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Sub-problem 1c - Page 4 of 5 |
ID# C401C04 |
Sub-problem 1c: Analysis of
the Eastbound Freeway Section
Next, we should
examine the impacts that result from varying other variables. If we increase
the truck percentage to 10%, two things happen (see
Dataset 9):
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First, the ET
for the trucks drops from 5 to 3.5. That’s because the trucks have less
impact if they are a higher percentage of the traffic stream, up to a point.
At first glance this may seem counter-intuitive, but upon reflection you can
see that it reasonably reflects conditions we all see quite regularly in the
field: as the percentage of trucks in the traffic stream becomes greater,
the trucks themselves begin to fill in the gaps that other trucks create,
and so their individual impact (in terms of the equivalent number of
passenger cars each truck represents) actually goes down. At the same time,
the cumulative effect of the trucks will continue to rise because of the
higher percentage of trucks in the traffic stream. This is also quite
reasonable and reflective of conditions seen regularly in the field.
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Second,
density increases, from 26.2 pcpmpl to 27.3 pcpmpl.
The countervailing trend in the value of ET is keeping
this growth in the truck percentage from having a more significant impact.
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The fact that these
two trends offset one another is a very important thing to notice.
To complete the
sensitivity analyses, let’s look at the impacts from varying the driver
familiarity adjustment factor. Let’s try using 0.85 instead of 1.0, assuming
that the analysis is being done in the summer when a higher percentage of
the drivers are vacationers. The flow rate becomes 1,695 pcphpl, the density
becomes 30.8 pcpmpl, the LOS is D, and the average passenger car speed is 55
mph. This is a 17.6% increase over the value we found for the initial
conditions we studied. So there is
an impact on operations, even though the LOS is
still D (see
Dataset 10).
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