Time-to-collision and Collision Avoidance Systems

In research on Traffic Conflicts Techniques, Time-To-Collision (TTC) has proven to be an effective measure for rating the severity of traffic conflicts and for discriminating critical from normal behaviour. The results of several studies point to the direct use of TTC as a cue for decision-making in traffic. The current development of driver support systems based on the application of modern technologies makes it necessary to have detailed knowledge on how drivers operate, and at what level and when a system should warn the driver or take action. An important issue in developing Collision Avoidance Systems (CAS) is to define a proper warning strategy, that warns the driver only when the driver is really at danger and immediate action is required. Misses should be avoided, but too many false alarms may easily cause the system to become a nuisance to the driver. The present paper deals with the use of the TTC measure to define an adequate criterion for activating a driver support system such as CAS in order to reduce the number of rear-end collisions on a motorway. Reduced visibility conditions (e.g. due to fog) frequently cause multivehicle crashes with very severe consequences. The results of a field experiment where subjects approaching a stationary object were instructed to start braking at the latest moment, reveal that both the decision to start braking and the control of braking may well be based on TTC information available from the optic flow field. First tests with Collision Avoidance Systems (CAS) indicate that warning strategies based on a TTC-criterion are preferred by the drivers and seem to be most in line with what drivers expect to get from a CAS. These tests only refer to a fixed 4 s TTC-criterion. Recently, some studies on driver behaviour in fog enable a more precise definition of CAS-critical situations and relevant criterion values for TTC. Based on the analysis of driver behaviour in adverse visibility conditions, it is concluded that it is worthwhile to investigate a TTC criterion in the range of 4.5 to 5 s for activating a CAS. Especially in the visibility range between 40 and 120 m free-driving speeds are too high, and therefore, special attention should be given to this distance range when developing a CAS. 6 ICTCT workshop Salzburg Proceedings