Traffic circle

From Academic Kids

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 had the right-of-way, though many circles in New Jersey gave (and still give) right-of-way to the primary roads. In modern 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 328.084 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 Allow 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 [1] (http://www.odot.state.or.us/techserv/engineer/pdu/Roundabouts/general.htm))

1. For countries that drive on the right-hand side of the road.

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