A roundabout, a rotary and a traffic circle are types of circular road in which traffic is permitted to flow in one direction around a central island. In the United States, engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate 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 through horizontal deflection and minimising 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 petrol or diesel 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, Canada, the United Kingdom and France.
History
Origins and demise of traffic circles Circular junctions existed before roundabouts, including:
1768 United Kingdom: 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 designed the original plan of Washington, D.C., which contained several large street crossings. Within each intersection was a plaza – some had rectangular outlines and some had circular outlines. 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 in Görlitz. 1904 US: Columbus Circle in Manhattan, New York. 1905 US: American architect William Phelps Eno favoured 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. In 1924 the architect H.V.Lanchester proposed "gyratory traffic control" (i.e. one-way flow) for squares and complex junctions in London. This was first adopted on 12 October 1925 on Aldwych and extended in early 1926 to Parliament Square, Trafalgar Square and Hyde Park Corner. In the absence of a "priority rule", such junctions were not free-flowing and continued to be controlled by police at busy times, until traffic signals replaced them. Gyratory control improved flow by simplifying traffic movements and reducing the number of points in the junction where traffic needed to be controlled. In the UK, signals continue to be used at large roundabouts with heavy traffic flows.
1960s development of modern roundabouts
Modern roundabouts are distinguished from earlier ones by:
the "priority rule": vehicles joining a roundabout give way to those already on it; the use of deflection, to slow vehicles on the approach and make them turn so that they are pointing in the direction of flow around the roundabout. From about 1956, many local authorities in England introduced the priority rule; among the first were Poole in Dorset, Newcastle and Swindon. Official trials began in 1963: the priority rule was found to improve traffic flow by up to 10%. The United Kingdom adopted the rule in 1966, mandating road markings which required entering traffic to give way to circulating traffic. During the early 1960s, engineers at the UK's Transport Research Laboratory redesigned and standardised circular intersections. Frank Blackmore led the research and invented the mini-roundabout. A Transportation Research Board guide reports that the modern roundabout represents a significant improvement, in terms of both operations and safety, when compared with older rotaries and traffic circles. The design became mandatory in the United Kingdom for all new roundabouts in November 1966. Australia and other British-influenced countries were the first outside the UK to build modern roundabouts.
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