Concept of roundabout capacity

The capacity of each entry to a roundabout is the maximum rate at which vehicles can reasonably be expected to enter the roundabout from an approach during a given time period under prevailing traffic and roadway (geometric) conditions. An operational analysis considers a precise set of geometric conditions and traffic flow rates defined for a 15-minute analysis period for each roundabout entry. While consideration of Average Annual Daily Traffic volumes (AADT) across all approaches is useful for planning purposes as provided in Exhibit 1-13 and Chapter 3, analysis of this shorter time period is critical to assessing the level of performance of the roundabout and its individual components.

The capacity of the entire roundabout is not considered, as it depends on many terms. However, Exhibit 1-13 provides threshold average daily traffic volumes for the various categories of roundabouts, assuming four legs. Below these thresholds, a four-legged roundabout with roadways intersecting perpendicularly should have adequate capacity (provided the traffic volumes are reasonably balanced and the geometry does not deviate substantially from those shown on the design templates in Exhibits 1-7 through 1-12). The focus in this chapter on the roundabout entry is similar to the operational analysis methods used for other forms of unsignalized intersections and for signalized intersections. In each case, the capacity of the entry or approach is computed as a function of traffic on the other (conflicting) approaches, the interaction of these traffic streams, and the intersection geometry.

For a properly designed roundabout, the yield line is the relevant point for capacity analysis. The approach capacity is the capacity provided at the yield line. This is determined by a number of geometric parameters in addition to the entry width. On multilane roundabouts it is important to balance the use of each lane, because otherwise some lanes may be overloaded while others are underused. Poorly designed exits may influence driver behavior and cause lane imbalance and congestion at the opposite leg.