Revised Guidelines for Designing Median Openings to Accommodate Simultaneous Left Turns

When designing median openings at divided highway intersections, it is important to consider the impacts of simultaneous (noncrossing) left turning movements by vehicles from opposing directions. The AASHTO Green Book recommends minimum lengths of median opening for different design vehicles and median widths. These minimum lengths are calculated based on assumed control radii for left turns from the divided highway into the crossroad and vice versa, and do not account for accommodation of simultaneous left turns. The Green Book minimum median opening lengths can prevent the capability for simultaneous left turns under certain circumstances. This study uses simulated vehicle turning paths to define the minimum conditions for accommodating simultaneous left turns and proposes corresponding revisions to the Green Book guidelines. The new method consists of determining the required control radius as a function of median width and calculating the minimum length of median opening by equations based on geometric relationships. The paper also describes the limitation of the Green Book methodology in addressing median openings with left-turn lanes, which is resolved by application of the proposed method.