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Roundabout interchange: Wikis

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A roundabout interchange is a type of interchange between a freeway and a minor road in which the ramps to and from the freeway mainlines converge at a single roundabout, which is grade-separated from the freeway lanes with bridges. A roundabout interchange is similar to a rotary interchange, which uses a rotary rather than a roundabout. Roundabouts may also be used in conjunction with other interchange types such as a standard or folded diamond interchange, though such use should not be confused with a roundabout interchange.

Roundabout interchanges are extremely common in the United Kingdom, with hundreds on the motorway network alone. However, recent cost-cutting has meant that "dumbbell interchange" are increasingly used instead. These are essentially diamond interchanges with roundabouts instead of signals or stop signs where the slip roads meet the minor road. They are cheaper than roundabout interchanges as only one bridge is required instead of two. Roundabout interchanges are much less common in North America, but have been built more frequently since 1995, to improve safety, and to reduce traffic delays and bridge widening costs. However, and conversely, many of the older and more dangerous rotary-style overpass interchanges have been signalized to improve throughput and safety, such as the Drum Hill Rotary in the New England state of Massachusetts, where such interchanges are unusually commonplace.

A "divided diamond," where the minor road is separated into four intersections, rather than two, also acts like a roundabout interchange, however is more square in shape, and typically has traffic light control.

Examples

Three-level stacked roundabout

The three-level stacked roundabout is a variation on the roundabout interchange in which both roads are grade-separated. It is similar to the three-level diamond interchange except that the small square of that latter interchange is enlarged to a true roundabout.

Three-level stacked roundabouts are quite common in Britain because they use less land than other 4-way junctions where both roads are grade separated. However, they have lower capacity for turning movements – some have had direct-linking slip roads added later in an attempt to solve this problem. Examples of such junctions are found at Lofthouse, West Yorkshire (M1/M62)53°43′55″N 1°30′47″W / 53.73194°N 1.51306°W / 53.73194; -1.51306 (Lofthouse, West Yorkshire) and Swanley, Kent (M20/M25) 51°23′19″N 0°11′38″E / 51.38861°N 0.19389°E / 51.38861; 0.19389 (Swanley, Kent).

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