THE HYSTERESIS PHENOMENON IN TRAFFIC FLOW

The application of aerial survey methods to record and study the movement of traffic on an urban freeway led to the isolation of the phenomenon of traffic flow hysteresis. This phenomenon is manifested as a generally retarded behavior displayed by a platoon of vehicles after emerging from a kinematic disturbance as compared to the behavior of the same vehicles approaching the disturbance. Although data suitable for the study of the phenomenon is still quite scarce, the appearance of the retardataion effect has been confirmed for all ten platoons studied to date. This paper undertakes an analysis of the nature of the hysteresis phenomenon from both a macroscopic and a microscopic point of view. Macroscopically attention is concentrated on the relationships existing between speed and density, volume and density, kinetic energy and density and mean headway and density. Each relationship is shown to dispaly distinctly different characteristics for disturbance approaching and disturbance leaving conditions. Microscopically consideration is given to the acceleration-deceleration asymmetry which underlies the hysteresis phenomenon and the concepts of car following theory are employed to provide further insight into the fundamental nature of the phenomenon. It is thought that the hysteresis phenomenon has substantial implications for the design of modern freeway control systems and further efforts toward its exploration are planned. /Author/