Guide for Roundabouts (2023)

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2023 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 1043 Guide for Roundabouts Kittelson & Associates, Inc. Portland, OR Sunrise Transportation Strategies, LLC Portland, OR Texas A&M Transportation Institute College Station, TX Kimley-Horn and Associates, Inc. Peachtree Corners, GA Accessible Design for the Blind, LLC Fairbanks, AK Subscriber Categories Highways • Design Research sponsored by the American Association of State Highway and Transportation Ofcials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agree- ment No. 693JJ31950003. The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, univer- sities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists in highway transportation matters to bring the findings of research directly to those in a position to use them. The program is developed on the basis of research needs iden- tified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Direc- tors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB. The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs. Published research reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet by going to https://www.mytrb.org/MyTRB/Store/default.aspx Printed in the United States of America NCHRP RESEARCH REPORT 1043 Project 03-130 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-69840-5 Library of Congress Control Number 2023933997 © 2023 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trade- marks of the National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. 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The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board does not develop, issue, or publish standards or speci- fications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. 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e National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. e National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. e National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. e three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. e National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. e Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. e mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. e Board’s varied activities annually engage about 8,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. e program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS This document was prepared under NCHRP Project 03-130, “Guide for Roundabouts.” It provides guidance for planning, designing, and implementing roundabouts that is based on integrating user needs within the widest array of project contexts. It expands upon a performance-based approach centered on developing project solutions to best meet and balance each user’s needs. Kittelson & Associates, Inc. (Kittelson), served as the prime contractor. Brian Ray, Sunrise Transportation Strategies, LLC (formerly with Kittelson), served as the principal investigator. Lee Rodegerdts, Kittelson, initiated the project as principal investigator and continued to serve as a key author of NCHRP Research Report 1043: Guide for Roundabouts (guide) content. Julia Knudsen (Kittelson) managed the guide’s devel- opment and provided invaluable project coordination. Michael Alston, Alek Pochowski, Krista Purser, Justin Bansen, Ed Myers, and Gene Hawkins completed the Kittelson team. Subcontractors provided valuable contributions to research tasks and guide content: Mark Lenters and Jay VonAhsen of Kimley-Horn and Associates, Inc.; Janet Barlow and Beezy Bentzen of Accessible Design for the Blind, LLC; and Marcus Brewer of Texas A&M University, Texas Transportation Institute. Quality Counts, LLC, supported data collection for research tasks that informed guide content. Guide development was supported by Ralph Bentley, Mindy Weber, Dimitryan Shadrin, Caleb Cox, and Anthea Fernandes of Kittelson. Andrew Ooms, Andy Duerr, Liz Byrom, Aditya Inamdar, Radu Nan, Margaret Kent, and Liz Flanagan (all of Kittelson) contributed to guide content, assisted with graphical information, and conducted early project tasks. CRP STAFF FOR NCHRP RESEARCH REPORT 1043 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Amir N. Hanna, Senior Program Officer Emily Griswold, Program Coordinator Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 03-130 PANEL Field of Traffic—Area of Operations and Control Michael J. Dugas, New Hampshire Department of Transportation (formerly), Concord, NH (Chair) Asma Ali, T3 Design Corporation, Fairfax, VA Christina D. Barry, Georgia Department of Transportation, Atlanta, GA Stephen A. Bass, Kansas Department of Transportation, Topeka, KS Richard B. Crossler-Laird, Oregon Department of Transportation, Salem, OR Rachel S. Price, Roundabotix, San Diego, CA Eugene Robert Russell, Sr., Kansas State University, Manhattan, KS Brian J. Walsh, Washington State Department of Transportation, Olympia, WA Hillary Nicole Isebrands, FHWA Liaison Bernardo B. Kleiner, TRB Liaison

