What is a Roundabout?

A Roundabout is a type of circular intersection that has been successfully implemented in Europe and Australia, and more recently in the USA. The lack of acceptance can generally be attributed to the negative experience with traffic circles or rotaries built in the earlier half of the twentieth century. Safety and operational problems caused these traffic circles to fall out of favor by the 1950's. However, substantial progress has been achieved in the subsequent design of circular intersections, and a Modern Roundabout should not be confused with the traffic circles of the past.
Roundabouts have the following features preventing the safety and capacity problems associated with older rotaries and traffic circles:

Yield at Entry: In roundabouts, entering motorists must yield to those inside the circle. Since circulating traffic is always moving, the roundabouts can be kept compact.

Deflection: Because roundabouts are compact (usually 25 to 70 meters in diameter), and entering motorists are deflected around a central island, speeds are low and safety of pedestrians and vehicles is high.

High Capacity: Roundabouts can handle high traffic flows (frequently 2,000 vehicles per hour for a single-lane design) because the alternation of flows at the intersection is entirely traffic responsive (i.e. motorists do not always have to stop). Flaring of the roundabout entries can further increase entry capacity.

Large Vehicles: Roundabouts are of sufficient size to allow large vehicles to turn left by travelling around the circle, the same as with other traffic.

Splitter Islands: Splitter islands are required on each approach to ensure motorists travel around them in the proper direction, and to provide refuge areas for pedestrians to cross the approach to a roundabout.

Well-designed roundabouts in appropriate locations also increase safety, reduce delay and provide environmental benefits.

Increased Safety: A study of 24 intersections conducted for the Insurance Institute for Highway Safety, entitled "Crash Reductions Following Installation of Roundabouts in the United States", concluded that roundabouts reduced collisions of all types by 39%, reduced injury collisions by 76%, as well as reduced fatal and incapacitating collisions by about 90%. This is due to the fact that roundabouts force vehicles to travel at slower speeds, and provide fewer conflict points and collision angles for for motorists and pedestrians.

Reduced Delay: Since motorists do not always have to stop, delays and queues are usually less compared to Stop signs and traffic signals (especially signals with long cycle lengths and multiple phases). This is true under non-peak traffic conditions and especially evident at high traffic flows.

Environmental Benefits: With reduced delay, roundabouts can reduce noise, air pollution and fuel consumption. They also provide opportunities for landscaping and gateway treatments.

When societal costs associated with crash incidence are accounted for, roundabouts are often less expensive than other intersection control alternatives. A high-capacity roundabout can reduce the need to widen adjacent roads, a significant consideration with highway interchange tunnels or overpasses. Increased safety means lower emergency response, health care and other societal costs. Roundabouts do not specifically require periodic maintenance like traffic signals, and there is no need to be concerned with power outages!