Relation between traffic density and capacity drop at three freeway bottlenecks

Three freeway bottlenecks, each with a distinct geometry, are shown to share a relation between vehicle density and losses in discharge flow. Each bottleneck suffered reductions in discharge once queues formed just upstream. This so-called “capacity drop” was related to the density measured over some extended-length freeway segment near each bottleneck. Pronounced increase in this density always preceded a capacity drop. For each bottleneck, the densities that coincided with capacity drops were reproducible. When normalized by a bottleneck’s number of travel lanes and averaged across observation days, the density that coincided with capacity drop was even similar across bottlenecks. (These densities were nearly identical for two of the bottlenecks and the more notable difference observed for the third may be only an artifact of how the data were collected.) The findings indicate that traffic-responsive schemes to control density hold promise for increasing bottleneck discharge flows. Standardized control logic might even suffice for bottlenecks of various forms. With an eye toward future testing and deployment of such control schemes, we present and validate in an Appendix A to this paper a simple algorithm for the real-time measurement of density over freeway links of extended lengths.