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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, a phase overlap can occur.
That is, the green in the A Ring transitions from movement 1 to
movement 2, while movement 5 is still green. When movement 5 terminates, a
second overlap occurs, as the green in the B ring transitions from movement
5 to 6. An alternate sequence of overlaps can occur if the signal
transitions 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 (two overlaps); 1&5 to 2&5 to 2&6 (again two
overlaps); 1&5 to 2&6 simultaneously (no overlaps); 2&6 alone
(1 and 5 both skipped, no overlaps). To the right of the barrier, four
similar transition sequences are possible. (Thus, this is referred to as an 8-phase
controller because eight phases – combinations of protected movements – can
occur: 1&5, 1&6, 2&5, 2&6, 3&7, 3&8, 4&7, and 4&8.)
Discussion:
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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