Evaluating the Impacts of Unconventional Outside Left-Turn Lane Design on Traffic Operations at Signalized Intersections

This study evaluated the operational impacts of an unconventional outside left-turn lane design used at some signalized intersections in China. Using traffic data collected from 10 approaches to five signalized intersections where both traditional inside and unconventional outside left-turn lanes are installed, the research team compared turning speeds and saturation flow rates for left-turning vehicles from inside and outside left-turn lanes. The team found that vehicles turning from outside left-turn lanes had relatively higher turning speeds than those turning from inside left-turn lanes. The differences in speed were statistically significant for both passenger cars and heavy vehicles. However, the higher speeds did not result in higher left-turn lane capacity. Unconventional outside left-turn lanes provided slightly lower, but generally comparable, left-turn saturation flow rate when compared with traditional inside left-turn lanes. The differences in discharge headways from inside and outside left-turn lanes were not statistically significant. A binary logit model was developed to quantify the impacts of various factors on the probability of a driver selecting the outside left-turn lane to make a left turn. The chance of a left-turning driver choosing the outside left-turn lane would increase with an increase in the traffic volume of the major street and the queue length in the inside left-turn lane and would decrease with an increase in the distance from the upstream right-side side street to the signalized intersection. Drivers of heavy vehicles and those from upstream right-side side streets were more likely to select outside left-turn lanes.