This paper addresses the stability of traffic flow on a highway when the vehicles operate under an adaptive cruise-control (ACC) system. These systems are commonly designed to maintain a constant time gap between vehicles during vehicle following. Previous researchers in the literature have produced contradictory results on whether the traffic flow is stable when the constant time-gap spacing policy is used. This paper resolves the contradiction and shows that the boundary conditions used at the inlets and exits influence traffic-flow stability in the case of the constant time-gap policy. Further, this paper shows that it is possible to design an unconditionally stable spacing policy, i.e., a spacing policy that guarantees traffic stability under all boundary conditions. The practical implications of instability are shown through traffic-simulation results. The advantages of an unconditionally stable spacing policy over the constant time-gap policy are demonstrated. The answer to the question "Should ACC systems be designed to maintain a constant rime gap between vehicles?" is "No" from a traffic-flow stability perspective. It is quite easy to develop alternate spacing policies with superior stability properties.
Bibliographical noteFunding Information:
Manuscript received September 14, 2002; revised July 11, 2003, December 19, 2003, and March 1, 2004. This project was supported in part by a Research Grant from the Intelligent Transportation Systems Institute, University of Minnesota, Minneapolis. The authors are with the Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: email@example.com). Digital Object Identifier 10.1109/TVT.2004.832386
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