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A roundabout is a type of circular intersection or junction in which road traffic flows almost continuously in one direction around a central island.〔''The New Shorter Oxford English Dictionary,'' Volume 2, Clarendon Press, Oxford (1993), page 2632〕 The modern form was standardised in the United Kingdom. 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.〔(【引用サイトリンク】url=http://www.wsdot.wa.gov/safety/roundabouts/benefits.htm )〕 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.〔(【引用サイトリンク】title=Roundabout Rules of the Road )〕 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.〔(2008 National Roundabout Conference, B. Guichet's presentation )〕 == History == 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 differs 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.〔http://twofeet.weebly.com/walking-blog/the-five-circles-of-hanchett-park The Five Circles of Hanchett Park, 7 June 2014〕 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. Widespread use of the modern roundabout began when the UK's Transport Research Laboratory engineers re-engineered circular intersections during the 1960s. Frank Blackmore led the development of the offside priority rule and subsequently invented the mini-roundabout to overcome capacity and safety limitations. The design became mandatory in Britain for all new roundabouts in November 1966. This give-way requirement has been the law in New York state since the 1920s. In the United States modern roundabouts emerged in the 1990s. They faced some opposition from a population mostly unaccustomed to them. American confusion at how to enter and especially how to exit a roundabout was the subject of mockery such as featured in the film ''European Vacation'' and the television series ''The Simpsons''. By 2011, however, some 3,000 roundabouts had been established, with that number growing steadily.〔 The first modern roundabout in the United States was constructed in Summerlin, Nevada in 1990. This roundabout occasioned dismay from residents, and a local news program said about it, "Even police agree, they (roundabouts) can be confusing at times." As of the beginning of the twenty-first century, roundabouts were in widespread use in Europe. For instance, in 2010 France had more than 30,000 roundabouts. In the United States, municipalities introducing new roundabouts often are met with some degree of public resistance, just as in the United Kingdom in the 1960s. Surveys show that negative public opinion reverses as drivers gain experience with them. A 1998 survey of municipalities found public opinion 68% opposed prior to construction; changing thereafter to 73% in favour.〔(Roundabout Practice in the United States )〕 A 2007 survey found public support ranging from 22% to 44% prior to construction, and several years after construction was 57% to 87%.〔Retting, Richard A., Sergey Y. Kyrychenko, and Anne T. McCartt. "Long-Term Trends in Public Opinion Following Construction of Roundabouts." Journal of the Transportation Research Board 2019 (2007): 219–24.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「roundabout」の詳細全文を読む スポンサード リンク
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