System Considerations

Roundabouts have been considered as isolated intersections in most other international roundabout guides and publications. However, roundabouts may need to fit into a network of intersections, with the traffic control functions of a roundabout supporting the function of nearby intersections and vice versa. The purpose of this chapter is to provide some guidance on potentially difficult, but not uncommon, circumstances or constraints.

Many countries whose initial design and driver experience was with isolated roundabouts have since extended their application to transportation system design and operation. This chapter addresses the appropriate use of roundabouts in a roadway network context and the benefits obtained. Since the design of each roundabout should generally follow the principles of isolated roundabout design, the discussion is at a conceptual and operational level and generally complements the planning of isolated roundabouts discussed in Chapter 3. In many cases, site-specific issues will determine the appropriate roundabout design elements.

To establish some fundamental understanding for subsequent discussion, three design issues at an isolated roundabout are presented. First, this chapter will describe the requirements and effects of signal control of one or more legs of a roundabout, as well as the entire roundabout. It is noted that fully signalized roundabouts are not desirable. Next, modified designs that incorporate at-grade rail crossings are discussed. It is noted that intersections with rail lines passing through them or near them are not desirable. However, these situations do occur and would then need to be analyzed.

Building upon this understanding, the next sections address design and performance of two closely spaced roundabouts and the specific application to roundabout interchanges. This is followed by issues pertaining to the use of roundabouts on an arterial or network that may include or replace coordinated signalized intersections. Finally, the role of microscopic simulation models in assisting with analysis of these system effects is reviewed.