The effect of left turns on the capacity of a traffic intersection.

A model is proposed for the traffic flow through a fixed cycle traffic signal on a narrow two lane highway. The average flows in the two opposing lanes are computed when the queues are arbitrarily long assuming that left turns are permitted and occur with fixed probabilities in the two lanes. It is found that the existence of left turns tends to favor short cycles of the light and, under certain conditions, the competition between this effect and the obvious advantages of the long cycle light gives rise to an optimal cycle time at which the average flows have a maximum value. Some simple models for multilane highways are also considered and show optimal cycle times.