Next Generation Simulation (NGSIM)
-Improved Simulation of Stop Bar Driver Behavior at Signalized Intersections-
Los Angeles, CA and Atlanta, GA Data Sets

Working Papers

A periodic scan computer simulation program was developed to investigate the effects of signal phasing and length of left-turn bay on capacity. After the simulation program was tested, inputs (phase sequence, volume, cycle length, and length of left-turn lane) were varied to evaluate their interrelationships under a range of conditions. Additional analysis was conducted by using a modified Poisson approach. The results show that, for a left-turn bay, traffic delay increases and signal capacity decreases when traffic interactions and flow blockages occur between left-turning and through vehicles. High left-turn volumes and short bay storage lengths experience the most severe reduction in capacity. We developed mathematical relationships between reductions in left-turn capacity and geometric and traffic conditions and provide design guidelines to minimize capacity reductions. Judicious selection of signal phasing reduces the loss in capacity to some extent, although all phasings can experience large losses under some geometric conditions. 

This article appeared in Transportation Research Record No. 644; Highway Capacity, Traffic Flow, and Traffic Control Devices [Year of Publication 1974].

Summary

This paper presents the results of a study which shows that traffic delay increases and signal capacity decreases for a left turn bay when traffic interactions and flow blockages occur between left turning and through movements. Longer cycle lengths, higher turn movements and shorter storage bay lengths exacerbate the problem, causing the most severe reductions in capacity.

The authors provide charts as recommendations for configuring left turn storage bay lengths based on left turning volumes, saturation ratios and cycle lengths.