Two-bridge roundabout interchange

There are two basic types of roundabout interchanges. The first is a large diameter roundabout centered over or under a freeway. The ramps connect directly into the roundabout, as do the legs from the crossroad. This is shown in Exhibit 8-5.

This type of interchange requires two bridges. If the roundabout is above the freeway as shown in Exhibit 8-5, then the bridges may be curved. Alternatively, if the freeway goes over the roundabout then up to four bridges may be required. The number of bridges will depend on the optimum span of the type of structure compared with the inscribed diameter of the roundabout island and on whether the one bridge is used for both freeway directions or whether there is one bridge for each direction. The road cross-section will also influence the design decision. Exhibit 8-6 shows an example from the United Kingdom. The designer should decide if the expected speeds of vehicles at larger roundabouts are acceptable.