Roundabouts and Traffic Circles

History

The first traffic circle in the United States was constructed in New York City in 1905. Located at the southwest edge of Central Park in Manhattan and designed by William Phelps Eno, it features a large monument to Christopher Columbus in the center of the circle and hence is called Columbus Circle. Many similar, large-scale circles (often with a diameter of more than 300 feet) were constructed in other U.S. and European cities, but they proved to have several problems that made them fall out of favor by the 1950s. In these early traffic circles, entering vehicles had the right of way, and this encouraged people to drive at high speeds (often above 30 mph) in the circle, leading to frequent high-speed collisions. In addition, the circles tended to become congested, so that they sometimes impeded the flow of traffic rather than facilitating it.

In the United Kingdom (UK), a rules change in 1966 that gave priority to vehicles already in the circle acted to reduce recurrence of both of these problems—requiring vehicles entering the [Page 1184]traffic circle to yield to those already in it discouraged high-speed entry and reduced the number of crashes, and also helped reduce congestion within the circle by forcing vehicles to wait for gaps in traffic before they could enter. At about the same time, traffic engineers began designing smaller traffic circles, called roundabouts, that also required entering and circulating vehicles to travel more slowly. These improvements in safety made the roundabout a popular intersection design adopted all over the world, although the United States is a fairly late adopter of this design.

Design

Although each roundabout should be designed to meet the needs of a specific situation, taking into account factors such as the volume of traffic expected, the size of vehicles expected to use the roundabout (for example, buses, semi-trailers) and the amount of space available, there are certain commonalities to each roundabout. A typical roundabout replaces an intersection, so multiple roads will approach the roundabout. Around the point where the roads would otherwise intersect, there is a large circular traffic island, often landscaped to alert drivers to the existence of the roundabout. Surrounding this island is a circular roadway; traffic moves in a counterclockwise direction in right-drive countries like the United States, and in a clockwise direction in left-drive regions like the UK.

Each of the roads will have a splitter island (separator island) or median dividing traffic into entry and exit lanes and alerting drivers to the existence of the roundabout. Exit lanes are typically wider than entry lanes to minimize congestion from cars slowing to exit, although this must be balanced against the need for pedestrian access and safety. If truck use is anticipated, the central island usually includes a traversable truck apron, a wide, sloped and slightly raised (for example, 2 to 3 inches off the ground) ring around the island to give trucks additional road space, while being sufficiently elevated to discourage cars from using it.

...