Abstarct: Roundabouts are intersections designed to improve safety, reduce congestion, and increase traffic flow. Compared to signalized and stop-sign intersections, roundabouts significantly reduce the number and severity of accidents. This thesis examines the risk assessment approach baxsed on the Collision to Time (TTC) safety index at urban roundabouts. The Collision to Time index is used when two vehicles are positioned one behind the other. The collision time is the moment when the front bumper of the following vehicle hits the rear bumper of the leading vehicle. To address this challenge, a frxamework is proposed that utilizes the collision safety index, examined through statistical probability models. In this research, accident analysis is conducted at the Dolvaran roundabout in Mashhad city, using filmed data. The desired data is extracted using Adobe Premiere software and calculated using Excel. The data includes actual time, vehicle speeds, time gaps, spatial gaps, and vehicle lengths. By monitoring this data, the collision time for 500 vehicles is calculated, with values ranging between 0 and 3, indicating critical conditions in this range where a collision may occur. Finally, by providing an appropriate probability distribution function using Fit Easy software, the percentage of accidents within this range is determined. The closer the collision time is to 3, the safer the driver; conversely, the closer it is to zero, the more hazardous the driver becomes. Using the appropriate probability function, three high-risk ranges are examined (0.1, 0.3, 0.5 = TTC). If the collision time is less than 0.1 seconds, the collision percentage at this roundabout is 0.227%; if less than 0.3 seconds, the collision percentage is 0.481%; and if the collision time is less than 0.5 seconds, the probability of collision and accident reaches 0.737%.