Identification and Ranking of Black Spots: Sensitivity Analysis

In Flanders, approximately 1014 accident locations are currently considered as ‘dangerous’. These ‘dangerous’ accident sites or ‘black spots’ are selected by means of their historic accident data for the period 1997-1999. More specifically; a combination of weighting values, respectively 1 for each light injury, 3 for each serious injury and 5 for each deadly injury (1_3_5), is used to rank and select the most dangerous accident locations. In this paper a sensitivity analysis is performed to investigate the impact on the identification and ranking of black spots of 3 different weighting value combinations representing a different attitude towards the traffic safety problem: avoiding all accidents (1_1_1), all deadly accidents (1_1_10) and all accidents with serious or deadly injuries (1_10_10). Furthermore, effects of using the expected number of accidents, estimated from a hierarchical Bayesian model, instead of the historic count data to rank and select the accidents sites are evaluated. Results show that a different attitude towards the traffic safety problem and the choice of the corresponding injury weighting values on the one hand and using estimates in stead of count values on the other hand do have important consequences for the selection and ranking of black spots. Not only will this have an important impact on the number of accident locations that will receive a different ranking order, it will also have an important effect on the type of accident locations that are selected as ‘dangerous’ and accordingly on the resulting future traffic safety decisions. Geurts, Wets, Brijs and Vanhoof 3