Saturation flow at signalized intersections during longer green time

Traffic signal timing is one of the most important tasks traffic engineers face in the United States. Saturation flow is a key input for optimal signal timing. A small variation in saturation flow values could affect changes in cycle length and thereby affect the efficiency and operation of an urban street system. A great deal of research on estimating saturation values has been done, taking into account prevailing conditions at signalized intersections. Various methods exist for determining saturation flow by observing queues of vehicles at an intersection and determining their respective headways, but little research has been carried out to evaluate the effect of longer green periods on saturation flow. A few studies have suggested a decrease in saturation flow after 30 to 40 s of green. This paper examines the effects of longer greens on saturation flow at signalized intersection approaches in the Dallas-Fort Worth (DFW) metropolitan area of Texas. Five signalized intersection approaches in the DFW metropolitan area were studied to determine these effects. Headways were observed for different queue positions including and excluding the effect of heavy vehicles. Analysis of the data did not indicate a significant effect of longer green time when compared with expected values. One signalized intersection approach showed an increase in headway after about 60 s of green time. But this result was not significant in concluding that longer green times had an effect on vehicle headways.