Comparative Assessment of Safety Indicators for Vehicle Trajectories on Highways
Safety measurement and its analysis have been challenging and well researched topics in transportation. Conventionally, surrogate safety measures have been used as safety indicators in simulation models for safety assessment, in control formulations for driver assistance systems, and in data analysis of naturalistic driving studies. However, surrogate indicators give partial insights on traffic safety; that is, these indicators only indicate a predetermined set of possible precrash situations for an interacting vehicle pair. Recently, a safety indicator called the "driving safety field," based on field theory, was proposed for two dimensional vehicle interactions. However, the objectivity of its functional form and its validity have yet to be tested. A qualitative and quantitative comparison of different safety indicators was provided as a risk measure to demarcate their mathematical properties and evaluate their usefulness in quantifying trajectory risk. Five relevant safety indicators were compared: inverse time to collision, postencroachment time, potential indicator of collision with urgent decceleration, warning index, and safety field force. Their formulations were mathematically analyzed to yield qualitative insights and their values over simulated vehicle trajectories were evaluated to yield quantitative insights. The results acknowledge the limitations and demarcate the functional utilities of the selected safety indicators.