Navid Nadimi

Traffic safety estimations can be done in three different approaches: 1- Using crash frequency and crash rates over a long-term or short-term period, 2- Indirect safety measures and 3- Statistical analysis. Among these, indirect methods have become popular in recent years for their advantages. Indirect methods mostly include traffic conflict technique (TCT) and do not use traffic accident data. TCT has changed gradually from subjective methods which rely on human observations to objective ones which apply surrogate safety measures to detect dangerous situations. Till now, different surrogate safety measures have been developed. In this paper first important measures are introduced and then a comprehensive review is done on time-to-collision (TTC). TTC is first defined as the time remaining before the collision if the course and speed of vehicles are maintained constant. In literature, there are so many researches on TTC, therefore it is well extended and now there are different indicators originated from it. However there is not a good connection between all these new concepts and it is not obvious which one must be used in safety evaluations. Thus, here after a review on TTC and its extensions, a useful framework is proposed for better application of this indicator

Time-to-collision, Safety, Traffic conflict technique, Surrogate measure, Motion Equation