Ruihua Tao (rtao@sha.state.md.us;)

and

Virginia P. Sisiopiku (sisiopik@egr.msu.edu)

Department of Civil and Environmental Engineering

Michigan State University

3546 Engineering Building

East Lansing, MI 48824

Individual vehicular speed change is a continuous process with respect to both time and distance. When a vehicle follows closely another, the individual vehicle speeds affect each other. This speed interaction can propagate upstream, when vehicles in a traffic stream are close enough to one another. On the other hand, if traffic is not heavy, following vehicles do not necessarily respond to temporary speed changes of leading vehicles, and small speed disturbance can be simply absorbed by the traffic stream. Lorenz and Elefteriadou provided the evidence of this phenomenon through their field observations⁽¹⁾. Field data confirm that the car following behavior is affected by variations in the extent of the speed disturbance, the time required by the lead vehicle to recover its speed, and the spacing between the lead and following vehicles.

Despite the popularity of car-following theories little research has been reported on interactive car-following with emphasis on speed disturbance and spacing concepts. In this respect, two major issues need to be addressed. First, what is the amount of speed disturbances that a traffic stream can absorb, and second, whether or not there exists an upper limit on the magnitude of speed disturbance that can be absorbed at a certain flow rate.

This paper presents the results from a theoretical analysis of interactive car-following behavior that examined a lead vehicle with speed disturbance and speed recovery, and the response of the following vehicle. Microscopic car following models were developed to dynamically calculate spacing between two consecutive vehicles. Moreover, the upper limits on the magnitude of speed disturbance that a spacing can absorb, without causing a breakdown, were obtained. The following three car-following scenarios were considered:

A spacing-speed disturbance plane was developed to illustrate the relationship between spacing and speed disturbance for each scenario. Finally, a time-speed disturbance plane was created to depict the relationship between the time interval, during which the following vehicle is allowed to maintain its original speed, and the upper limits on the magnitude of speed disturbance of the lead vehicle.

The findings in this paper are expected to enhance the current understanding of car-following behavior and provide the background for further research on interactive car-following theories.

1. Lorenz, M., and Elefteriadou, L., A Probabilistic Approach to Defining Freeway Capacity and Breakdown. Fourth International Symposium on Highway Capacity, Proceedings, Transportation Research Board, 2000.