Wide nodes and narrow roads

The ultimate manifestation of roundabouts in a system context is to use them in lieu of signalized intersections. Some European cities such as Nantes, France, and some Australian cities have implemented such a policy. It is generally recognized that intersections (or nodes), not road segments (or links), are typically the bottlenecks in urban roadway networks. A focus on maximizing intersection capacity rather than widening streets may therefore be appropriate. Efficient, signalized intersections, however, usually require that exclusive turn lanes be provided, with sufficient storage to avoid queue spillback into through lanes and adjacent intersections. In contrast, roundabouts may require more right-of-way at the nodes, but this may be offset by not requiring as many basic lanes on the approaches, relative to signalized arterials. This concept is demonstrated in Exhibit 8-10.

Analysis tools, such as those provided in Chapter 4, should be used to evaluate the arterial or network. These may be supplemented by appropriate use of microscopic simulation models as discussed next. Supplemental techniques to increase the capacity of critical approaches may be considered if necessary, such as bypass lanes, flaring of approaches and tapering of exits, and signalization of some roundabout approaches.