6: Queue Discharge Flow and Speed Models for Signalised Intersections

Traditional traffic theory focuses on modelling of queue discharge flow rates (or headways) at signalised intersections with relatively little consideration of the corresponding queue discharge speed characteristics. A simplified queue discharge flow model is commonly employed for estimating capacity and various performance statistics (delay, queue length, etc). This model uses a constant saturation flow rate and converts the displayed green time to an effective green time using start loss and end gain time parameters associated with the saturation flow rate (Webster and Cobbe 1966, Akçelik 1981, Teply, et al. 1995, TRB 2000).

Akçelik, Besley and Roper (1999) described exponential queue discharge flow rate and speed models that represent queue discharge behaviour without resorting to various simplifying assumptions needed to derive saturation flows and effective green times. Modelling of queue discharge speed, in addition to the queue discharge flow rate (or headway), makes it possible to derive relationships for traffic parameters such as vehicle spacing, gap length, density, time and space occupancy ratios, gap time, occupancy time, space time as well as vehicle acceleration characteristics. Thus, a complete set of fundamental relationships for queue discharge behaviour at traffic signals is obtained. These relationships are particularly useful for adaptive control purposes (Akçelik 1997, AUSTROADS 1993, Lowrie 1996), and important in relation to microsimulation modelling (Akçelik and Besley 2001a).