From Wikipedia, the free encyclopedia.

A roundabout, rotary, or gyratory circus is a type of road junction (or traffic calming device) at which traffic streams circularly around a central island after first yielding to the circulating traffic. Roundabouts are also sometimes referred to as traffic circles, though technically a distinction between the two was made in the mid-1960s:

roundabout	traffic circle
Entering vehicles yield	Stop sign, stop signal, or giving priority to entering vehicles
Vehicles in the roundabout have priority over the entering vehicle	Allow weaving areas to resolve conflicted movement
Use deflection to maintain low speed operation	Some large circles provide straight path for higher speed
No parking is allowed	Some large circles permit parking within the circle
Pedestrians prohibited from the central island	Some large circles allow pedestrians on central island
All vehicles circulate around the central island	Mini-traffic circles with left-turning vehicles passing to the left of the central island.
(Source for table: Oregon Department of Transportation [1])

British engineers reengineered the traffic circle in the mid-1960s to overcome its limitations of capacity and for safety issues. Unlike traffic circles, roundabouts operate with yield control to give priority to circulating traffic and eliminate much of the driver confusion associated with traffic circles and driver wait associated with signalized intersections. Roughly the same size as signalized intersections with the same capacity, roundabouts also are significantly smaller in diameter than traffic circles, separate incoming and outgoing traffic with pedestrian islands and therefore encourage slower and safer speeds (see traffic calming).

Roundabouts are safer than both traffic circles and traditional intersections -- having 40% fewer vehicle collisions, 80% fewer injuries and 90% fewer serious injuries and deaths (compared with a sampling of roundabouts in the United States with the intersections they replaced). Roundabouts also significantly reduce potential points of conflict between pedestrians and motorized traffic and are therefore considered to be safer for them. Roundabouts, especially large fast moving ones, are however unpopular with, and can be dangerous for cyclists.

In addition to improved vehicle and pedestrian safety, and in spite of lower speeds, roundabouts dramatically outperform traffic circles in terms of vehicle throughput and, because circular traffic is always moving, in roundabouts, they outperform signalized traffic signals as well.

However, due to the fact that vehicle traffic must yield instead of stop, there are some safety concerns for bicyclists who ride on pedestrian walkways and especially for persons with visual impairments. Safety concerns for the second group of people is especially important in countries that have legislation protecting the rights of people with disabilities.

In addition, roundabouts do not cope with the traffic on motorways or similar roads, thus leading to long tailbacks when they are encountered by the motorist. Britain's strategic road network has many isolated roundabouts on otherwise almost motorway-like roads (for example, A1/A421) and even on motorways in the UK (for example, the A601(M), A627(M), M271 and even the M60 have roundabouts on the main line). Some of these roundabouts, as well as other busy roundabouts, have been signalized.

Types of Roundabout

In the UK, there are many variations in the design of roundabouts. Large roundabouts such as those used at motorway intersections typically have two to four lanes around the central hub, and frequently have traffic lights regulating flow during peak hours. Mini-roundabouts have a central island which is usually domed in shape and flush with the road surface at its edge, enabling drivers of larger vehicles to cut across the center whilst turning.

Some roundabouts have a divider between traffic turning left (in right-hand drive countries) and other traffic, enabling those making left turns to bypass the roundabout entirely. Another type of roundabout is the through-about roundabout or "hamburger" junction. This type of roundabout enables straight-through traffic on one road to cross over the central island, whilst all other traffic must drive around the island. As a consequence this junction must always be controlled by traffic lights. Examples of this type exist in Reading, England and Bracknell