Roundabout

A roundabout is a type of circular intersection or junction in which road traffic flows almost continuously in one direction around a central island.

So-called "modern" roundabouts require entering traffic to give way to traffic already in the circle and optimally observe 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 by reducing traffic speeds and minimizing T-bone and head-on collisions. Variations on the basic concept include integration with tram and/or train lines, two-way flow, higher speeds and many others.

Traffic exiting the roundabout comes from one direction, rather than 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 on average spend less time idling at roundabouts than at signalled intersections, using a roundabout potentially leads to less pollution. Also, 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 produce less work to regain the initial speed, resulting in lower emissions. Additionally, slow moving traffic in roundabouts makes less noise than traffic that must stop and start, speed up and brake.

Modern roundabouts are commonplace throughout the world. Half of the world's roundabouts are in France (more than 30,000 as of 2008), although the United Kingdom has more as a proportion of the road than any other country.

Circular junctions existed before roundabouts, including the Circus in the English city of Bath, Somerset, a world heritage site completed in 1768; the 1907 Place de l'Étoile around the Arc de Triomphe in Paris; the 1904 Columbus Circle in Manhattan; and several circles within Washington, D.C. The operating and entry characteristics of these circles differ considerably from modern roundabouts. In 1907 architect John McLaren designed one of the first American roundabouts for both autos and street cars (trams) in the Hanchett Residence Park in what is now San Jose, California. The first British circular junction was built in Letchworth Garden City in 1909. Its centre originally was intended partly as a traffic island for pedestrians. It was featured in the film The World's End. In the early 20th century, numerous traffic circles were constructed in the United States, particularly in the northeast. Examples include a circle in Atherton, California.

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