Estimation of Critical Gap of U-turns at Uncontrolled Median Openings

Abstract

Critical gap is an important traffic parameter which useful to estimate the capacity of U-turning vehicle at uncontrolled median openings. It is also considered as crucial factor in designing the U-turn facilities. Designing a U-turn using inappropriate driver gap acceptance data could result in traffic congestion and potentially lead to a severe traffic accident. This article aims to study the critical gap estimation at uncontrolled median opening U-turns by using the real traffic behavior gathered from 3 locations in Bangkok area. A comparative analysis of critical gap estimation was performed using various methods. The results showed that the critical gaps calculated by the Raff, Greenshield, Acceptance Curve Method and Maximum Likelihood Method were ranging between 3.24-5.39, 4.75-6.75, 6.75-9.30 and 4.29-6.69 seconds, respectively. From all techniques, Maximum Likelihood is the most suitable method for critical gap estimation as it provides a more consistent results than other techniques. The results also point out that, as the direct traffic volume increases, the estimated critical gap form all methods tends to decrease. In addition, it was found that the critical gaps of different U-turn patterns were different.