Saturation Flow Rate Analysis at Signalized Intersections for Mixed Traffic Conditions in Motorcycle Dependent Cities

This article describes the results of a comprehensive saturation flow rate analysis at signalized intersections in motorcycle dependent cities. Most of current capacity analysis methods for signalized intersections consider vehicles moving on lanes, and they assume lane discipline and high driver discipline regarding traffic regulations. Moreover, for modeling mixed traffic conditions, the not lane-based heterogeneous real traffic flow is transferred into lane-based homogeneous flow by using Passenger Car Units. So far, available methods of capacity analysis do not consider the specific conditions of driver behaviour and traffic flow which are dominant e.g. in Vietnam or other countries where motorcycles have the major share in traffic. Consequently, there is a need to develop a proper method to allow such saturation flow rate analysis. The methodology of this research was developed with consideration of the specific traffic situation in Hochiminh City, Vietnam, a motorcycle dependent city. First, the article describes the unique aspects of traffic flow and saturation flow rate analysis under such mixed traffic conditions compared with car-dominated traffic, using data collected by observations of saturated flow conditions at 12 signalized intersections. Then, saturation flow rate models using a regression method are presented and described. The term of Motorcycle Unit (MCU) is introduced, and a set of factors which mainly affect the saturation flow in such mixed conditions is developed. These factors reflect major differences between the traffic flows under saturated conditions at signalized intersections in motorcycle dependent cities and car-dominated cites. For example, this includes the influence of the approach width, the impact of four-wheel vehicles (including car, bus, and truck) on motorcycles, the very specific interactions of left-turning and opposite straight-moving traffic streams, the relationship between right-turning 4-wheel vehicles and motorcycles moving straight in the same direction, and the phenomenon of capacity drop within the green time. Finally, a procedure to calculate the saturation flow rate for specific traffic situations is provided, and the application of the proposed model is depicted. The conducted research indicates that the proposed saturation flow rate analysis model is an appropriate approach to calculate the saturation flow rate for traffic streams at signalized intersections under such mixed traffic conditions. The set of motorcycle equivalent units used for analysis is also proved to be suitable for this model. Equations and graphs to determine the appropriate values for the identified influencing factors are presented. According to each specific traffic situation, calculation procedures are provided to determine the value of the saturation flow rate.