Subproblem 5a - Page 1 of 5
Sub-problem 5a: Two-Phase
Traffic Signal Control
Step 1. Setup
A two-phase control plan
provides no protected phases for any of the left turns that are opposed by
through traffic. The westbound approach has the only left-turn movement in
this category. With a volume of 120 vph, it is conceivable that this
movement could be accommodated without a protected phase.
Some agencies would decide
to provide a protected left-turn phase for this movement, without further
analysis, because of the high speed (50 mph) of the approaching traffic.
This is a legitimate issue. The
two-phase alternative is therefore presented in this sub-problem primarily
as an illustration of the details of the HCM procedures. For some agencies,
the outcome of this analysis will be a moot point with respect to the decision itself.
We will examine the
two-phase alternative using both the Quick Estimation Method (QEM) presented
in HCM Chapter 10 and the full operational analysis procedure presented in
HCM Chapter 16.
Consider the following as you proceed through this