Safety assessment before and after implementation of roundabouts through microsimulation

Roundabouts are internationally recognized as a viable alternative to signalized and stop-sign controlled intersections, and the frequency of roundabout implementations continues to increase. The safety advantages associated with roundabouts are often cited as significant reasons for the conversion of traditional intersection designs to roundabouts. Safety evaluations at real-world sites are costly and cannot be performed for evaluating alternative geometric designs unless those designs are actually built. Microsimulation that is commonly used for the evaluation of alternatives with regards to their operational performance can also be used for safety assessment. The Federal Highway Administration developed the Surrogate Safety Assessment Model (SSAM) that uses vehicle trajectories obtained through microsimulation to estimate the number of conflicts based on several surrogate measures of safety such as time to collision and speed differential. While SSAM has been extensively used for safety evaluations at other types of intersections, applications of SSAM at roundabouts are limited. In particular, a double single lane roundabout has never been evaluated using SSAM. The objective of this study is to assess the accuracy of SSAM in predicting potential number of conflicts at roundabouts that vary in their geometric design and traffic demand characteristics. In addition, a before and after study is performed to assess safety benefits when signalized intersections and stop-sign controlled intersections are converted to roundabouts. The results show that SSAM is an adequate tool for estimating the total number of conflicts, type of conflicts, and location of conflicts found from the real-world observations at both the single lane roundabout and double single lane roundabout. The before and after study showed that roundabouts significantly decrease the number of conflicts when signalized and stop controlled intersections are converted to roundabouts.