The Study of Preventive Role of Speed Cameras in Road Accidents (Application of PSM Method Using STATA Software)

This paper aims to evaluate the impacts of speed limit enforcement cameras on reducing road accidents by accounting for both confounding factors and the selection of proper reference groups. The propensity score matching (PSM) method is employed to do this. The propensity score can be used as the criteria for selecting the reference group in before-after control studies. The propensity score matching can be done using the program psmatch2 in STATA software. A total of 771 sites and 4787 sites for the treatment and the potential reference groups respectively that are observed for a period of 9 years in England¡ are applied in this paper as a case study. The PSM method shows that there are significant reductions in the number of accidents of all severities at speed camera sites. Speed cameras were found to be most effective in reducing accidents up to 200 meters from camera sites and no evidence of accident migration was found.