Roundabout

A roundabout (also called a traffic circle, road circle, rotary, rotunda or island) 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. Modern roundabouts 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 greatly 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. 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. 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 circle and rotaries. Since then they have spread and modern roundabouts are commonplace throughout the world.:2 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 city of Bath, Somerset, England, completed in 1768, part of a world heritage site; the 1899 Brautwiesenplatz in Görlitz, Germany; 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. French architect Eugène Hénard was designing one-way circular intersections as early as 1877. American architect William Phelps Eno favored small traffic circles. He designed New York City's famous Columbus Circle, which was built in 1905. In 1907, architect John McLaren designed one of the first American roundabouts for both autos and streetcars (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. Other circular intersections were subsequently built in the United States, though many were large diameter 'rotaries' that enabled high speed merge and weave maneuvers. They may control entering traffic by stop signs or traffic lights. Many older traffic circles allow entry at higher speeds without deflection, or require a stop and a 90-degree turn to enter. These designs were doomed to failure for two primary reasons. The first is that it takes a large diameter circle to provide enough room for merging at speed. Although some of these circles were huge (many were in excess of 100 meters or 300 feet in diameter), they were not large enough for high-speed merging. Secondly, giving priority to entering traffic means that more vehicles can enter the circulatory roadway than it can handle. The result is congestion within the circle which could not clear without police intervention. The experience with traffic circles and rotaries in the US was almost entirely negative, characterised by high accident rates and congestion problems. By the mid 1950s, construction of traffic circles and rotaries had ceased entirely. The experience with traffic circles in other countries was not much better until the development of the modern roundabout in the United Kingdom during the 1960s. Widespread use of the modern roundabout began when the UK's Transport Research Laboratory engineers re-engineered and standardised circular intersections during the 1960s. Frank Blackmore led the development of the 'Priority Rule' and subsequently invented the mini-roundabout to overcome capacity and safety limitations. The priority rule was found to improve traffic flow by up to 10%. The design became mandatory in the United Kingdom for all new roundabouts in November 1966. This give-way requirement has been the law in New York state since the 1920s.