A review of constant time headway policy for automatic vehicle following

This article reviews the relevant issues in constant time headway (CTH) schemes in adaptive cruise control (ACC) systems based on system-level considerations of stability, capacity and safety. From the perspective of stability, CTH schemes can be implemented autonomously while guaranteeing that errors in spacing do not amplify during any maneuver of the lead vehicle in the string. The corresponding control efforts of following vehicles are filtered versions of their immediately preceding vehicles. The primary benefit of ACC systems in terms of relieving congestion is the improvement of throughput; however, this will require a reliable distance regulation at traffic capacity.