Comparison between Surrogate Safety Assessment Models (SSAM) and Accident Models on Unconventional Roundabouts

This paper describes the comparison between the Surrogate Safety Assessment Model (SSAM) of the Federal Highway Administration (FHWA) and the predicted number of accidents calculated through analytical models, regarding Unconventional Roundabouts. The novelty of this comparison lies precisely in the fact that the three roundabouts analyzed fall into the category of so-called Unconventional Roundabouts, i.e., arrangements with "roundabout circulation", which do not fall within the types listed in the Italian Guidelines (Ministerial Decree 19-04-2006). In roundabout intersections, among the various types of accidents that may occur, those of the rear-end collision type occur more frequently, for which it was decided to use the formulas of the accident models relating to this type of conflict. In particular, the conflicts type "Approach" for the Maycock & Hall model and the conflict type "Rear end" for the Arndt & Troutbeck model were taken into consideration. As mentioned, in addition to the application of analytical models, possible points of conflict (of the same category, i.e., "Rear end") were evaluated using dynamic simulation models. In particular, the dynamic simulation software Aimsun™ was used as a means to obtain the necessary inputs for the evaluation of the surrogate safety carried out through SSAM, a software application that reads the trajectory files generated by the simulation programs. In the final part of this paper, the conclusions on the comparison and some possible future ideas for further research developments have been included.