01372750

Component

A properly set speed limit establishes a reasonable and acceptable threshold that the majority of drivers can follow. It is fair and allows officers of the law to go after those who deviate from the rest of the pack. When a speed limit is too low, drivers are more likely to speed in order to make up for what they may believe is an unfair limit, creating a “speed trap.” Exactly what happens when a speed limit is high is subject to much contention. Some believe that setting speed limits too high invites a higher rate of accidents and that the ensuing accidents tend to be more severe than at a site with a lower speed limit. Others argue that there are many contributing factors and that road conditions and driver behavior – not speed – are the main indicators of accident frequency and severity. Much literature has been devoted to this subject but the results have been widely variable. It is speculated that the variance of these conclusions can be attributed to the endogeneity of the two variables (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 in order to eliminate the influence of the endogenous variables and obtain unbiased predictor variables. By running single equation models individually involving crash counts, speed limits and then comparing them with a simultaneous equation model (SEM) 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 298 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.

This paper was sponsored by TRB committee ANB20 Safety Data, Analysis and Evaluation

01362476

12-4132

English

18p

2012

Transportation Research Board 91st Annual Meeting

Digital/other

References; Tables

Crash rates; Highway safety; Simultaneous equations; Speed limits; Traffic crashes

Endogeneity

Highways; Safety and Human Factors; I80: Accident Studies

Transportation Research Board Annual Meeting 2012 Paper #12-4132

TRIS, TRB

Feb 8 2012 5:22PM