Traffic circle

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Columbus Circle in New York City. Unlike a modern roundabout, the circle is quite large and pedestrians have access to the center island. Access is controlled by traffic lights.
DeSoto Fountain sits in the center of a traffic circle in the City of Coral Gables, Florida.
There are eight lanes of traffic negotiating the traffic circle around the Arc de Triomphe in Paris.

A traffic circle or rotary is a type of intersection that has a generally circular central island. Entering traffic must typically alter direction and speed to avoid the island, creating a circular flow in one direction. In most applications, traffic circles replace the stop lights and traffic signs that regulate flow in other intersections.

In English-speaking countries other than the United States, circular junctions or intersections are typically in the form of roundabouts which give priority to circulating traffic and which are physically designed to slow traffic entering the junction to improve safety, so that the roads typically approach the junction radially; whereas rotaries are frequently designed to try to increase speeds, and thus have roads that enter the traffic circle tangentially.

In the U.S., traffic engineers use the term roundabout for intersections in which entering traffic must yield to traffic already in the circle, reserving the term traffic circle for those in which entering traffic is controlled by stop signs, traffic signals, or is not formally controlled.[1] Colloquially, however, roundabouts are often referred to as circles. [2]

Design[edit]

Design criteria include:

  • Right-of-way—whether entering or circling vehicles have the right of way. The New Jersey Driver's Manual recommends that, in the absence of flow control signs, traffic yields based on "historically established traffic flow patterns",[3] and there are no set rules.[4] In New England,[5] Washington, D.C. and New York State,[6] entering traffic yields, as is the norm in virtually all countries outside of the U.S.
  • Traffic speed—High entry speeds (over 30 mph / 50 km/h) require circulating vehicles to yield, often stopping, which lowers capacity and increases crash rates than modern roundabouts.[8]
  • Lane changes— Allowed or not
  • Diameter—The greater the traffic, the larger the circle.[7]
  • Island function—Parking, parks, fountains, etc.[7]

History[edit]

French architect Eugène Hénard was designing one-way circular intersections as early as 1877.[9] American architect William Phelps Eno favored small traffic circles. He designed New York City's famous Columbus Circle, which was built in 1905. Other circular intersections were subsequently built in the United States, though many were large diameter 'rotaries' that enabled high speed merge and weave maneuvers. These designs were doomed to failure for two primary reasons:

  • 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 weren't large enough for high-speed merging.[citation needed]
  • 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,[citation needed] characterized 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.

Examples of traffic circles[edit]

The Western Rotary in Zagreb, Croatia with tram lines passing underneath.

United States[edit]

Massachusetts[edit]

Traffic circles are referred to as "rotaries" in Massachusetts as well as parts of Connecticut, New Hampshire & Vermont; see Rotaries in Massachusetts

Other states[edit]

Elsewhere[edit]

See also[edit]

References[edit]

  1. ^ U.S. Department of Transportation: Roundabouts: an Informational Guide para 1.5
  2. ^ Johnson, Jeffrey A. (6 August 2012). "Salem Four Corners traffic circle to start rounding into shape". The Day. 
  3. ^ http://www.state.nj.us/mvc/manuals/chap_04_06.html
  4. ^ http://www.state.nj.us/mvc/pdf/Licenses/Driver%20Manual/Chapter_4.pdf
  5. ^ Commonwealth of Massachusetts, Registry of Motor Vehicles. "Sharing the Road: A User's Manual for Public Ways". Retrieved 2009-05-13. 
  6. ^ :http://www.safeny.com/rowa-vt.htm#1145
  7. ^ a b c Modern Roundabouts, an Informational Guide
  8. ^ Shashi S. Nambisan, Venu Parimi. "A Comparative Evaluation of the Safety Performance of Roundabouts and Traditional Intersection Controls". Institute of Transportation Engineers. Retrieved 2007-11-27. 
  9. ^ P. M. Wolf, Eugene Henard and the Beginning of Urbanism in Paris, 1900–1914, International Federation for Housing and Planning, The Hague, 1969, cited by Ben Hamilton-Baillie & Phil Jones, Improving traffic behaviour and safety through urban design, Proceedings of ICE – Civil Engineering, volume 158 Issue 5 May 2005 p. 41 http://www.hamilton-baillie.co.uk/papers/ICE_paper_April05.pdf