Design process

The process of designing roundabouts, more so than other forms of intersections, requires a considerable amount of iteration among geometric layout, operational analysis, and safety evaluation. As described in Chapters 4 and 5, minor adjustments in geometry can result in significant changes in the safety and/or operational performance. Thus, the designer often needs to revise and refine the initial layout attempt to enhance its capacity and safety. It is rare to produce an optimal geometric design on the first attempt. Exhibit 6-2 provides a graphical flowchart for the process of designing and evaluating a roundabout.

Because roundabout design is such an iterative process, in which small changes in geometry can result in substantial changes to operational and safety performance, it may be advisable to prepare the initial layout drawings at a sketch level of detail. Although it is easy to get caught into the desire to design each of the individual components of the geometry such that it complies with the specifications provided in this chapter, it is much more important that the individual components are compatible with each other so that the roundabout will meet its overall performance objectives. Before the details of the geometry are defined, three fundamental elements must be determined in the preliminary design stage:

1. The optimal roundabout size;
2. The optimal position; and
3. The optimal alignment and arrangement of approach legs.