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
. In modern traffic circles, entering traffic must yield to traffic already in the circulatory roadway.
French architect Eugene Henard was designing one-way circular intersections as early as 1877. American architect William Eno favored small traffic circles. He desigend 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:
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.
- 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 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.