Identification of Spatial and Temporal Dilemma Zone at Mid-Block Median Openings: A Gap Acceptance Based Approach

A U-turn at a mid-block median opening (MBMO) is a complex maneuver. The U-turning vehicle has to find a suitable gap between two fast-moving vehicles in the approaching through traffic (ATT) stream and decide whether to accept or reject the available gap. Incorrect judgment of an available gap may lead to congestion or a collision between the U-turning vehicle and ATT. Frequently, U-turning vehicles are in a state of dilemma while deciding whether to reject or accept the available gap. Under these scenarios, the decision-making process is a challenging task for U-turning vehicles. This article thus aims to analyze the dilemma zone of U-turning vehicles at MBMO by collecting field data from 14 locations. The dilemma zone has been estimated using three methods: the cumulative distribution method; the binary logit method; and the support vector machine (SVM) method. The length and location of the dilemma zone have been observed to be influenced by different types of vehicle. The dilemma zone has been observed to shift away from the nose of the MBMO as the size of the vehicle increases. Furthermore, a practical yet straightforward equation has been also suggested for estimating the dilemma zone based on the composition of the ATT stream. The outcome of the present study will be beneficial in understanding the dilemma associated with a U-turning driver at an MBMO. The estimated dilemma zone boundary can be used further to develop a warning system in association with the intelligent transportation system for helping U-turning vehicles complete the U-turn movement safely.