From Roader's Digest: The SABRE Wiki
A roundabout is a circular road junction with traffic flowing clockwise along the circulatory carriageway around a central island. Traffic wishing to enter the roundabout must give way to traffic already on it. In parts of the Midlands and the North West of England, the junctions are colloquially known as islands, not to be confused with pedestrian refuges. Elsewhere the term island refers to the central area within the junction.
The first roundabout in Great Britain was built at Sollershott Circus in Letchworth Garden City in 1909.
Roundabouts are designed in accordance with TD16 of DMRB. Key features of the design include entry deflection, where the geometry of the roundabout is designed to slow vehicles down on the approach to the roundabout. Capacity of the roundabout is dependent on its size, usually measured across the Inscribed Circle Diameter (the largest circle that can be drawn touching the outer edge of the circulatory carriageway; on the entry width of the approach roads; and on the width of the circulatory carriageway (usually 1.0 to 1.2 times the widest entry width). Other factors include the flare length on each approach road (which is a measure of widening of each approach) and the entry angle and entry radius of each approach road.
There are a number of forms of roundabout, namely:
- Normal Roundabout
- Compact Roundabout
- Mini Roundabout
- Signallised Roundabout
- Grade Separated Roundabout
A normal roundabout has a centre island of at least 4m in diameter (often much bigger), and usually has flared entries to allow two or three vehicles to enter the roundabout at the same time. They may be found in urban and rural areas, and are often the junction of choice between rural main roads.
Compact roundabouts are found in urban areas, and have a small kerbed island, often less than 4m in diameter and with single lane approaches. They often have overrun strips around the central island in order to allow larger vehicles to negotiate the junction.
A mini-roundabout is a "priority to the right" control, used at junctions where a kerbed island cannot be accommodated. They usually consist of a painted central island, sometimes slightly raised, with arrows to indicate the direction of circulation.
A signalised roundabout is based on a conventional roundabout, but with one or more entries signalised to control entry. Signals may be part time or full time operation, and are used to increase capacity of the junction, allowing busier roads a higher level of priority at the junction. Some signalised roundabouts are conversions of existing roundabouts, whereas others are built with signals from the outset. Larger signalised roundabouts almost always have full-time signals as wide entries and wide circulatory carriageways make it very unlikely that the roundabout would comply with deflection requirements for an unsignalised roundabout.
There are many examples of signalised roundabouts, for example Oldings Corner.
Grade Separated Roundabout
A grade separated roundabout is a large roundabout, with one or more roads passing over or under the roundabout, and connected to it by means of slip roads. The roundabout may be signalised. As a large roundabout, vehicle speeds can be high, and the roundabout may not fully comply with roundabout deflection standards.
An example of a grade separated roundabout is Priory Wood Roundabout
An example of a large signalised grade separated roundabout is Bignells Corner.