Roundabout

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, 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 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 of Indianapolis, Indiana ;
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 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.

1960s development of modern roundabouts

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%. In 1966, the United Kingdom adopted a rule at all circular junctions that required entering traffic to give way to circulating traffic. 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.

Spread in Europe and North America since 1970s

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.
In 1973, the U.S. city of Seattle began installing small traffic circles within existing intersections to calm traffic., the city has installed over 1,200 traffic circles, primarily in residential neighborhoods.
In 1980, Switzerland had 19 roundabouts.
In 1980, Norway had 15 roundabouts.
In the early 1980s, single-lane roundabouts were also introduced in the Netherlands. 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.
In 1983, France adopted the yield-at-entry rule on national routes; since then the country's roundabouts have proliferated.
In 1985, Norway put up yield signs at the entries to all its roundabouts. After this, safety and traffic flow rapidly improved, and Norwegian roundabouts increased from 15 in 1980, to 350 in 1990, to 500 in 1992.
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.
In the late 1980s, the Netherlands saw significant growth with about 400 roundabouts constructed in just 6 years. 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.
In 1990, the US constructed its first modern roundabout, although older roundabouts have been somewhat common in parts of the Northeast.
In 1991, France was building 1,000 roundabouts every year.
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.
In 1999, Canada built its first modern roundabout.
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.
* There were around 25,000 in the United Kingdom in 2015.
North American introduction since 1990s

In the United States modern roundabouts emerged in the 1990s after years of planning and educational campaigning by Frank Blackmore and Leif Ourston, who sought to bring the by then well-established increased safety and traffic flow in other countries to America. The first was constructed in Summerlin, Nevada, in 1990 and was followed by another the following year. This roundabout occasioned dismay from residents, and a local news program said about it, "Even police agree, can be confusing at times." Between 1990 and 1995, numerous modern roundabouts were built in California, Colorado, Florida, Maryland, Nevada, and Vermont.
Municipalities introducing new roundabouts often were met with some degree of public resistance, just as in the United Kingdom in the 1960s.

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.
A 1998 survey of municipalities found public opinion 68% opposed prior to construction, changing thereafter to 73% in favour.
A 2007 survey found public support ranging from 22% to 44% prior to construction, and several years after construction was 57% to 87%.
By 2011, however, some 3,000 roundabouts had been established, with that number growing steadily.
By 2022, it was estimated that there were about 8,800 roundabouts in the United States.

In the mid-2010s, about 3% of the then circa 4,000 U.S. modern roundabouts were located in Carmel, Indiana, whose mayor James Brainard had been actively promoting their construction; because of increased safety, injuries caused by car accidents in the city dropped by 80% after 1996. there were about 4,800 modern roundabouts in the United States. As an example, Washington state contained about 120 roundabouts as of 2016, all having been built since 1997, with more planned.
The first Canadian traffic circles were in Edmonton. There were seven such by 1954. However, they didn't gain popularity in the rest of the country until the 1990s. They became increasingly popular amongst traffic planners and civil engineers in the 15 years thereafter due to their success in Europe. By 2014 there were about 400 roundabouts in Canada at the time, or one per 90,000 inhabitants.

Modern roundabout

A "modern roundabout" is a type of looping junction in which road traffic travels in one direction around a central island and priority is given to the circulating flow. Signs usually direct traffic entering the circle to slow and to give way to traffic already on it.
Because low speeds are required for traffic entering roundabouts, they are physically designed to slow traffic entering the junction to improve safety, so that the roads typically approach the junction radially; whereas older-style traffic circles may be designed to try to increase speeds, and have roads that enter the circle tangentially.
Roundabouts are normally not used on controlled-access highways because of the low speed requirement, but may be used on lower grades of highway such as limited-access roads. When such roads are redesigned to incorporate roundabouts, traffic speeds must be reduced via tricks such as curving the approaches.
Many traffic circles have been converted to modern roundabouts, including the former Kingston traffic circle in New York and several in New Jersey. Others have been converted to signalised intersections, such as the Drum Hill Rotary in Chelmsford, Massachusetts, which is now six lanes wide and controlled by four separate intersections.
Japan was first introduced to roundabouts in September 2014 to stop major accidents and traffic jams.