Sub-problem 4c: Clifton Country Road 2004 PM - With vs.
Mitigation The next question is: what can be done to mitigate this impact?
Adding lanes is the usual solution. But at this intersection it
would be difficult. Reconstruction would be expensive and disruptive, so
we will explore other options. Look at
42to see the analysis of the PM With condition plus30% higher traffic and the poorer performance.
Dataset 43contains the analysis of the dataset
in the enhanced condition. The results from these two analysis are
differences derive from three sources. First, we assumed that coordination
would be introduced for the eastbound flows. We changed the
from 3 to 5 for the eastbound through and right.
considered decreasing the
unit extension from 3 seconds (the default)
to 2, which the HCM predicts will reduce delays; but in the field, such reductions often produce premature phase
terminations, which actually increase delay instead of decreasing it.
also debated reducing the
start-up lost time from 3.0 to 2.0 seconds for
the northbound and southbound approaches but failed to see how the change
could be defended without reconstructing the intersection.
did consider increasing the cycle length beyond 108 seconds, but chose to reduce the
cycle time instead. (See later text.)
substantial change was to the lane utilization coefficients. We assumed
they were all at default values,
because the site-generated traffic is going straight ahead (not onto
I-87), so the lane utilization will be better than the condition
originally observed in the field. The lower
v/c ratio eastbound made it
possible to remove green time from phase three and shorten the cycle
length. It also made it possible to add time to phase 5.
The end result is an intersection that is at LOS D or better for all
movements, even under the 30% higher site-generated traffic conditions. This
is a substantial improvement; and it illustrates the value of carefully
organizing the flows, if possible.