign

The proposed use of the YIELD sign in the Guide is generally consistent with the

MUTCD. However, the MUTCD contains language that generally discourages the

use of YIELD signs for controlling the major flow at an intersection and the use of

YIELD signs on more than one approach (MUTCD, §2B-8). This language predates

the consideration of roundabouts and should be modified in the next edition of the

MUTCD.

C.2 Roundabout Ahead Sign

As an alternative to the Circular Intersection sign, a Roundabout Ahead sign has

been proposed. This sign, along with a supplemental advisory speed plate (W13-1),

is shown in Exhibit C-1.

Exhibit C-1. Roundabout

Ahead sign with advisory

speed plate (W13-1).

This sign should be used on all approaches to a roundabout. The purpose of a

Roundabout Ahead sign is to convey to a driver that the driver is approaching an

intersection with the form of a roundabout. The intent of this sign is to be similar in

function to the other intersection warning signs (e.g., CROSS ROAD (W2-1) signs),

for example, which convey that the driver is approaching intersections of those

forms. Unlike those signs, however, the Roundabout Ahead sign is recommended

for all roundabouts, not just visually obscured locations.

C.2.1 Need

The 1988 edition of the MUTCD provides no sign related to roundabouts. The closest applicable sign is the YIELD AHEAD sign, either in word message or symbolic

form (W3-2 or W3-2a, respectively). While this sign is necessary for indicating an

upcoming traffic control device, it does not provide any information to the driver

that the upcoming yield sign is for a roundabout. Driver behavior, lane assignments,

Appendix C MUTCD Recommendation
266 Federal Highway Administration

and driver expectation are much different for roundabouts than for traditional yieldcontrolled locations (typically low-volume streets or right-turn bypass lanes). Identification that a roundabout is upcoming is particularly important for multilane approaches so that drivers can anticipate and move into the proper lane in advance of

the roundabout. Therefore, some indication that a driver is approaching a roundabout is essential, especially given the relative rarity of roundabouts in the United

States.

The National Committee on Uniform Traffic Control Devices (NCUTCD) has adopted

the Circular Intersection sign shown in Exhibit C-2, and this sign is being considered for adoption by FHWA.

Exhibit C-2. Circular

Intersection sign.

C.2.2 Existing Practice

Due to the lack of a standard Roundabout Ahead sign, jurisdictions in the U.S.

have experimented with a variety of warning signs, sometimes with multiple variations within the same jurisdiction. Examples of these are shown in Exhibit C-3. As

can be seen from the figure, the lack of standardization from jurisdiction to jurisdiction is evident.

Exhibit C-3. Sample of

existing Roundabout Ahead

signs in United States.

(g) (g)

(b)

(c) (d) (e)

(a)

(f) (h)

Bradenton Beach, FL

Mary Esther, FL

Mary Esther, FL

Lisbon, MD

 Leeds, MD

 Lothian, MD

 Naples, FL

West Boca Raton, FL

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)
Roundabouts: An Informational Guide • Appendix C: MUTCD Recommendations 267

Exhibit C-4. Sample of

Roundabout Ahead signs used

internationally.

C.2.3 Recommendation

Based on a review of existing signs in the U.S. and current international practice,

a recommended Roundabout Ahead sign was developed, as presented previously

in Exhibit C-1. This sign is similar in concept to those shown in (b), (c), and (j) of

Exhibit C-3 and is shown fully dimensioned in Exhibit C-5. This sign has been

developed based on the following criteria:

• The recommended sign is symbolic, consistent with current MUTCD practice.

• The recommended sign uses the internationally recognized circular ring of arrows to represent a roundabout and is almost an exact mirror image of the sign

used in Australia (Exhibit C-4).

• The recommended sign gives advanced notice of the proper direction of circulation. The NCUTCD-adopted sign in Exhibit C-2 does not convey this information and could give the driver the incorrect impression that the circulatory roadway is bidirectional.

International practice varies from country to country but is generally more consistent than current U.S. practice. Sign shapes and coloration vary depending on the

standards of that country, but the one consistent feature is a simple ring of arrows,

oriented to the direction of traffic flow. Examples from the United Kingdom and

Australia are given in Exhibit C-4.

(i)

(n)

(j) (k)

Exhibit C-3 (continued).

(i) Santa Barbara, CA

(j) Tallahassee, FL

(k) Taneytown, MD

(l) Tavares, FL

(m) Vail, CO

(n) West Vail, CO

(l) (m)

United Kingdom Australia
268 Federal Highway Administration

Exhibit C-5. Dimensions of

Roundabout Ahead sign.

• The recommended sign can be used for roundabouts with any number of legs,

including intersections with one-way approaches. Many of the signs in Exhibit

C-3 and the NCUTCD-recommended sign in Exhibit C-2 are unique to four-leg

roundabouts with legs at right angles and would be inappropriate for roundabouts with three or five legs, for example.

• The recommended sign can be supplemented by an advisory speed plate. An

advisory speed plate would not be appropriate for a YIELD AHEAD sign because

of the need for the driver to proceed only when clear.

• The recommended sign is simple with no extraneous or distracting elements to

confuse a driver. Some of the signs in Exhibit C-3 are perhaps too complex for

higher speed environments.

• Mini-roundabouts cannot be easily signed to show the proper direction of circulation. The recommended sign provides guidance to the driver as to the proper

direction of circulation.