CONGESTION AND ACCIDENT RATES AT SIGNALIZED INTERSECTIONS. FINAL REPORT

A relationship may exist between the level of congestion and the accident rates and accident frequency at signalized intersections. Current practices for determining the accident reduction (or increase) caused by changes in the geometry or volume-to-capacity (v/c) ratio may not be adequate. In this study, an attempt was made to show a relationship between the approach volume and the frequency and rates of accidents at signalized intersections. In an attempt to hold constant as many variables as possible, the equations were developed for various geometric conditions and time periods. Accident prediction equations were developed for four accident types (rear end, left turn leaving, left turn entering, and cross traffic) and for two severities (Property Damage Only and Personal Injury). The study gave a number of equations that would be useful for predicting accident frequencies and rates at signalized intersections. In all, 1,210 equations were tested by using the Statistical Analysis System (SAS) and Microsoft Excel computer softwares. For each of the accident types, severities and time periods, one equation to predict the accident frequency based on the traffic volume and one equation to predict the accident rate based on v/c ratio was selected, for a total of 288 equations. Of these equations, 66 showed a moderate to strong relationship with a coefficient of determination of greater than 0.300. Furthermore, a series of equations were developed to predict the average annual number of accidents by type and severity as a function of v/c ratios. Although these equations did not show any significant improvements compared to the first set of equations, they seem to be more practical and easy to use in predicting the effects of congestion (as represented by v/c ratio) on the number of accidents. Extreme caution and the use of engineering judgment is recommended when any of the equations in this report are used for predicting the number of accidents as a function of v/c ratio or traffic volume. This is because of the low coefficient of determination of the equations and the limited scope of this study.