Safety performance functions using traffic conflicts

Recent research has shown that traffic conflicts provide useful insight into the failure mechanism that leads to road collisions while being more frequent and of minor social cost. However, the relationship between collisions and conflicts must first be established in order to use traffic conflicts as surrogates to collisions for safety analysis. To investigate the relationship between conflicts and collisions, a two-phase model is proposed where a lognormal model is employed in the first phase to predict conflicts using traffic volume, area type (urban/suburban) and some geometric-related variables as covariates. In the second phase, a conflicts-based negative binomial (NB) safety performance function (SPF) is then employed to predict collisions. The proposed model was applied to a dataset corresponding to 51 signalized intersections in British Columbia. The results show that a significant proportional relationship exists between conflicts and collisions where the moderating effects of conflicts on collisions are non-linear with decreasing rates. The scaled deviance and Pearson χ2 goodness of fit measures indicated that the proposed NB model has adequately fitted the data. The finding that conflicts can be used to represent collisions calls for further research on the countermeasures needed to reduce conflicts as effective means for decreasing collision frequency. Apart from the traffic- and geometric-based traditional countermeasures, new driving-behavior-based measures should be devised that would hopefully have a downward influence on collisions.

• A significant proportional relationship was found between conflicts and collisions.
• The moderating effects of conflicts on collisions are non-linear with decreasing rates.
• The results show that conflicts are significantly more likely in urban than suburban areas.
• Conflicts were found to be less likely whenever right turn lanes are present at the minor approach.