Revisiting the Cycle Length

During oversaturation, a popular objective in traffic signal operations is to maximize throughput to keep traffic moving. As cycle lengths are increased, the proportion of lost time used to transition between signal phases is reduced. This factor is often a rationale for programming long cycle lengths into signal timing plans. An investigation of the impact of cycle length revisited the concept of the use of critical lane analysis to calculate throughput and applied the technique to data collected at an oversaturated intersection in Indianapolis, Indiana. Traffic volumes were measured for 10 weeks while various cycle lengths, ranging from 80 to 135 s, were tested at the intersection. During saturated conditions, no clear increase in the sum of critical lane throughput was observed, even when the cycle length increased by more than 50%. At 135 s, there was a slight reduction in the total critical lane sum volume. These findings concur with a recent study by Denney et al. The decrease in throughput during the longer cycle lengths is attributed to the reduction of saturation flow during long green times. Possible results of use of a time-dependent saturation flow rate are discussed. Additionally, critical lane analysis may have applications to evaluation and ranking of intersections within corridors as under, near, or over saturation.