Saturation flows of exclusive double left-turn lanes

Abstract

The objectives of this study were to develop reliable estimates of the saturation flows of exclusive double left-turn lanes, and to investigate the physical and operating characteristics of the intersection which may affect those flows. The results are based on observations of 3,458 completed left turns from exclusive double left-turn lanes on 14 intersection approaches in three Texas cities. The sample data were used to test specific statistical hypotheses concerning the saturation flow region of the queue, variability in the estimates of left-turn departure headways, and some of the factors which may affect left-turn departure headways. The analyses indicate that saturation flow for each lane of the double left-turn movements studied is obtained after the second vehicle in the queue. The analyses suggest that average left-turn departure headways vary significantly between the intersection approaches studied. However, much of the variability in the departure headways can be accounted for by grouping the headways according to city size. Also, the study indicates that, within each city, the departure headways do not differ significantly between the two lanes of a double left-turn movement. Based on the results of this study, and a review of the data from a limited number of related studies, an average double left-turn saturation flow rate on the order of 1600 vehicles per hour of green per lane would appear to be a reasonable value for most planning applications. This flow rate can be assumed to be achieved for the third vehicle in the queue and beyond. Also, this flow rate appears to be applicable for mixed traffic conditions where heavy vehicles constitute as much as 3 to 5 percent of the left-turn traffic volumes. The study suggests that average double left-turn saturation flows may be substantially higher than previously thought. However, the results are based on data from only three cities in Texas. Consequently, in assessing some site-specific problem, engineers and planners may find it useful to collect local data to "spot check" the applicability of the flow rates recommended in this study.