Queue length

Queue length is important when assessing the adequacy of the geometric design of the roundabout approaches. The average queue length (L-vehicles) can be calculated by Little’s rule, as shown in Equation 4-8 (17):

Average queue length is equivalent to the vehicle-hours of delay per hour on an approach. It is useful for comparing roundabout performance with other intersection forms, and other planning procedures that use intersection delay as an input.

For design purposes, Exhibit 4-10 shows how the 95th-percentile queue length varies with the degree of saturation of an approach (18, 19). The x-axis of the graph is the degree of saturation, or the ratio of the entry flow to the entry capacity. Individual lines are shown for the product of T and entry capacity. To determine the 95th-percentile queue length during time T, enter the graph at the computed degree of saturation. Move vertically until the computed curve line is reached. Then move horizontally to the left to determine the 95th-percentile queue length. Alternatively, Equation 4-9 can be used to approximate the 95th-percentile queue. Note that the graph and equation are only valid where the volume-to-capacity ratio immediately before and immediately after the study period is no greater than 0.85 (in other words, the residual queues are negligible).