Calibration and Application of Crash Prediction Models for Safety Assessment of Roundabouts Based on Simulated Conflicts

There has been significant interest in recent research in evaluating road safety by means of surrogate measures. This paper contributes to that research by investigating the use of simulated conflicts as a possible surrogate safety measure for roundabouts, for which it has proven difficult to relate crashes to geometric characteristics. The evaluation of the capability of conflicts to predict crashes at a roundabout approach was carried out by seeking a formal link between these two safety measures. The idea is that if a link can be established that is insensitive to design characteristics the relative safety of roundabouts with different designs or of various designs for the same roundabout can be evaluated by examining differences in conflicts estimated through simulation, and relating those differences to changes in crash frequency. The microsimulation package VISSIM was applied to estimate the number of peak hour conflicts for roundabout approaches using a database of U.S. roundabouts. For this, roundabout design characteristics were considered directly by graphically replicating the roundabout in the simulation, and indirectly though their effect on speed measures required as a simulation input. Conflict prediction models and several approach-level crash-conflict prediction models were successfully calibrated, suggesting that simulated conflicts could be considered as a surrogate measure for crashes at roundabout