Queue Length at Signalized Intersections from Red-Time Formula and the Highway Capacity Manual Compared with Field Data

Abstract

This study compared the Highway Capacity Manual 2010 (HCM 2010) procedure (the procedure in HCM 2016 is the same) and the red-time formula (RTF) estimations of the back-of-queue with field data results. The comparisons were made for the 50th and 95th percentile field queue lengths at four signalized intersections along a corridor in one off-peak period and a.m., noon, and p.m. peak periods. For the 50th percentile queue length, the HCM estimates had significant differences from the field data in 52% of the cases (major and minor street cases combined); in 93% of which the HCM overestimated the queue length and in 7% it underestimated the queue length. For the major street, in 28% of the cases the HCM significantly overestimated the queue length on average by 66%, and in 4% of the cases the HCM significantly underestimated on average by 42%. For minor streets, in 70% of the cases the HCM significantly overestimated the queue length on average by 44%, and in 5% of the cases it significantly underestimated on average by 20%. To lower the number of cases with significant discrepancy and to balance the frequency of overestimation and underestimation, a multiplicative correction factor of 0.93 for the major street and 0.78 for the minor streets could be applied to the HCM estimates. For the 95th percentile queue length comparison, in general the HCM presented a better estimation than the RTF. And for minor streets, the RTF tends to overestimate the queue length at higher-volume approaches, while it underestimates at lower-volume approaches. But on a major street with heavier traffic than the minor, such a trend is not clear.