Type of interchange between highways and minor roads
A roundabout interchange is a type of
interchange between a controlled access highway, such as a
motorway or
freeway, and a minor road. The slip roads to and from the motorway carriageways converge at a single
roundabout, which is
grade-separated from the motorway lanes with bridges.
Design
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, but such use should not be confused with a roundabout interchange.
Roundabout interchanges are extremely common in the
United Kingdom and
Ireland with hundreds on the
motorway network alone. However, recent
cost cutting has meant that
dumbbell interchanges 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, many of the older and more dangerous
rotary-style overpass interchanges have been signalized to improve throughput and safety, such as the former Drum Hill Rotary (now Drum Hill Square) in
Chelmsford, Massachusetts, in
New England, where such interchanges are unusually common.
A
divided diamond, in which the minor road is separated into four intersections, rather than two, also acts like a roundabout interchange, but it is more square in shape and, typically, has traffic light control.
Washington Circle, in
Foggy Bottom,
Washington, D.C., is another example of a roundabout interchange similar to the one in Dupont Circle. Here, K Street traverses beneath the intersection.
A roundabout is used as an interchange between
Quebec Autoroute 20 and
Quebec Autoroute 520 in
Montreal,
Quebec. The roundabout is known as the Dorval Circle. Ramps to side streets are added as well. The interchange is currently[when?] being replaced.
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. If the roundabout is sufficiently large, the interchange may require only two levels.[1]