Roundabouts have been shown to reduce fatalities and injuries at intersections. However, there is still room for improvement and a better understanding of their safety performance is needed, especially considering the anticipated increase in roundabout construction in the future. This study aims to determine the critical Time to Collision (TTC) value for conflicts between entering and circulating vehicles at modern roundabouts. Three modern roundabouts in Izmir, Türkiye, were chosen for data collection. Video data was collected using Unmanned Aerial Vehicles (UAVs) and processed using the AIMSUN simulation program to obtain vehicle trajectories. The Surrogate Safety Assessment Model (SSAM) was then used to determine the critical TTC value for each roundabout. The calibrated TTC values for the three roundabouts were found to be 1.58 s, 3.00 s, and 4.9 s. These differences are believed to be due to variations in traffic volume, composition, and geometry. The results of this study suggest that there is no single critical TTC for conflicts between entering and circulating vehicles at modern roundabouts. Instead, it is important to consider the specific characteristics of each roundabout, such as traffic volume, composition, and geometry. This study also implies directions for future research and practical applications, suggesting the need for pilot studies across various intersection types to validate its findings.

Modern Roundabouts, Time-to-Collision, Conflict Analysis