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

Step 2: Results

The QEM represents an extension of a technique known as Critical Movement Analysis, or CMA. This technique has appeared in the literature in several forms and is intended primarily for planning level analysis. The QEM produces an estimate of the status of the overall intersection with respect to its capacity, based on the assumption that the signal timing plan will produce an equal degree of saturation among the critical movements on each phase. The intersection status is determined from the sum of the v/c ratios for the critical movements on each phase.

The QEM produces, as a by product, a synthesized signal operating plan consisting of:

• A phasing plan determined by the left turn treatments for each approach. The left-turn treatments may be specified, or they may be synthesized, based on the volumes of the left-turn movement and its opposing through movement.

• A cycle length that will produce a target v/c ratio of 90%.

• An allocation of phase times that will equalize the degree of saturation among the critical movements on each phase.

The HCM offers the caveat that the synthesized plan may not be suitable for implementation because it does not include important considerations such as minimum green times. Nevertheless, it usually provides a good starting point for an operational analysis, which requires the signal timing plan to be specified along with several other items of geometric, operational, and traffic data that are not always available at the planning level.

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