SAFETY AND OPERATIONAL ANALYSIS FOR MEDIAN U-TURN INTERSECTIONS IN THAILAND

Abstract

At major conventional uncontrolled intersections along high-speed divided highways, road users experience serious accident risks and long travel delay. Specifically, vehicles from minor roads at unsignalized intersections have to make a direct crossing at the main intersection. To solve safety and mobility problems at such conventional intersections. a median U-turn intersection design has recently proposed among highway authorities in Thailand. This design removes the median opening at the main intersection, restricts vehicles from making a direct crossing, guides them to make a U-turn at the downstream median U-turn opening, and returns to the main intersection. For such design, the distance between the median U-turn opening downstream and the main intersection (called a median U-turn offset) is the most important design parameter that influences the safety and operating efficiency. Too short median U-turn offset will result in harsh lanechange conflict for minor-road traffic, while too long median U-turn offset will increase vehicle travel time. This paper proposes the framework to rationally recommend the median U-turn offsets of such design. The microscopic traffic simulation models are developed to estimate vehicle travel time, and the crash surrogates are used to estimate the number of vehicles involved in conflicts. The median U-turn offsets that balance the operating and safety efficiencies are recommended. It is found that the recommended median U-turn offsets are sensitive to volumes on major and minor roads, vehicle speed, and vehicle composition. The proposed framework is then applied to the real-world highway improvement projects.