Should Adaptive Cruise Control (ACC) Systems Be Designed to Maintain a Constant Time-Gap Between Vehicles?

This paper addresses the stability of traffic flow on a highway when the vehicles operate under an adaptive cruise control (ACC) system. ACC systems are commonly designed to maintain a constant time-gap between vehicles during vehicle following. Previous researchers in 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, the paper shows that it is possible to design an unconditionally stable spacing policy, i.e. a spacing policy which 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 time gap between vehicles?” is NO. It is quite easy to develop alternate spacing policies with superior stability properties.