Roundabouts in the United States (2007)

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TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2007 www.TRB.org 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 REPORT 572 Subject Areas Highway and Facility Design Roundabouts in the United States Lee Rodegerdts Miranda Blogg Elizabeth Wemple Edward Myers KITTELSON & ASSOCIATES, INC. Portland, OR Michael Kyte Michael Dixon UNIVERSITY OF IDAHO Moscow, ID George List NORTH CAROLINA STATE UNIVERSITY Raleigh, NC Aimee Flannery GEORGE MASON UNIVERSITY Fairfax, VA Rod Troutbeck TROUTBECK AND ASSOCIATES Brisbane, Australia Werner Brilon Ning Wu RUHR-UNIVERSITY BOCHUM Bochum, Germany Bhagwant Persaud Craig Lyon PERSAUD AND ASSOCIATES Etobicoke, Canada David Harkey Daniel Carter UNIVERSITY OF NORTH CAROLINA Chapel Hill, NC Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. 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CRP STAFF FOR NCHRP REPORT 572 Robert J. Reilly, Director, Cooperative Research Programs Crawford F. Jencks, Manager, NCHRP B. Ray Derr, Senior Program Officer Eileen P. Delaney, Director of Publications Natalie Barnes, Editor Beth Hatch, Editor NCHRP PROJECT 3-65 PANEL Field of Traffic—Area of Operations and Control Beatriz Caicedo-Maddison, Florida DOT, Ft. Lauderdale, FL (Chair) Maria G. Burke, Texas DOT, Austin, TX Jerry Champa, California DOT, Sacramento, CA Leonard Evans, Science Serving Society, Bloomfield Hills, MI Steve King, Kansas DOT, Topeka, KS Robert R. Limoges, New York State DOT, Albany, NY Richard Long, Western Michigan University, Kalamazoo, MI Brian J. Walsh, Washington State DOT, Olympia, WA Mohsin A. Zaidi, Kansas City Department of Public Works, Kansas City, MO Joe Bared, FHWA Liaison Richard A. Cunard, TRB Liaison 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

Based on a comprehensive evaluation of roundabouts in the United States, this report presents methods of estimating the safety and operational impacts of roundabouts and updates design criteria for them. The report will be useful to geometric designers and traffic engineers who are considering improvements to an intersection. Presentation materials that may be helpful in public meetings and similar forums are available on the TRB website (http:// www.trb.org/news/blurb_detail.asp?id=7086). Although traffic circles have been used in the United States since 1905, their use has been limited since the 1950s because many were found to work neither efficiently nor safely. The modern roundabout was developed in the United Kingdom in the 1960s to address these problems. Two key characteristics of the modern roundabout are (1) entering traffic that yields to circulating traffic and (2) geometric constraints that slow entering vehicles. Many studies have shown that modern roundabouts (hereafter referred to as roundabouts) are safe and effective, and they are now widely used internationally. Because roundabout design is relatively new to the United States, there has been some reluctance to use them. Perceived differences in driver behavior raise questions about how appropriate some international research and practices are for the United States. Therefore, additional information on the safety and operation of roundabouts in the United States will be very helpful to planners and designers in determining where roundabouts would reduce intersection crashes and congestion and in refining the design criteria currently being used. These design refinements can be particularly important for bicyclists and pedestrians using the intersection. Under NCHRP Project 3-65, Kittelson & Associates, Inc. and their subcontractors reviewed existing safety and operational models. After compiling a comprehensive inven- tory of roundabouts in the United States, they traveled to several representative ones to gather geometric, operational, and safety data. Particular emphasis was placed on collect- ing data at roundabouts with significant pedestrian and bicycle volumes. They then evalu- ated the different analytical models to determine how well they replicate U.S. experience. The best models were then refined. During the course of the project, the research team also gathered information on transportation agencies’ experiences with different design configurations. NCHRP Web-Only Document 94 (http://www.trb.org/news/blurb_detail.asp?id=7274) contains the appendixes to this report and includes detailed reviews of the literature on safety performance and operational models, the master inventory of U.S. roundabouts assembled for this project, and the results of the statistical testing of various models. F O R E W O R D By B. Ray Derr Staff Officer Transportation Research Board

