Capacity Drop: a Relation between the Speed in Congestion and the Queue Discharge Rate

It has been empirically observed for years that the queue discharge rate is lower than the pre-queue capacity. This is called the capacity drop. The magnitude of capacity drop varies over a wide range depending on the local traffic conditions. However, right now it is unknown what determines the capacity drop value. In fact, there is still no thorough empirical analysis revealing a quite reliable relation between the capacity drop and the congestion level. Therefore, this paper tries to fill in the gap by revealing the relation between the vehicle speed in congestion and the queue discharge rate through empirical analysis. Our research studies congested states where speed ranges from 6 to 60 km/h. The queue discharge rate is shown to increase considerably with increasing speed in the congestion. In contrast to previous research, this study draws the relation based on empirical data collected on freeways and the data presents a sufficiently large observation sample. A discussion about the influence of weather and study site characteristics on the discharge rate indicates that the relation needs site-specific calibrations. This study sheds light on a better prediction of capacity drop and a better understand in a theoretical sense on the fluctuation of capacity drop. Yuan, Knoop, Hoogendoorn 3