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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.

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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