A Microscopic Investigation into the Capacity Drop: Impacts of the Bounded Acceleration and Reaction Time

Abstract

The capacity drop indicates that the queue discharge rate is lower than the free-flow capacity. Studies show that queue discharge rates vary under different traffic conditions. Empirical data show that the queue discharge rate increases as the speed in congestion increases. Insights into the underlying behavioral mechanisms that result in such variable queue discharge rates can help minimize traffic delays and eliminate congestion. However, to the best of the authors’ knowledge, few efforts have been devoted to testing impacts of traffic behaviors on the queue discharge rate. This paper tries to fill this gap. We investigate to what extent the acceleration spread and reaction time can influence the queue discharge rate. It is found that the (inter-driver) acceleration spread does not reduce the queue discharge rates as much as found empirically. Modelling reaction time might be more important than modeling acceleration for capacity drop in car-following models. A speed-dependent reaction time mechanism for giving variable queue discharge rates is proposed. That is, decreasing reaction time as the speed in congestion increases can give the same queue discharge rate as found empirically. This research suggests that motivating drivers to speed up earlier could increase the queue discharge rate and thereby minimize delays.