Subproblem 5a - Page 1 of 5

ID# C305A01

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 problem:

	The Quick Estimation Method (QEM) uses the critical movement technique (also known as the critical movement analysis) to estimate the intersection capacity. At a conceptual level, how would you use the critical movement technique to estimate the capacity of an intersection?
	The QEM is similar to the critical movement analysis in that they are both planning level analyses. What factors will cause variation between the QEM and the full operational analysis?

Discussion:
Take a few minutes to consider these questions. Click continue when you are ready to proceed.

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