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Cycle Length Determination
As part of a signal design team, you have been
assigned to find the optimum cycle length for a three-phase cycle. Field observations
and calculations by yourself and others are the basis for your work. So far, you know
the critical flow ratio for each phase and the intergreen time for each
phase. Calculate the optimum cycle length for your signal, given the critical flow
ratios and intergreen times below. What would the optimum cycle length be if all of
the critical flow ratios were near zero?
Phase Number |
Critical Flow Ratio |
Intergreen Time |
1 |
0.233 |
6 sec |
2 |
0.13 |
4 sec |
3 |
0.256 |
7 sec |
[Solution Shown Below]
Solution
Webster's optimum cycle length equation, which is shown below, has two variables on the
right-hand side. These are the total cycle lost time (L) which is usually taken as
the sum of all the intergreen times, and the sum of all the critical flow ratios (S(V/s)).
Co= 1.5L + 5
1 - S (V/s)
In this case, the sum of the intergreen period lengths is 17
seconds (L=17). The sum of the critical flow ratios is 0.619. When we substitute
these values into the equation above we obtain an optimum cycle length of 80
seconds. If all of the critical flow ratios were very near zero, the denominator in
Webster's equation would approach unity and the optimum cycle length would be 31 seconds.
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