The use of surrogates for actual crashes could significantly reduce the time and resources required to obtain safety data. One such surrogate is Time to Collision (TTC), which is defined as the expected time for two vehicles to collide if they remain at their present speed and on the same path.TTC can be used to determine when a conflict occurs. A conflict occurs when the TTC is less than a pre-selected threshold value. The study determined to what extent different threshold values of TTC, could adequately describe the occurrence of inter-vehicle crashes on interstate highways that are subjected to truck lane restrictions. A part of an urban Interstate highway in Northern Virginia, at which trucks were restricted from using the leftmost lane, was used as the test site. The expected number of conflictsper hour was obtained for different threshold values of TTC by simulationusing Paramics, which is a stochastic, microscopic, time step and behavior-based simulation program. Two different definitions of TTC were used. The number of simulated conflicts obtained for each hour was then compared with the average number of actual crashes for the same hour over a 5-year period. Models relating actual crashes with conflicts were also developed. The results of a sensitivity analysis identified the best TTC threshold Values. The results indicated that there is a correlation between crashes and the proposed surrogates. The models and the methodology of this study can be used to determine the effects of various truck strategies on safety before they are implemented.
Abstract