NCHRP Research Report 1043: Guide for Roundabouts provides information and guidance on all aspects of roundabouts and supersedes NCHRP Report 672: Roundabouts: An Informa- tional Guide—Second Edition. The information contained in NCHRP Research Report 1043 will help highway agencies and other organizations address relevant issues when consider- ing the planning and implementation of roundabouts. Since its publication in 2010, NCHRP Report 672: Roundabouts: An Informational Guide— Second Edition has served as a national guide on roundabout planning, analysis, design, and construction. However, in the years since NCHRP Report 672 was published, technology has changed, substantial research on roundabouts has been performed, and many roundabouts have been constructed. The findings and experience gained from these developments have contributed to knowledge on implementing roundabouts. However, additional research was needed to address the gaps in available roundabout guidance and incorporate the informa- tion on new technologies, the findings of new and earlier research, and the lessons learned from constructed projects into a guide that provides updated information and guidance on all aspects of roundabouts. Under NCHRP Project 03-130, “Guide for Roundabouts,” Kittelson & Associates, Inc., was tasked with developing a guide to supersede NCHRP Report 672 and provide guidance on many aspects of roundabouts. To accomplish this objective, the research team reviewed relevant literature, including national and state research and guidance documents and other sources; sought and incor- porated practitioners’ experiences; and conducted research on designing for trucks to assess roundabout design decisions for serving large trucks and research on designing for bicycles to better address bicycle treatments. The research team also synthesized information pertain- ing to roundabout design and implementation in the following areas: oversized/overweight trucks, retrofitting of existing roundabouts, mini-roundabouts, pedestrian crossings, traffic control devices, illumination, and economic impacts. Finally, the research team developed a comprehensive guide that integrates a performance-based design approach, incorporates research findings, and provides roundabout-specific guidance. A conduct of research report summarizing the work performed to develop NCHRP Research Report 1043 together with several appendices that provide further elaboration on the research are available as NCHRP Web-Only Document 347: Background and Summary of a Guide for Roundabouts from the National Academies Press website (nap.nationalacademies.org). F O R E W O R D By Amir N. Hanna Staff Officer Transportation Research Board

P A R T I Introduction to Roundabouts 1-1 Chapter 1 Introduction 1-1 1.1 Roundabout Guide Purpose and Intended Audience 1-2 1.2 Organization of the Guide 1-3 1.3 History and Practice 1-4 1.4 Policy and Practice Considerations 1-8 1.5 References 2-1 Chapter 2 Roundabout Characteristics and Applications 2-2 2.1 Roundabout Definition and Characteristics 2-2 2.2 Other Types of Circular Intersections 2-7 2.3 Roundabout Categories 2-16 2.4 Considerations in Building Roundabouts 2-22 2.5 Innovative Contexts 2-23 2.6 References P A R T I I Planning and Stakeholder Considerations 3-1 Chapter 3 A Performance-Based Planning and Design Approach 3-2 3.1 Ties to the Project Development Process 3-3 3.2 Project Goals 3-6 3.3 Performance Measures 3-7 3.4 Decision-Making Framework 3-8 3.5 Project Considerations 3-10 3.6 Roundabouts in a System Context 3-12 3.7 Retrofits of Existing Circular Intersections 3-16 3.8 References 4-1 Chapter 4 User Considerations 4-2 4.1 Pedestrians 4-7 4.2 Bicyclists and Micromobility Users 4-10 4.3 Passenger Cars and Motorcycles 4-11 4.4 Large Vehicles 4-17 4.5 Emergency Vehicles 4-17 4.6 Railroads and Light Rail Transit 4-18 4.7 Connected and Automated Vehicles 4-18 4.8 References C O N T E N T S

