Sub-problem 5a: Two-Phase Traffic Signal Control

Both the QEM and the operational analysis procedure have the same underlying logic. In many places, the QEM uses assumed or default values and the operational analysis procedure uses more precise site-specific data. Therefore, some differences can be expected in the results, and those differences are generally attributable to the approximate nature of the QEM.

The QEM provides two checks to evaluate the need for a protected left turn on each approach. The first involves computing the product of the volumes for the left turn and its opposing through movement. The cross product criterion has been described in the literature as a popular technique that generally preceded the availability of more complex computational models. Different cross product thresholds have been adopted by different agencies, and the threshold values are generally dependent on the number of available lanes. Thresholds of 50,000, 90,000 and 110,000 are typically recommended for 1,2 and 3, through lanes respectively.