An Examination of the Endogeneity of Speed Limits and Accident Counts in Crash Models

A properly set speed limit establishes a reasonable and acceptable threshold that the majority of drivers can follow. Much literature has been devoted to investigating the relationships between speed limit and accident number, but the results have been widely variable. It is speculated that the variance of these conclusions can be attributed to the endogeneity of speed limit and accident count. Traffic volumes and crash counts at a total of 298 intersections in the City of Corona were collected and analyzed using simultaneous equation models to eliminate the influence of the endogenous variables and obtain unbiased predictor variables. By running single-equation models individually involving crash counts and speed limits and then comparing them with a simultaneous equation model that evaluates these same variables, it was possible to determine the effect of endogeneity on the resultant estimator variables. It was found that although the difference between the estimator variables in the single and simultaneous equation models was not statistically significant in the locations observed in this study, the presence of endogenous variables was confirmed. It is therefore anticipated that endogeneity might need to be accounted for in transportation models involving crash histories and speed limits in the future.