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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.
