Traffic circle

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. Traditionally, traffic entering a circle has the right-of-way, although some circles give right-of-way to the primary roads. In roundabouts, as opposed to traffic circles, entering traffic must yield to traffic already in the circulatory roadway.

History

French architect Eugène Hénard was designing one-way circular intersections as early as 1877. American architect William Phelps 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. 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.

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 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.

The largest traffic circle in the world is claimed to be one of those in the Dammam coastal road, Al-Khaleej Street, in Saudi Arabia with a length of approximately 1.9 km (1.20 miles).

Composition

Traffic circles are often composed of concrete or asphalt although more recently rubber curbing is being used to create traffic circles as well, primarily in residential areas. Rubber curbing consists of units of flexible rubber that are bent and installed around a landscaped area to create traffic circles.