Roundabouts are an increasingly common form of intersection control that are typically circular, use yield control, and have geometric features that promote desirable vehicular speeds and provide accessibility to all roadway users. Common geometric features of a roundabout include a central island, traversable apron, splitter islands (channelizing islands), a circulatory roadway, entrance lines, pedestrian crossings, and landscape strips. There are three types of modern roundabouts:

Roundabouts vary in size, with mini-roundabouts and compact roundabouts ranging from 70 feet to 110 feet in inscribed circle diameter (ICD). Single lane and multilane roundabouts range from 110-feet to 200-feet ICD (refer to

RDM Chapter 14

). Geometric elements of roundabouts, e.g., are only partially addressed in part with the available operational analysis models. Number of entry lanes is dominant key predictive variable of roundabout capacity.

Roundabouts can handle a wide range of vehicular volumes and provide operational and safety benefits in many circumstances.

Figure 12-1

shows the typical mini-roundabout features,

Figure 12-2

shows the typical single-lane roundabout features, and

Figure 12-3

shows the typical multilane roundabout features. For information regarding other roundabout design principles, refer to

RDM Chapter 14

and NCHRP 1043, Chapter 2. For information regarding roundabout interchanges, see

Chapter 11

in this manual.