The roundabout concept was first invented in the early 1900’s and deployed throughout Europe and America. A modern roundabout has three major characteristics compared to its predecessors, traffic circles and rotaries:
- The roundabout gives vehicles in the circular travel way the right-of-way.
- Roundabouts are small, generally from 70 to 160 feet in diameter compared to 300 to 400 feet and more for traffic circles and rotaries.
- Roundabouts have a raised entry "splitter" island that slows down or constrains speed just before entry, duplicating the curvature the driver will experience within the roundabout itself.
During the 1950’s there was a loss of confidence in roundabouts, due mainly to the problem of traffic locking and the increasing number of accidents. Many were replaced by traffic signals.
In 1966, the off-side priority rule (an entering vehicle gives way to vehicles in the roundabout) and the yield at entry operation enhanced roundabout capacity and safety performance. The success of this modern roundabout inspired renewed interest in roundabouts worldwide.
The modern roundabout finally arrived in the United States in 1990 in Summerlin, a major Las Vegas residential subdivision. When the first roundabout freeway interchange in the nation was built in 1995 (at the I-70 interchange in Vail, Colorado), roundabouts then numbered about a dozen nationally. Roundabouts have since been constructed in every state including Alaska and Hawaii.
As of January 2006, the number of modern roundabouts in the USA had leaped to around 1,000. Roundabout proponents anticipate that roundabouts will be built in the United States by the thousands in the near future.
Sources: RoundaboutsUSA, Oregon Department of Transportation
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