Capacity Analysis for Urban Roundabouts in Sri Lanka Using Modified SIDRA Model

Abstract

Traffic performance of an intersection can be more effectively controlled by roundabouts than other methods due to increased safety and efficiency. Vehicle flows merge and diverge at small angles with low speeds in one circulatory direction within the roundabout. Designing a roundabout requires thorough analysis of its capacity and other performance parameters. There are many methods to estimate the capacity of roundabout entry. However these methods are developed in countries with homogeneous traffic conditions which consist of higher percentage of passenger cars. Traffic flows in Sri Lanka consists of 45–55% of light vehicles such as motor bikes and three wheelers. Therefore, most available roundabout capacity estimation methods cannot be directly applied to the traffic condition in Sri Lanka. An adjustment should be done to the mixed traffic condition before applying the existing estimation methods. This research aims to introduce an adjustment method to estimate the capacity of urban roundabouts in Sri Lanka by modifying the SIDRA roundabout analysis method. Statistical analyses were done to identify the significance of the adjustment methods. The results show that the existing SIDRA model can be used when the traffic flow is adjusted using passenger car equivalency factors. It can be concluded that the adjustment method gives more sustainable estimations to the roundabout entry capacity for local traffic flow conditions.