Some Observed Features of Dual Left-Turn Flow Rates at Signalized Intersections

This study focuses on the features and operational characteristics of dual left-turn flow at signalized intersections. Based on the field data collected in Beijing city, the saturation headway and saturation flow rates of dual left-turn lanes at signalized intersections are studied. Grand mean of headways, trimmed average value of headways, and median value of headways are computed and compared, and it is found that the trimmed average value of queuing vehicle departure headway is reasonable to estimate the saturation headway and saturation flow rates. Combined with spot-observation and data analysis, the influence of heavy vehicles, lane width, left-turn radius and U-turn behavior on the dual left-turn saturation flow rates are studied. It is recommended that heavy vehicle equivalency factors and turn-radius effects on saturation should be studied more so that it can be used in practice. The study of the effects of lane width and U-turn behavior on the saturation flow rates is recommended as well. Comparisons between dual left-turn saturation flow rates and single left-turn lane saturation flow rates are also made.