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Columbus Circle, New York City, NY. Unlike a modern roundabout, the circle is quite large, pedestrians have access to the center island, lane changes can be made in the circle, and the intersecting streets are at 90 degree angles. Access control is controlled by traffic lights.
DeSoto Fountain sits in the center of a traffic circle in the City of Coral Gables, Florida.
Signage for a traffic circle or rotary in New Jersey
Rotary signage in Lowell, Massachusetts. Note the Yield sign

A traffic circle or rotary is a type of circular intersection in which traffic must travel in one direction around a central island. Typically, traffic entering the circle has the right-of-way and drivers in the circle must yield. Other common characteristics include large diameters (over 100 m or 300 ft) and minimal horizontal deflection so as to facilitate speeds of 50 km/h (30 mph) or more.[1]

Traffic circles should not be confused with roundabouts, in which entering traffic must always yield to traffic already in the circle, and generally operate at much lower speeds.[2]

Contents

Characteristics of Traffic Circles

The term "traffic circle" is used to describe circular intersections that have the following characteristics:

  • Often, circulating traffic yields to entering traffic at one or more approach points. The New Jersey Driver's Manual advises drivers to yield to cars on the right (thus the circulating traffic would yield to entering traffic.)[3]. In New England, rotaries operate such that entering traffic yields[4].
  • Tangential approaches between approach roadways and the circulatory roadway allow full-speed entry. Conversely, some traffic circles do not have channelized approaches at all, and roads intersect the circles at 90 degree angles.[1]
  • High circulating speeds (over 30 mph / 50 km/h) mean that large gaps are needed in the circulating traffic to allow stopped vehicles to safely enter, resulting in lower capacities and higher crash rates than modern roundabouts.[5]
  • Lane changes may be made within circle road.
  • The circles are generally of a very large diameter.[1]
  • There is sometimes pedestrian access to the center island, or parking on the circle.[1]

History

French architect Eugène Hénard was designing one-way circular intersections as early as 1877[6]. 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.
  • 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, 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.

Among the most famous traffic circles in the world is that of Canberra, Australia, where a large traffic circle encircles Parliament House. This circle has traffic lights at each major intersection within the circle. However, it is not a true traffic circle, as the circular road in question does not form a full circle.

Examples of Traffic Circles

Pop culture

Traffic circles are currently featured in the 2009 Mathematical Contest in Modeling. The problem is to determine an optimal design for a traffic circle in terms of traffic flow. This includes technical reports to engineers interested in implementing the models.[7]

Sources

  1. ^ a b c d Modern Roundabouts, an Informational Guide
  2. ^ http://www.tfhrc.gov/safety/00068.pdf
  3. ^ http://www.state.nj.us/mvc/manuals/chap_04_06.html
  4. ^ Commonwealth of Massachusetts, Registry of Motor Vehicles. "Sharing the Road: A User's Manual for Public Ways". http://www.mass.gov/rmv/dmanual/index.htm. Retrieved 2009-05-13. 
  5. ^ Shashi S. Nambisan, Venu Parimi (March 2007). "A Comparative Evaluation of the Safety Performance of Roundabouts and Traditional Intersection Controls". Institute of Transportation Engineers. http://findarticles.com/p/articles/mi_qa3734/is_200703/ai_n18755716/pg_1. Retrieved 2007-11-27. 
  6. ^ 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
  7. ^ 2009 Mathematical Contest in Modeling: Problems

See also

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