Modification of SIDRA Model for Delay Estimation of Roundabouts under Mixed Traffic Condition

Roundabouts are commonly used as an intersection traffic control device due to its safety and improved operational characteristics. Several methods have been developed to analyze the performance of roundabouts. Most of these methods are related to countries where the traffic flow is homogeneous. SIDRA is one such method. This research aims to propose an improvement of SIDRA roundabout delay estimation model to cater to the mixed traffic condition in most of the developing countries. An urban roundabout in Kandy, Sri Lanka with four approaches was considered in this study. Data collection was done by video recordings and inventory surveys. Collected data shows that traffic flow at the considered location contains 40%–55% of motorbikes and three-wheelers. Two improvement methods were proposed to incorporate the mixed traffic condition in circulating flow and the entry capacity. The delay at each entry to the roundabout was determined using these adjustment methods and the original SIDRA method. Then the three methods were compared. Statistical analysis was done and the results showed that there is no significant difference between two mixed traffic adjustment methods. However, the adjusted methods estimate higher delay values than the original SIDRA method. Additionally, the modified methods and original SIDRA method have a significant difference only for three entries. When the entry width and the number of entry lanes are higher the three methods do not show any significant difference.