C O N T E N T S 1 Summary 4 Chapter 1 Introduction and Research Approach 4 Problem Statement and Research Objective 4 Scope of Study 5 Research Approach 7 Literature Review 12 Chapter 2 Data Characteristics 12 Site Inventory 12 Safety Data 16 Operational Data 19 Geometric Data 20 Speed Data 20 Pedestrian and Bicyclist Data 22 Conclusion 23 Chapter 3 Safety Findings 23 Ability of Existing Non-U.S. Models to Represent U.S. Data 26 Models Calibrated for U.S. Data 29 Development of Speed-Based Prediction Models Using U.S. Data 31 Before-After Analysis 32 Conclusion 34 Chapter 4 Operational Findings 34 Assessment of Existing Capacity and Delay Models 38 Gap Acceptance Analysis 46 Parametric Analysis 54 Capacity Model Calibration 58 Capacity Model Development 64 Level of Service 64 Conclusion 65 Chapter 5 Design Findings 65 Speed Analysis 78 Pedestrian Analysis 89 Bicyclist Analysis 94 Other Design Findings 101 Conclusion 102 Chapter 6 Interpretation, Appraisal, and Applications 102 Application of Intersection-Level Safety Performance Models 104 Estimation of the Safety Benefit of a Contemplated Conversion of an Existing Intersection to a Roundabout

106 Application of Approach-Level Safety Models 107 Incorporation of Safety Models into Other Documents 107 Application of Operational Models 109 Chapter 7 Conclusions and Suggested Research 109 Safety Performance 109 Operational Performance 109 Geometric Design 110 Pedestrian and Bicyclist Observations 111 Suggested Research 113 References 115 Appendixes (All appendixes have been published as NCHRP Web-Only Document 94, available on the TRB website [http://www.trb.org/news/blurb_detail.asp?id=7274]). AUTHOR ACKNOWLEDGMENTS Numerous individuals contributed to this effort. Mr. Lee A. Rodegerdts of Kittelson & Associates, Inc. directed the project as Principal Investigator. Mr. Bruce Robinson served as the co-Principal Investigator for the early stages of the project. Many staff members at Kittelson & Associates, Inc. made significant contributions, including Dr. Miranda Blogg, Ms. Elizabeth Wemple, Mr. Edward Myers, Ms. Karen Giese, Mr. Sagar Onta, Mr. Del Huntington, and Mr. Ralph Bentley. Dr. Michael Kyte and Dr. Michael Dixon of the University of Idaho, Dr. George List of North Carolina State University (who was with Rensselaer Polytechnic Institute during the conduct of this project), and Dr. Aimee Flannery of George Mason Uni- versity led the data collection efforts. The field data collection team consisted of students from three universities: including Mr. Phil Rust and Mr. Yuri Mereszczak from the University of Idaho; Ms. Stacy Eisenman, Mr. Hyunwoo Cho, Ms. Alix Demers, and Ms. Rebecca Brown from Rensselaer Polytechnic Institute; and Mr. William Johnson and Ms. Angie Martin from George Mason University. The opera- tional data extraction team from the University of Idaho included Ms. Christina Hemberry, Mr. Gary Haderlie, Mr. Brent Orton, Ms. Chittemma Potlapati, Ms. JoeAnn Brazill, and Ms. Audra Sherman. Dr. Rod Troutbeck, Dr. Werner Brilon, and Dr. Ning Wu made significant contributions to the opera- tional analysis. Dr. Bhagwant Persaud and Mr. Craig Lyon led the safety analysis, with Dr. Richard Hall of the University of Southampton providing senior review. Mr. David Harkey and Mr. Daniel Carter con- ducted the pedestrian and bicycle evaluation. Dr. John M. Mason, Jr., of Pennsylvania State University and Mr. Wayne Kittelson of Kittelson & Associates, Inc. provided senior review and guidance. The authors appreciate the many contributions of data that support the findings of this project. People from agencies all across the United States, too numerous to mention individually, supplied crash data, design plans, and other information, as well as facilitated our field visits during scouting and data collec- tion. The authors also thank Mr. Srinivas Mandavilli of Kansas State University for being available on short notice to assist with data collection in Kansas and Mr. Michael Wallstedt of TranSystems, Inc. and Mr. Leif Ourston of Ourston Roundabout Engineering for providing supplementary video footage. The authors especially thank the NCHRP 3-65 panel members for long and diligent service in review- ing and guiding this project and greatly appreciate the guidance and patience from the Senior Program Officer of the National Cooperative Highway Research Program.