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Roundabout

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Roundabout in La Crosse, Wisconsin
UK Roundabout
Movement within a roundabout in left-hand traffic; note the clockwise circulation
Sparta Army National Guard
Movement within a roundabout in right-hand traffic; note the anticlockwise circulation
The Hotel Indonesia traffic circle in Jakarta, Indonesia
A ring junction in Hemel Hempstead, UK

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.[1][2]

In the United States, engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate design rules to increase safety.[3][2] 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.[4][5] 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[citation needed] and reduced queuing associated with traffic lights. They allow U-turns within the normal flow of traffic, often not possible at other forms of junction. Moreover, since vehicles with internal combustion engines typically spend less time idling at roundabouts than at signalled intersections, using a roundabout potentially leads to less pollution.[6][7] When entering vehicles only need to give way, they do not always perform a full stop; as a result, by keeping 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.[8]

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,[2]: 2  including Australia, Canada, the United Kingdom and France.[7]

History

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Origins and demise of traffic circles

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Circular junctions existed before roundabouts, including:

Although some may still be referred to as roundabouts, the operating and entry characteristics of these traffic circles differed considerably from modern roundabouts.[14]

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.[15]: 3:02 

In 1924 the architect H.V.Lanchester proposed "gyratory traffic control" (i.e. one-way flow) for squares and complex junctions in London.[16] This was first adopted on 12 October 1925 on Aldwych and extended in early 1926 to Parliament Square, Trafalgar Square and Hyde Park Corner.[17][18] 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

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The United Kingdom's first roundabout (1909) in Letchworth Garden City

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%.[19] The United Kingdom adopted the rule in 1966, mandating road markings which required entering traffic to give way to circulating traffic.[20]

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[21][22]. 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.[14] The design became mandatory in the United Kingdom for all new roundabouts in November 1966.[23] Australia and other British-influenced countries were the first outside the UK to build modern roundabouts.[3]

Spread in Europe and North America since 1970s

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  • In 1951, Cyprus, British Crown colony back then, adopted the roundabout in the main cities.
  • In the 1970s, France and Norway adopted the modern roundabout.[24]
  • In 1973, the U.S. city of Seattle began installing small traffic circles within existing intersections to calm traffic.[25] As of 2021, the city has installed over 1,200 traffic circles, primarily in residential neighborhoods.[26]
  • In 1980, Switzerland had 19 roundabouts.[3]
  • In 1980, Norway had 15 roundabouts.[3]
  • In the early 1980s, single-lane roundabouts (or mini-roundabouts) were also introduced in the Netherlands.[24] It began in the relatively sparsely populated northern and eastern Netherlands because of fears that the roundabouts would not be able to cope with the traffic density of the Randstad; however, when it appeared the single-lane roundabouts had an even higher capacity than signalised intersections, they were constructed en masse in the western Netherlands as well.[24]
  • In 1983, France adopted the yield-at-entry rule on national routes; since then the country's roundabouts have proliferated.[3]
  • In 1985, Norway put up yield signs at the entries to all its roundabouts. After this, safety and traffic flow rapidly improved,[3] and Norwegian roundabouts increased from 15 in 1980, to 350 in 1990, to 500 in 1992.[3]
  • In 1987, Switzerland introduced the yield-at-entry rule; since then its roundabouts increased from 19 in 1980 to 220 in early 1992, while 500 more were being considered.[3]
  • In the late 1980s, the Netherlands saw significant growth with about 400 roundabouts constructed in just 6 years.[3] Construction accelerated in the 1990s, and by 2001, there were an estimated 1,500 to 1,800 roundabouts in the Netherlands, more than half of which were located within built-up areas.[24]
  • In 1990, the US constructed its first modern roundabout, although older roundabouts have been somewhat common in parts of the Northeast (called rotaries or traffic circles).
  • In 1991, France was building 1,000 roundabouts every year.[3]
  • As modern roundabouts rose in popularity in the 1980s, the old traffic circles fell out of favour and many were converted into modern roundabouts or other types of intersections.[3]
  • In 1999, Canada built its first modern roundabout.[27]
  • As of the beginning of the 21st century, roundabouts were in widespread use in Europe. For instance:
    • In 2010, France had more than 30,000 roundabouts.[28]
    • There were around 25,000 in the United Kingdom in 2015.[29]