Rationale for Incorporating Queue Discharge Flow into Highway Capacity Manual Procedure for Analysis of Freeway Facilities

The freeway facilities methodology in the 2010 Highway Capacity Manual (HCM) is the only HCM methodology that encompasses undersaturated and congested flow regimes over multiple periods. However, the methodology is limited by its assumption of a fixed capacity threshold between the two flow regimes. The method does not consider the two-capacity phenomenon, which suggests that a drop in the throughput from theoretical capacity is observed after breakdown has occurred. A summary of the available literature on empirical evidence of the capacity drop under queue discharge conditions offers a theoretical evaluation of the impact on queue discharge flow based on shock wave theory relationships that form the basis of the queuing model used in the HCM freeway facilities method. Examples illustrate incorporating queue discharge into a freeway facilities analysis, and implications for practice are discussed.