Traffic circles have been part of the transportation system in the United States since 1905, when the Columbus Circle designed by William Phelps Eno opened in New York City. Subsequently, many large circles or rotaries were built in the United States. The prevailing designs enabled high–speed merging and weaving of vehicles. Priority was given to entering vehicles, facilitating high–speed entries. High crash experience and congestion in the circles led to rotaries falling out of favor in America after the mid–1950’s. Internationally, the experience with traffic circles was equally negative, with many countries experiencing circles that locked up as traffic volumes increased.
The modern roundabout was developed in the United Kingdom to rectify problems associated with these traffic circles. In 1966, the United Kingdom adopted a mandatory “give–way” rule at all circular intersections, which required entering traffic to give way, or yield, to circulating traffic. This rule prevented circular intersections from locking up, by not allowing vehicles to enter the intersection until there were sufficient gaps in circulating traffic. In addition, smaller circular intersections were proposed that required adequate horizontal curvature of vehicle paths to achieve slower entry and circulating speeds.
These changes improved the safety characteristics of the circular intersections by reducing the number and particularly the severity of collisions. Thus, the resultant modern roundabout is significantly different from the older traffic circle both in how it operates and in how it is designed. The modern roundabout represents a substantial improvement, in terms of operations and safety, when compared with older rotaries and traffic circles. Therefore, many countries have adopted them as a common intersection form and some have developed extensive design guides and methods to evaluate the operational performance of modern roundabouts.
Maryland State Highway Administration (SHA) began to consider roundabouts as an intersection control in the late 1980’s. The SHA became particularly interested because of the international experience regarding the safety of roundabouts. After several years conducting research and developing roundabout guidelines and site selection procedures, SHA built their first roundabout in 1993. This roundabout, at the intersection of MD 94 and MD 144 in Lisbon, Howard County, was built primarily to reduce accidents. Prior to conversion to a roundabout, the intersection was listed as a high accident intersection, with an average of eight police reported accidents per year. The intersection has averaged a total of 2 police reported accidents/year since the roundabout was installed. In 2000, there was 1 police–reported accident at this intersection.
Federal Highway Administration, Roundabouts: An Informational Guide, Report No. FHWA–RD–00–067, June 2000.
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