Research On Capacity And Influencing Factors Of Left-Turn Lanes At Signalized Intersection

Nowadays, with the booming increase in left-turn traffic flow, exclusive multiple left-turn lanes have been installed at urban signalized intersections in China. Generally, exclusive double left-turn lanes are widely used. And triple left-turn lanes have also been set up in some cities. However, the feasibility of operational characteristics, safety and effciency concerns of exclusive multiple left-turn lanes have rarely been discussed.The limited literature and spot results were summarized and compared about the performance of single, double, and triple left-turn lanes at home and aboard. Data were collected at signalized intersections with exclusive left turn lanes and protected left-turn phases in Beijing. The impact factors on the performance of multiple left-turn lanes were analyzed and the operational efficiency was greatly considered. This work will provide reliable references for the planning, design, construction, and administration of signalized intersections.Saturate flow rate method of the 2000 Highway Capacity Manual was employed to estimate the capacity and impact factors of the left-turn lanes. Saturate flow rate and time headway are considered to be the key issues of the analysis. Differently, considering the difference of operation between left-turn and through movements, it is suggested that the estimation for capacity of left-turn lanes should be solely based on the left-turn lanes. Through analyzing the data collected, some specific characteristics are found in multiple left-turning operations including left-turning radius, lane widening during turning, u-turning movements and lanes usage of left-turn lanes and so on. Those characteristics have been studied via both statistical analysis and simulations. The relation model has been build between saturation flow rate and left-turning radius. And, the adjustment factors of left-turning radius were given. Also, the results represent that the passenger-car-equivalent of u-turns relate to left-turn small passenger cars, the adjustment factors of impact with u-turns and factors of heavy left-turning vehicles are obtained. Respectively, the observed saturation flows showed the range from 1408 to 1679 passenger cars per hour of green per lane (pcu/h/ln), 1490 to 1785 pcu/h/ln for inner and outer lane on dual left-turn lanes, and the range are from 1444 to 1875 pcu/h/ln, 1595 to 1895 pcu/h/ln and 1466 to 1813 pcu/h/ln for inner, middle and outer on triple left-turn lanes. In addition, the analysis of independent samples T test and variance test were used to determine statistical significance on saturation time headway among different lane position of double left-turn and triple left-turn lanes. Finally, all the impact factors mentioned above have been introduced into the saturate flow rate model of the left-turn lanes and corrections have been made for existed models.