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Sub-problem 1a - Page 6 of 9

ID# C201A06

Sub-problem 1a: Maxwell Drive PM Peak Hour - Existing Conditions

Skipped Phases: General Ideas
Actuated signals not only vary the phase durations but they also skip phases when specific movements aren’t called, as at this intersection. The eastbound protected left (phase 2 in Exhibit 2-5) is sometimes skipped. Before we see what effect that has on how you should do the analysis, let’s discuss movements and phases. We need to make sure that you understand how the word “phase” is used in the context of the HCM.

Eight-phase, dual ring, NEMA controllers are often set up as shown in Exhibit 2-11. The diagram shows the movement sequences in the A and B rings. The green indications progress through the A and B rings simultaneously, in parallel, and the two barriers (between movements 2&6 and 3&7 and between movements 4&8 and 1&5) are crossed simultaneously in both rings.

Exhibit 2-11. Typical Dual-Ring NEMA Phasing

The typical way in which the signal indications progress is as follows: greens are first displayed for movements 1&5, the eastbound and westbound protected lefts. These lefts lead the through movements assuming both are called. When actuations cease for either movement 1 or 5, for example movement 1 in this case, the green in the A Ring changes from movement 1 to movement 2, while movement 5 is still green. When movement 5 terminates, the green in the B ring changes from movement 5 to 6. An alternate sequence of distinct intervals can occur if the signal changes from 1&5 to 1&6 and then 2&6. Thus, the following phase sequences can occur left of the middle barrier: 1&5 to 1&6 to 2&6 (three intervals); 1&5 to 2&5 to 2&6 (again three intervals); 1&5 to 2&6 simultaneously (two intervals); 2&6 alone (1 and 5 both skipped). To the right of the barrier, four similar sequences are possible.

Consider some signalized intersections that you know well. Write down the dual ring phasing for them. Then construct the sequence of phases that are the input to the HCM analysis. Make sure you understand how they relate to one another.

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