Identification and Ranking of Black Spots: Sensitivity Analysis

In Flanders, Belgium, approximately 1, 014 accident locations are considered dangerous. These dangerous accident sites, or black spots, are selected by means of historic accident records for the period 1997 to 1999. A combination of weighting values, respectively, 1 for each light injury, 3 for each serious injury, and 5 for each deadly injury (1₃₅), is used to rank and select the most dangerous accident locations. A sensitivity analysis was performed to investigate the effect on the identification and ranking of black spots as based on three different weighting value combinations, representing a different attitude toward the traffic safety problem: avoiding all accidents (1₁₁), all deadly accidents (1₁₁0), and all accidents with serious or deadly injuries (1₁0₁0). Furthermore, effects of use of the expected number of accidents, estimated from a hierarchical Bayesian model, instead of the historic count data, to rank and select the accidents sites, were evaluated. Results show that a different attitude toward the traffic safety problem and the choice of the corresponding injury weighting values on the one hand and use of estimates instead of count values on the other hand do have important consequences for the selection and ranking of black spots. This is important not only for the number of accident locations that will receive a different ranking order but also for the effect on the type of accident locations that are selected as dangerous and accordingly for the resulting future traffic safety decisions.