Within the context of this chapter, capacity is the maximum number of vehicles that can pass through a roundabout entry during an interval of time. The computational steps in

Section 12.2.2

explain how to determine approximate daily service volumes and peak hour service volumes for a roundabout. Software such as SIDRA, Synchro, or Junctions may also be used for a more detailed determination of capacity. Vissim and SimTraffic do not have a direct entry capacity output and cannot be used to determine exact capacity values for a roundabout.

The volume-to-capacity (V/C) ratio describes the number of vehicles entering from one approach to the roundabout (volume) divided by the maximum number of vehicles that can pass through that same entry during an interval of time, usually one hour. The V/C ratio is typically used as an indicator of the level of saturation of a roundabout entry. If V/C ratio exceeds 0.85, consider delay and queue length more carefully and conduct sensitivity testing using alternate models, geometry and/or unanticipated traffic flows).