The impact of adaptive cruise control systems on highway safety and traffic flow

This paper deals with the design of new adaptive cruise control (ACC) systems that can improve traffic flow while at the same time ensuring safe operation on today's highways. ACC systems are commonly designed to maintain a constant time-gap (CTG) between vehicles during vehicle- following. In this paper, a new inter-vehicle spacing policy in which the inter-vehicle spacing is a non-linear function of vehicle speed is developed. The new spacing policy, referred to as a variable time-gap (VTG) policy, is shown analytically to lead to better traffic flow and a higher highway capacity. Practical advantages of using the new spacing policy are demonstrated through traffic simulations. However, a detailed analysis of safety shows that the traditional CTG policy is superior in several scenarios. The VTG policy is then modified by explicitly taking inter-vehicle relative velocity into account in the definition of desired spacing. The resulting new spacing policy is shown to retain the advantages of stable traffic flow and a higher capacity while providing the same level of safety as the CTG policy.