On the Mechanism of Capacity Drop at Freeway Sag and Tunnel Bottlenecks

Abstract

The "capacity drop (CD)" phenomenon, which means the queue discharge flow rate is less than the traffic capacity before congestion, is observed at freeway bottlenecks in general. Although this phenomenon has been known for a long time, its precise mechanism is not necessarily clear. This study conducts an empirical analysis of the CD phenomenon based on the recent advanced continuum traffic flow theory (Jin, 2018; Wada et al., 2020) that describes the CD endogenously for freeway sag and tunnel bottlenecks. Specifically, we first show the ideas, components, and consequences of the theory while reviewing the conventional hypotheses for the CD and their issues. We then compare several theoretical predictions with real data (i.e., detector data and ETC2.0 probe data), qualitatively and quantitatively. The results validate the theory and bring us new interpretations of observations, which will help to advance various traffic control methods for mitigating the CD in the future.