An important issue in developing Collision Avoidance Systems (CASs) is to define a proper warning strategy, that warns the driver only when he or she is really in danger and immediate action is required. The results of several studies point to the direct use of Time-To-Collision (TTC) as a cue for decision-making in traffic. Therefore, TTC could be an adequate criterion for activating a CAS. First tests indicate that drivers prefer TTC-based warning strategies. Recently, some studies on driver behaviour in fog enable a more precise definition of CAS-critical situations and relevant criterion values for TTC. TTC values in the range of 4.5 to 5 appear to be the most suitable. Special attention should be given to visibility conditions between 40 and 120 meter, since in that range driving speeds are found much too high to stop in time for an unexpected obstacle. (A)
Abstract