Roundabouts: An Informational Guide

1 Publication Project Manager:Joe InformationalGuideKittelson & Associates, University of TechnologyRuhr-University BochumUniversity of FloridaUniversity of IdahoPennsylvania State UniversityHurst-Rosche EngineersEppell Olsen & PartnersBuckhurst Fish & Departmentof Transportation Federal Highway AdministrationForewordRoundabouts are a form of intersection control in common use throughout the world. Until recently, manytransportation professionals and agencies in the United States have been hesitant to recommend andinstall 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 wereinterested in roundabouts had to rely on foreign roundabout design guides, consultants with roundaboutexperience, or in some States, statewide roundabout design guides.

2 To facilitate safe, optimal operationand designs that are both consistent at a national level and consequential for driver expectation andsafety, the Federal Highway Administration (FHWA) developed this Informational Guide on information supplied in this document, Roundabouts: An Informational Guide , is based on establishedinternational and practices and is supplemented by recent research. The Guide is comprehensive inrecognition of the diverse needs of transportation professionals and the public for introductory materialthrough design detail, as well as the wide range of potential applications of roundabout operation and safety performance are particularly sensitive to geometric design regarding evaluation procedures can result in over-design and less safety.

3 The design prob-lem is essentially one of determining a design that will accommodate the traffic demand while minimizingsome combination of delay, crashes, and cost to all users, including motor vehicles, pedestrians, andbicyclists. Evaluation procedures are suggested, or information is provided, to quantify and cost how wella design achieves each of these there is no absolutely optimum design, this Guide is not intended as an inflexible rule book, butrather 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 pro-vided in this Guide , combined with the expert heuristic knowledge of a designer.

4 Adherence to theseprinciples still does not ensure good design, which remains the responsibility of the F. TrentacosteDirector, Office of Safety Research and DevelopmentNOTICEThis publication is disseminated under the sponsorship of the Department of Transportation in the interestof information exchange. The publication does not constitute a standard, specification, or regulation. Anytrade or manufacturers names that appear herein are included solely because they are considered essen-tial to the object of the : An Informational GuideList of Exhibits viiiPhoto Credits xi vChapter 1 - Introduction Scope of Guide Organization of Guide Defining Physical Features Key Dimensions

5 Distinguishing Roundabouts from Other Circular Intersections roundabout Categories References 20 Chapter 2 - Policy Considerations Characteristics Multimodal Considerations Costs Associated with Roundabouts Legal Considerations Public Involvement Education References 48 Chapter 3 - Planning Planning Steps Considerations of Context

6 Number of Entry Lanes Selection Categories Comparing Operational Performance of AlternativeIntersection Types Space Requirements 69 Table of ContentsFederal Highway Evaluation 76 Chapter 4 - Operation at roundabouts Requirements Analysis Software for Roundabouts 98 Chapter 5 - Prediction 6 - Geometric Design : An Informational GuideChapter 7 - Traffic Design and Landscaping1837.

7 1 Signing1857. 2 Pavement Markings1977. 3 Illumination2027. 4 Work Zone Traffic Control2057. 5 Landscaping2077. 6 References209 Chapter 8 - System Signals at Rail Spaced in an Arterial A: Operations Analysis Formulas251 Appendix B: Example roundabout Designs257 Appendix C: MUTCD Recommendations265 Federal Highway AdministrationVIIIList of ExhibitsChapter 1 - IntroductionExhibit of key roundabout of key roundabout of key roundabout of key roundabout dimensions. 7 Exhibit of roundabouts with traffic design elements at design characteristics for each of the sixroundabout urban urban single-lane urban double-lane rural single-lane rural double-lane roundabout19 Chapter 2 - Policy ConsiderationsExhibit 2-1.

8 Average annual crash frequencies at 11 converted to s chances of death if hit by a motor of vehicle-vehicle conflict points forintersections with four single-lane vehicle path through a double-lane of aesthetic of Informational straight through a left at a 3 - PlanningExhibit daily service volumes for afour-leg maximum daily service volumes : An Informational GuideExhibit of community enhancement of traffic calming of predicted rural roundabout injury crashes with rural TWSC of predicted injury crashes for single-laneand double-lane roundabouts with rural or urban delay per vehicle at the MUTCD peak hoursignal warrant of TWSC and single-lane roundabout capacity.

9 65 Exhibit hourly distribution of savings in delay of single-lane roundabout versusAWSC, 50 percent of volume on the major savings in delay of single-lane roundabout versusAWSC, 65 percent of volume on the major savings for roundabouts vs. signal, 50 percentvolume on major savings for roundabouts vs. signal, 65 percentvolume on major for spatial comparison of roundabouts and comparable conventional comparison: Urban compact roundabout signalized comparison: Urban single-lane roundabout signalized comparison: Urban double-lane roundabout signalized comparison: Urban flared roundabouts signalized costs for crashes of varying levelsof 4 - OperationExhibit factors for passenger car equivalents (pce).

10 84 Exhibit flow capacity of a single-lane capacity of a double-lane Highway AdministrationXExhibit reduction factors for short comparison of single-lane anddouble-lane reduction factor M for a single-laneroundabout assuming pedestrian reduction factor M for a double-laneroundabout assuming pedestrian delay as a function of capacity andcirculating queue length of roundabout software products foroperational 5 - SafetyExhibit conflict points for T Intersections withsingle-lane conflict point comparison for intersectionswith single-lane lane-use conflicts in double-lane turn conflicts in double-lane conflicts at signalized conflicts at single-lane conflicts at conventional conflicts at annual crash frequencies at 11 converted to crash reductions in various proportions of major crash types of collision types at depiction of collision types at percentage per type of user urbanroundabouts in 15 towns in western crash rates for pedestrians at roundabouts andsignalized reduction in the number of accidents bymode at 181 converted Dutch.