Introduction

Designing the geometry of a roundabout involves choosing between trade-offs of safety and capacity. Roundabouts operate most safely when their geometry forces traffic to enter and circulate at slow speeds. Horizontal curvature and narrow pavement widths are used to produce this reduced-speed environment. Conversely, the capacity of roundabouts is negatively affected by these low-speed design elements.

As the widths and radii of entry and circulatory roadways are reduced, so also the capacity of the roundabout is reduced. Furthermore, many of the geometric parameters are governed by the maneuvering requirements of the largest vehicles expected to travel through the intersection. Thus, designing a roundabout is a process of determining the optimal balance between safety provisions, operational performance, and large vehicle accommodation.

While the basic form and features of roundabouts are uniform regardless of their location, many of the design techniques and parameters are different, depending on the speed environment and desired capacity at individual sites. In rural environments where approach speeds are high and bicycle and pedestrian use may be minimal, the design objectives are significantly different from roundabouts in urban environments where bicycle and pedestrian safety are a primary concern. Additionally, many of the design techniques are substantially different for single-lane roundabouts than for roundabouts with multiple entry lanes.

This chapter is organized so that the fundamental design principles common among all roundabout types are presented first. More detailed design considerations specific to multilane roundabouts, rural roundabouts, and mini-roundabouts are given in subsequent sections of the chapter.