Traffic circle

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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 is controlled by traffic lights.
DeSoto Fountain sits in the center of a traffic circle in the City of Coral Gables, Florida.
Traffic circle in New Jersey showing the non-standard signs New Jersey uses before a circle.
Rotary sign in Lowell, Massachusetts. Note the Yield sign

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, traffic circles are typically in the form of roundabouts which give priority to traffic already on the junction 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]

Contents

[edit] Design

Design criteria include:

  • Right-of-way—whether entering or circling vehicles have the right of way. The New Jersey Driver's Manual recommends that circulating traffic yield to entering traffic[2] although there are no set rules. [3]. In New England[4], Washington, D.C. and New York State[5], entering traffic yields, as is the norm in virtually all countries outside of the U.S.
  • Angle of entry— Angles range from glancing (tangential) that allow full-speed entry to 90 degree angles (perpendicular).[6]
  • 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.[7]
  • Lane changes— Allowed or not
  • Diameter—The greater the traffic, the larger the circle.[6]
  • Island function—Parking, parks, fountains, etc.[6]

[edit] History

French architect Eugène Hénard was designing one-way circular intersections as early as 1877.[8] 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.

[edit] Examples of traffic circles

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

[edit] United States

[edit] Massachusetts

[edit] Other States

[edit] Elsewhere

[edit] Sources

  1. ^ U.S. Department of Transportation: Roundabouts: an Informational Guide para 1.5
  2. ^ http://www.state.nj.us/mvc/manuals/chap_04_06.html
  3. ^ http://www.state.nj.us/mvc/pdf/Licenses/Driver%20Manual/Chapter_4.pdf
  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. ^ :http://www.safeny.com/rowa-vt.htm#1145
  6. ^ a b c Modern Roundabouts, an Informational Guide
  7. ^ 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. 
  8. ^ 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

[edit] See also

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