An empirical study on traffic safety indicators for the analysis of car-following conditions

The likelihood that a car-following driving condition can lead to a rear-end collision is usually assessed through the Time Headway (TH) and Time To Collision (TTC), considered as safety indicators of traffic conditions. The present study computed and compared TH and TTC on the basis of data recorded by means of a traffic control system of an Italian highway. Specifically four measurements sites were investigated for two days and almost 200000 travelling vehicles were collected. About 50% of vehicles were travelling in car-following conditions. The overall objectives of this paper consisted in verifying if, on highways, TH and TTC are independent of each other and, therefore, do not provide the same information on the driver behaviour in car-following situation but could be used for evaluating different conditions. Moreover the effects of different visibility conditions (day or night driving), the type of lead vehicle (passenger car or heavy vehicle) and the type of travelling lane (left or right) on TH and TTC were evaluated in order to increase the knowledge of drivers behavior in car-following conditions. The results demonstrated that TH and TTC provide different information on driver behaviour in car-following conditions as they can be considered practically independent of each other. Moreover it was found that shorter THs are used by the following driver during daytime, behind passenger vehicle and driving on the passing lane. On the contrary only few cases with critical TTC values were recorded, demonstrating that TTC is more effective for detecting impending dangerous situations.