TRB's National Cooperative Highway Research Program (NCHRP) Report 672: Roundabouts: An Informational Guide - Second Edition explores the planning, design, construction, maintenance, and operation of roundabouts. The report also addresses issues that may be useful in helping to explain the trade-offs associated with roundabouts. This report updates the U.S. Federal Highway Administration's Roundabouts: An Informational Guide, based on experience gained in the United States since that guide was published in 2000.

The Traffic Engineering Division has been conducting a study to determine the feasibility and applicability of traffic circles as a speed control device. Experimental traffic circles were installed at three intersections in residential areas in the city. Before and after speed surveys show a very impressive drop in speeds close to the intersection. The general conclusion is that the traffic circle has merits in reducing speeds in residential areas but further in-depth studies need to be conducted.

The guidance 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. The following topics are addressed: definition of a roundabout and what distinguishes roundabouts from traffic circles; public acceptance and legal issues associated with roundabouts; consideration of all user modes, including heavy vehicles, buses, transit, bicycles, and pedestrians; a methodology for identifying appropriate sites for roundabouts and the range of conditions for which roundabouts offer optimal performance: methodologies for estimating roundabout capacity, delays, and queues with reference to the Highway Capacity Manual; design principles and guidance on safety and geometric design, with reference to applicable national standards such as the AASHTO Policy on Geometric Design of Highways and Streets; guidelines for control features such as signing and pavement markings, with reference to the Manual on Uniform Traffic Control Devices; illumination; and landscaping.

Despite traffic circles, four-way stop signs, lights regulated by timers or sensors, and other methods, the management of urban intersections remains problematic. Consider that transportation systems have all the features of so-called complex systems: the great number of state and control variables, the presence of uncertainty and indeterminism, th