What is a roundabout?
A modern roundabout is a circular intersection where drivers travel counterclockwise around a center island. There are no traffic signals or stop signs in a modern roundabout. Drivers yield at entry to traffic in the roundabout, then enter the intersection and exit at their desired street.
Studies by the Federal Highway Administration have found that roundabouts can increase traffic capacity by 30 percent to 50 percent compared to traditional intersections.
Traffic circles, traffic calming circles, and roundabouts
There are many differences between modern roundabouts, traffic circles (also known as rotaries) often found on the East Coast and in Europe, and neighborhood traffic calming circles.
Traffic circles, or rotaries, are much larger than modern roundabouts. The graphic at right shows the size of a traffic circle (in green) compared to the smaller modern roundabout (in grey). Traffic circles often have stop signs or traffic signals within the circular intersection. The Arc de Triomphe in Paris and Dupont Circle in Washington, D.C., are two examples of older-style traffic cicles.
Drivers enter a traffic circle in a straight line and do not have to yield to traffic already in the circle. Traffic circles typically become congested if many vehicles enter at the same time.
Neighborhood traffic calming circles (right) are much smaller than modern roundabouts and often replace stop signs at four-way intersections. They are typically used in residential neighborhoods to slow traffic speeds and reduce accidents but are typically not designed to accommodate larger vehicles. Many drivers often turn left in front of the circles rather than turning around them.
Modern roundabouts (right) are designed to accommodate vehicles of all sizes, including emergency vehicles, buses, and truck and trailer combinations. In a modern roundabout, drivers enter the intersection by navigating a gentle curve. Drivers yield at entry to traffic already in the roundabout, then proceed into the intersection and exit at their desired street.
A main feature of the modern roundabout is a raised central island. The circular shape is designed to control the direction of traffic and reduce speeds to 15 to 20 mph. It also reduces the likelihood of t-bone or head-on collisions.
The central island of many roundabouts includes a truck apron (bottom right), a raised section of concrete that acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn, while the raised portion of concrete discourages use by smaller vehicles.
In addition to the central island, roundabouts also feature triangular splitter islands designed to slow and direct traffic. The islands also provide a refuge for pedestrians. This means pedestrians can choose to cross one direction of traffic at a time and have a safe place to wait before crossing another direction of traffic.
Brian Walsh, Traffic Design Engineer