Exploring Properties of Networkwide Flow–Density Relations in a Freeway Network

Abstract

The objective of this study is to investigate the properties of network-level traffic flow relationships in a freeway network with the use of commonly available loop detector data. The impact of the spatial and temporal distribution of congestion in a selected network on the shape and properties of the flow–density relation is investigated, with emphasis on the formation and characterization of hysteresis patterns. Accordingly, a path-dependent characterization of hysteresis patterns in freeway networks is introduced and illustrated conceptually as well as through empirical observations. Comparison of the spatial and temporal distribution of congestion throughout a selected subnetwork on different days suggests a relationship between the size of the hysteresis loop and the inhomogeneity of the traffic distribution. The maximum network average flow is not a constant value but varies across different days. In addition, for the same value of average network occupancy, the variation of occupancy is higher during the recovery period compared with the loading period. The observed large variation in network occupancy during recovery implies the formation of fragmented queues and traffic instability. A chaotic pattern is also to be expected in the networkwide flow–occupancy plane when the spatial distribution of link densities is inhomogeneous and the average network occupancy remains consistently high and roughly unchanged for successive time intervals. Overall, the study results provide a deeper understanding of the properties of networkwide relations on freeway networks.