5-1 Chapter 5 Stakeholder Considerations 5-2 5.1 Identifying Stakeholders 5-4 5.2 Outreach at Each Project Development Stage 5-8 5.3 Outreach Approaches 5-16 5.4 References 6-1 Chapter 6 Intersection Control Evaluation 6-2 6.1 Policy and Legal Considerations 6-2 6.2 Typical ICE Steps 6-3 6.3 Connection to Project Development Process 6-4 6.4 Project Considerations 6-6 6.5 Evaluation of Alternatives 6-6 6.6 Interim and Ultimate Configurations 6-7 6.7 ICE Example 6-10 6.8 References P A R T I I I Roundabout Evaluation and Conceptual Design 7-1 Chapter 7 Safety Performance Analysis 7-2 7.1 Introduction 7-2 7.2 Conflict Types and Conflict Points 7-14 7.3 Roundabout Crash Types and Factors 7-16 7.4 Roundabouts Compared with Other Intersection Types 7-17 7.5 Safety Surrogate Measures 7-18 7.6 Safety Predictive Methods 7-20 7.7 Assessment of Existing Circular Intersections 7-22 7.8 References 8-1 Chapter 8 Operational Performance Analysis 8-2 8.1 Introduction 8-3 8.2 Operational Analysis Principles 8-6 8.3 Operational Performance Measures 8-8 8.4 Quality of Service and Level of Service 8-10 8.5 Reporting and Interpreting Results 8-10 8.6 Planning-Level Analysis Techniques 8-16 8.7 Highway Capacity Manual Analysis Techniques 8-17 8.8 Other Deterministic Methods and Software Implementations 8-18 8.9 Simulation Techniques 8-19 8.10 Assessment of Existing Roundabouts and Circular Intersections 8-22 8.11 References 9-1 Chapter 9 Geometric Design Process and Performance Checks 9-2 9.1 Design Process 9-4 9.2 Design Principles and Objectives 9-5 9.3 Performance Checks Overview 9-5 9.4 Geometric Speeds 9-13 9.5 Sight Distance and Visibility 9-20 9.6 Vehicle Path Alignment 9-23 9.7 Design Vehicles 9-26 9.8 Bicycle and Pedestrian Wayfinding and Crossing Assessment 9-29 9.9 Retrofitting Existing Circular Intersections 9-31 9.10 References

P A R T I V Horizontal, Vertical, and Cross-Section Design 10-1 Chapter 10 Horizontal Alignment and Design 10-3 10.1 Optimal Design for Project Context and User Type 10-4 10.2 Design Process and Principles 10-4 10.3 Horizontal Design Performance Influences 10-17 10.4 Design for People Walking and Biking 10-35 10.5 Design for Large Trucks 10-46 10.6 Single-Lane Roundabouts 10-58 10.7 Multilane Roundabouts 10-72 10.8 Design for Interim and Ultimate Configurations 10-76 10.9 Bypass Lanes 10-83 10.10 Interchanges 10-88 10.11 Access Management 10-94 10.12 Parking 10-95 10.13 Bus Stop Placement 10-95 10.14 Treatment for High-Speed Approaches 10-102 10.15 References 11-1 Chapter 11 Vertical Alignment and Cross-Section Design 11-2 11.1 Profile and Cross-Section Relationships 11-2 11.2 Profile Development 11-7 11.3 Roadways with Grades 11-10 11.4 Transverse and Cross-Slope Design 11-13 11.5 Pedestrian Design Influences 11-14 11.6 Trucks 11-17 11.7 References P A R T V Final Design and Implementation 12-1 Chapter 12 Traffic Control Devices and Applications 12-2 12.1 Principles of Traffic Control Devices 12-3 12.2 Transition Area 12-10 12.3 Entry Area 12-21 12.4 Circulating Area 12-24 12.5 Exit Area 12-25 12.6 Entry Metering Signals 12-27 12.7 At-Grade Rail Crossings 12-30 12.8 Pavement Marking Materials 12-32 12.9 Other Devices 12-32 12.10 References 13-1 Chapter 13 Curb and Pavement Details 13-1 13.1 Roadway Pavement Type 13-3 13.2 Truck Apron Material 13-5 13.3 Pavement Jointing 13-7 13.4 Curb Type 13-9 13.5 Splitter Islands with Sloped Noses 13-10 13.6 References

14-1 Chapter 14 Illumination, Landscaping, and Artwork 14-2 14.1 General Illumination Considerations 14-3 14.2 Lighting Levels 14-6 14.3 Illumination Equipment Type and Location 14-8 14.4 Landscaping 14-14 14.5 Art and Other Fixed Objects 14-18 14.6 References 15-1 Chapter 15 Construction and Maintenance 15-1 15.1 Introduction 15-2 15.2 Stakeholder Engagement and Construction 15-3 15.3 Construction Plans 15-3 15.4 Construction Coordination 15-4 15.5 Construction Staging 15-10 15.6 Work Zone Traffic Control 15-11 15.7 Maintenance 15-13 15.8 Snow Plowing and Storage 15-16 15.9 References A-1 Appendix Design Performance Check Techniques R-1 References AA-1 Abbreviations and Acronyms Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at nap.nationalacademies.org) retains the color versions.