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.