Left-Turn Lanes at Unsignalized Median Openings

2014PDFTech ReportSWUTC/14/600451-00044-1DTRT12-G-UTC06Left turn lanesUnsignalized intersectionsMedian openingsLengthHighway engineeringHighway safetyUrban areasAccess control (Transportation)Research HubLeft turn lanesUnsignalized intersectionsMedian openingsTexasUnited StatesSouthwest Region University Transportation Center (U.S.)Qi, YiChen, XiaomingWang, YubianLu, YanTexas Southern University. Center for Transportation Training and ResearchUnited States. Dept. of Transportation. Research and Innovative Technology AdministrationUS Transportation CollectionDue to the frequent presence of median openings in urban arterial settings, the requirements for the deceleration and storage of turning vehicles (e.g. AASHTO Green Book) often exceed the available length between two adjacent openings which leaves traffic engineers having to decide whether left-turn lanes, shorter than the standards, can be used or not. The goal of this research is to investigate the minimum required length for the left-turn lanes at the unsignalized median openings, and study the safety and operational impacts of such left-turn lanes with substandard lengths. To achieve this goal, researchers will: 1) synthesize existing related research; 2) develop models for storage lengths at unsignalized median openings; 3) develop models for estimating the delays caused by substandard deceleration lengths and the resulting excessive deceleration on main travel lanes; and 4) analyze safety impacts of substandard median left-turn lanes. The results of this study lead to following key findings: 1) at the operational impacts perspective- if a substandard length left-turn lane can accommodate the necessary storage length and the deceleration length assuming a 20mph speed differential, it will not affect the operational performance of median openings significantly and the delays caused by using substandard length left-turn lane are significantly less than the delays associated with the absence of dedicated left-turn lanes; 2) at the safety impacts perspective- substandard length left-turn lanes will affect the safety performance of median openings. However, when it is impractical to provide the Greenbook required length, use of substandard length left-turn lanes may still be an option because of operational benefits comparing the no dedicated left-turn lane option; and 3) based on traffic simulation study- the required storage length is less than that estimated by the AASHTO \u201ctwo-minute arrival\u201d rule-of-thumb method. The minimum required storage length can be estimated with the regression model developed in this research