Assessing Traffic Safety of Dutch Weaving Sections: Validation of the Surrogate Safety Assessment Model combined with VISSIM

Dutch road designers and safety experts are searching for more quantitative methods to evaluate the safety of a (proposed) weaving section than the traditional methods of expert judgement and use of accident records. An alternative would be to determine safety using VISSIM micro-simulation models in combination with the Surrogate Safety Assessment Model (SSAM). SSAM calculates the number of conflicts (i.e. observable situations in which two or more road users approach each-other resulting in a potential collision risk) that occurred in a micro-simulation model using surrogate safety measures. This study evaluates this method for Dutch weaving sections by comparing the number of conflicts observed from VISSIM microsimulation models combined with SSAM with the crash rate, and other criteria. Nine Dutch weaving sections were selected and ranked based on four criteria: (I) crash rate, (II) conflict rate calculated from VISSIM-simulations using SSAM, (III) number of crashes expected based on a previously developed crash prediction model, and (IV) road safety experts judgement. To examine the correlation between the different rankings, the Spearman Rank Correlation Coefficient was calculated between each two rankings. The correlation of 0.567 (휌푠=0.112) between the crash and conflict rate ranking suggests a reasonable, but insignificant correlation. In a sensitivity analysis the effects of some micro-simulation settings, conflict analysis thresholds, and the calibration method were assessed. Although different than expected, extending the calibration process resulted in a weaker correlation. Hence care should be taken when using VISSIM and SSAM to evaluate the conflict rates as (only) safety-predictor of Dutch weaving sections.