Comparison Between Existing Accident Models and Surrogate Safety Assessment Models (SSAM) on Unconventional Roundabouts, with Focused Applications of the latter to some Real Study Cases

This article has been divided into two main parts. The first and most substantial part describes the comparison between the expected number of accidents calculated through analytical models and the Surrogate Safety Assessment Model (SSAM) of the Federal Highway Administration (FHWA), regarding unconventional roundabouts. The novelty of this comparison lies precisely in the fact that the three roundabouts analysed 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. In any case, apart from this latest innovation coupled with the small size of the sample observed, the present work can be considered an exploratory study with a view to further development. Returning to talk about roundabouts, it is possible to state that 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 conflict type "Approach" for the Maycock & Hall model and the conflict type "Rear end" for the Arndt & Troutbeck model were taken into consideration. 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. The second part of the article instead focuses on the application of SSAM to two real case studies for which, thanks to the results obtained, it was possible to demonstrate the effectiveness of the proposed solutions. At the end of this work, both the conclusions on the comparison made and on the application of SSAM to real cases have been inserted.