A roundabout, also known as a rotary or traffic circle, is a type of circular intersection or junction in which road traffic is permitted to flow in one direction around a central island, and priority is typically given to traffic already in the junction.
Engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate various design rules to increase safety. Compared to stop signs, traffic signals, and earlier forms of roundabouts, modern roundabouts reduce the likelihood and severity of collisions greatly by reducing traffic speeds and minimizing T-bone and head-on collisions. Variations on the basic concept include integration with tram or train lines, two-way flow, higher speeds and many others.
For pedestrians, traffic exiting the roundabout comes from one direction, instead of three, simplifying the pedestrian's visual environment. Traffic moves slowly enough to allow visual engagement with pedestrians, encouraging deference towards them. Other benefits include reduced driver confusion associated with perpendicular junctions and reduced queuing associated with traffic lights. They allow U-turns within the normal flow of traffic, which often are not possible at other forms of junction. Moreover, since vehicles that run on gasoline typically spend less time idling at roundabouts than at signalled intersections, using a roundabout potentially leads to less pollution. When entering vehicles only need to give way, they do not always perform a full stop; as a result, by keeping a part of their momentum, the engine will require less work to regain the initial speed, resulting in lower emissions. Research has also shown that slow-moving traffic in roundabouts makes less noise than traffic that must stop and start, speed up and brake.
Modern roundabouts were first standardised in the UK in 1966 and were found to be a significant improvement over previous traffic circles and rotaries. Since then, modern roundabouts have become commonplace throughout the world, including Australia, the United Kingdom and France.
History
Origins and demise of traffic circles
Circular junctions existed before roundabouts, including: 1768 England: The Circus in the city of Bath, Somerset was completed. This was constructed based on architectural considerations and not for traffic purposes. 1780 (ca.) France: The Place de l'Étoile around the Arc de Triomphe in Paris. 1791 US: Pierre Charles L'Enfant (who came to America in 1776) designed a plan of Washington, D.C., which was then being planned. This design contained several large places/street crossings. Many of them had a rectangular outer outline, but within each was a place, around which the streets should be built, thus reducing the number of arms/legs of each crossing. 1821 US: The Governor's Circle (later renamed Monument Circle) of Indianapolis, Indiana (which gave the city the nickname "The Circle City"); 1877 France: The French architect Eugène Hénard was designing one-way circular intersections. 1879 Netherlands: The Keizer Karelplein in Nijmegen. 1899 Germany: Brautwiesenplatz (Bride Meadow('s) Place) in Görlitz. 1904 US: Columbus Circle in Manhattan, New York. 1905 US: American architect William Phelps Eno favored small traffic circles.He re-designed New York City's famous Columbus Circle, which was finished in 1905. 1907 US: Architect John McLaren designed one of the first American traffic circles for both autos and streetcars (trams) in the Hanchett Residence Park in what is now San Jose, California. 1909 United Kingdom: The first British circular junction was built in Letchworth Garden City.
Although some may still be referred to as roundabouts, the operating and entry characteristics of these traffic circles differed considerably from modern roundabouts.
Circular intersections were built in the United States, though many were large-diameter 'rotaries' that enabled high-speed merge and weave manoeuvres. Older-style traffic circles may control entering traffic by stop signs or traffic lights. Many allow entry at higher speeds without deflection, or require a stop and a 90-degree turn to enter. Because these circumstances caused a lot of vehicle collisions, construction of traffic circles and rotaries ceased in the 1950s, and some were removed.