Evaluation of Safety Effect of Roundabout on Intersection Crashes Reduction

Determining the numerical value of the safety effectiveness of countermeasures or the Crash Modification factor (CMF) is one of the issues that has received considerable attention in recent years in the world. In this regard, several studies have been carried out to determine the CMF for converting intersection to roundabout in different countries. The results of these studies have led to a reduction of about 35 to 70 percent of crashes at the intersections. Since there is no serious study regarding determining CMF of constructing a roundabout and the percentage of crash reduction due to this countermeasure in Iran, the researchers are seeking to determine the CMF and reduction percentage of the crashes due to this measure in Iran. For this purpose, Salman Farsi roundabout of Ilam city was evaluated. AADT rates were obtained from the available traffic counts and crash data from the police. For determining CMF, empirical Bayes method was considered. The reason for using empirical Bayes method is its high efficiency in removing the phenomenon of regression to mean and other time-alteration factors and the high precision of this method is due to the use of SPF in order to reasonably estimate the frequency of crashes in the period prior to countermeasure. During studying, a SPF from AASHTO 2010 was used. Using this tool and predicting crashes before and after the construction of the roundabout, as well as calculating the frequency of expected crashes before and after it, the effectiveness of safety was calculated. The results for crash frequency is as crash CMF and crash frequency reduction percentage indicates a CMF of 0.68, implying a 32 percent reduction in crash. The statistical significance evaluation of this reduction shows that the 32 percent reduction of crashes after constructing roundabout is significant at 90 percent confidence level.