FHA Signal Timing On A Shoestring

Split Issues
The split is the amount of time allocated to each phase in a cycle at each intersection. The
toolbox offers two ways to calculate splits manually, the Greenshields-Poisson method
previously described and the Critical Movement method which is described below.
Critical Movement Method
This method uses techniques that were employed in the 1984 Highway Capacity
Manual as a “Planning Analysis” (Figure 6) to estimate intersection capacity. We
have adapted elements of this analysis to use to develop traffic signal timing
parameters. To use this method, intersection turning movements, the signal
phasing and the cycle length for the intersection must be known. The designer must
determine the effective demand for each phase by applying various adjustment
factors to reduce the demand to passenger car equivalents per lane (PCEPL).
For the purposes of preparing traffic signal timing plans, a high level of precision is
not needed. Traffic demand can vary plus or minus 20% in just a few minutes at a
given location. Also a variation of 20% from day to day is not unusual. Our
objective, therefore, is to develop timing plans that are robust and that will perform
well through a wide range of demand conditions. The following steps are suggested:
1. If the left turn movement is not protected, multiply the left turn demand by
1.6.
2. If the number of trucks is known, multiply the trucks by 1.5.
3. Divide the traffic demand on
the four major and left-turn
approaches by the number of
lanes for each movement.
4. Determine the critical
movements for both the east-
west street and the north-south
street. Determine the
intersection critical movement
by adding these two together.
If this sum is less than 1500,
then continue. If it is over
1500 then the intersection is
probably over saturated and
the method may not be
applicable.
5. Determine the number of
critical movements in each
cycle. With no left-turn
protection, there would be two,
with left turn protection on one
street there would be three,
and with left turn protection on
both streets there would be
Figure 6.
Example.
Critial Lane Analysis
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