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Sub-problem 1b - Page 4 of 6

ID# C101B04

Sub-problem 1b: Analysis of the Proposed Signalized Intersection

What time periods should be analyzed? The data that have been collected for this site represent the peak 15-minute flow rates for the afternoon peak period. A 15-minute analysis is used here (as opposed to the one-hour analysis used for the existing stop sign control described in sub-problem 1a) because signalization is an expensive mitigation option that almost always adds to total system delay. Therefore, we want to evaluate it at a higher standard -- that is, we want to be sure it is able to perform adequately even during the peak 15 minutes of the peak hour. If there are other peak times during the day, such as the morning peak or sometimes a midday peak, these should also be included in an operational analysis. Since we are considering a decision that may take several years to implement, we will also consider traffic conditions that are expected over the next ten years. Review the traffic data to see the existing and projected afternoon peak period volumes for this site. This is also discussed in Problem 3 when we consider an analysis of event traffic following a football game.

How do we construct a signal timing plan for a proposed traffic signal using the HCM? The construction of a timing plan for a signalized intersection can be a complex process, though there are also some simple approaches that give very reasonable first-approximations. In this problem, we will assume that the proposed new signal will operate in fixed time mode, and the methods included in Appendix B of Chapter 16 of the HCM can be used to determine the signal timing plan for this condition. There are other tools that can be used for developing signal phasing and timing plans including the Institute of Transportation Engineers (ITE) Traffic Engineering Handbook, as well as the ITE guidelines for left turn phasing. The critical movement analysis technique is another good way to quickly develop a reasonable signal phasing and timing plan.

The decision to signalize the intersection or not does not depend on whether the traffic controller is fixed time or actuated. To simplify this analysis, then, we have chosen to assume that it will operate in fixed time mode. In Problem 4, we will illustrate how the HCM procedure considers actuated controller operational parameters.

The following signal timing was produced for this sub-problem, using the methods of Appendix B, chapter 16, of the HCM.

Exhibit 1-10. Signal Timing Data for sub-problem 1b
Phase Movement Green Yellow All red
1 NB/SB 35 4 1
2 EB/WB 15 4 1

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