Single-lane roundabout capacity

Exhibit 4-3 shows the expected capacity for a single-lane roundabout for both the urban compact and urban/rural single-lane designs. The exhibit shows the variation of maximum entry flow as a function of the circulating flow on the roundabout. The calculation of the circulating flow was described previously. The capacity forecast shown in the chart is valid for single-lane roundabouts with inscribed circle diameters of 25 m to 55 m (80 ft to 180 ft). The capacity forecast is based on simplified British regression relationships, which may also be derived with a gap-acceptance model by incorporating limited priority behavior.

Note that in any case, the flow rate downstream of the merge point (between the entry and the next exit) should not be allowed to exceed 1,800 veh/h. Exceeding this threshold may indicate the need for a double-lane entry.

The urban compact design is expected to have a reduced capacity, but has significant benefits of reduced vehicle speeds through the roundabout. This increases safety for pedestrians and bicyclists compared with the larger single lane roundabouts. Mini-roundabout capacities may be approximated using the daily maximum service volumes provided for them in Chapter 3, but in any case should not exceed the capacity of the urban compact design.