Sub-problem 1c - Page 5 of 9
Sub-problem 1c: Analysis of Future
For TWSC (see
The delays for the major street left turn movements
remain low, even with the increased flows expected in the future. Both movements remain at level of service A.
The movements on the minor street approach experience
high levels of delay. Consider the TH/RT lane on the westbound (Styner)
approach, for example. Today, the average delay is 27.6 seconds per
vehicle, or level of service D. But the delay is projected to more
than double to 80.9 seconds in ten years, or level of service F.
The v/c ratio for the minor movements are near or
exceeding 1.0. When the v/c ratio exceeds 0.8, it is possible to see short term breakdown of the operation at the intersection, which leads
to high delays and growing queues. And, the EB LT movement has a v/c
ratio that exceeds 1.0. This means that all of the demand for this
analysis time period will not be served, and that some will spill
over into the next period. We will consider how to deal with
oversaturated conditions in Problem 4 of this case study.
It is worth commenting here on several aspects of the
HCM model forecasts. First, while computer models will often report delay to the
nearest one-tenth of a second, this more precise than is
warranted by the accuracy of the model itself, especially when planning
year horizon volumes are used that are forecasts. It would probably
be more reasonable to round the delay estimates to the two most
significant digits. Second, the high
levels of delay forecasted by the model for the EB LT movement (movement
10) are probably not realistic. You should use delay estimates in
this range, when the v/c ratio exceeds one, with great care. While
they indicate that the delay will likely be high for these conditions, it
would not make sense to compare this delay estimate with another estimate
in the same high range and make solid conclusions regarding the relative
differences between such estimates.
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