Influence of Intersection Geometrics on the Operation of Triple Left-Turn Lanes

Triple left-turn lanes have been used to reduce vehicle delays, queue lengths, and left-turn storage bays at signalized intersections with relatively higher left-turn demand. Limited research has been conducted to study capacity issues and how they relate to various geometric configurations of triple left-turn lanes. This study analyzed the influence of a number of geometric factors found at 15 triple left-turn lane sites in Florida on saturation flow, lane usage, and lane utilization. A total of 2,395 lane cycles and 38,023 vehicles were observed. The observed saturation flows yielded a mean saturation flow of 1,859 vehicles per hour of green per lane (vphgpl) with the 95% confidence interval of 1,810-1,907 vphgpl. The analysis of variance (ANOVA) test was used to determine statistical significance of the variables' influence on saturation flows. In addition, multivariate analysis of variance (MANOVA) was conducted to determine the influence of interaction of variables. The results showed that triple left-turn lanes on downgrades and with angle of turn less than 90 degrees were the two factors that mostly contributed to the high saturation flow while triple left-turn lanes located on one-way streets and on approaches which are curved had the lowest saturation flow. It was also found that street type significantly influenced saturation flow results when combined with other variables. The lane utilization was dependent on the geometrics of the intersections in which shadowed left-turn lanes (one or two left-turn lanes added in the vicinity of the intersections as storage lanes) had lower utilization of the innermost lane compared to unshadowed lanes.