Traffic circle

Encyclopedia : T : TR : TRA : Traffic circle

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A traffic circle is an intersection with a circular shape and, usually, a central island. In some traffic circles two-way traffic is allowed within the circle. It is much more common, however, that traffic is allowed to go in one direction only around a central island. In some traffic circles, entering roads are controlled by stop signs or traffic signals. In other cases, traffic enters the circulatory roadway by merging, sometimes at relatively high speeds. Traditionally, traffic entering a circle has the right-of-way, though many circles in New Jersey give right-of-way to the primary roads. In roundabouts, as distinct from traffic circles, entering traffic must yield to traffic already in the circulatory roadway.

History

French architect Eugene Henard was designing one-way circular intersections as early as 1877. American architect William Eno favored small traffic circles. He designed New York City's famous Columbus Circle, which was built in 1905. Other traffic circles were subsequently built in the United States. Many were large diameter 'rotaries' that enabled high speed merge and weave, and gave priority to the traffic entering the circle. These designs were doomed to failure for two primary reasons:

• It takes a large diameter circle to provide enough room for merging at speed. Despite the fact that some of these circles were huge (many were in excess of 100 meters or 328 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.

The experience with traffic circles in the US was almost entirely negative, characterized by high accident rates and congestion problems. By the mid 1950s, construction of traffic circles had ceased entirely. The experience with traffic circles in other countries wasn't much better until the development of the modern roundabout in the United Kingdom during the 1960s.

roundabout	traffic circle
Entering vehicles yield	Stop sign, stop signal, or giving priority to entering vehicles
Vehicles in the roundabout have priority over the entering vehicle	weaving areas to resolve conflicted movement
Use deflection to maintain low speed operation	Some large circles provide straight path for higher speed
No parking is allowed	Some large circles permit parking within the circle
Pedestrians are (usually) prohibited from the central island	Some large circles allow pedestrians on central island
All vehicles circulate around the central island	Mini-traffic circles with left-turning vehicles passing to the left¹ of the central island.
(Source for table: Oregon Department of Transportation [link]) 1. For countries that drive on the right-hand side of the road.

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 patrolling each major intersection within the circle.

See also

• List of traffic circles in New Jersey
• List of Circles in Washington, D.C.

Types of road junction
Interchanges (grade separated)	Cloverleaf - Diamond - Directional T - Diverging diamond Parclo - Trumpet - SPUI - Stack
Intersections (at-grade)	Box junction - Continuous flow - Crossroads - Hook turn Jughandle - Michigan left - Roundabout - 3-way junction - Traffic circle

 

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