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Problem 3 Discussion - Page 1 of 1

ID# C2030D1

Problem 3: Shenendehowa Campus

So what have we learned? We’ve seen that you have to be careful in using the peak hour factor. It’s good to incorporate a peak hour factor, so that the conditions in the peak 15 minutes are examined. But unless you know the flows all peak simultaneously, it’s not good to use peak hour factor values that are movement specific. You’re better off using the value that pertains to the intersection as a whole during the peak hour. Even that value can lead to delay estimates that are higher than any real values obtained during the actual 15-minute intervals. The reason is that the overall peak hour factor, applied to all of the flows, still assumes implicitly that all of the movements peak simultaneously and proportionally as well. Sometimes, as is the case here, that doesn’t happen. If you find this is a significant issue, you might want to do analyses for each 15-minute period individually.

We’ve also seen that it is important to pay attention to the heavy vehicle percentages. This may be of particular importance in a situation like this, where the Shenendehowa intersection serves a lot of school buses. We might not initially realize the importance of accounting for their presence in the traffic stream, but doing so changes the delays considerably.

Lastly, we’ve seen that there are ways to check for impacts from site-generated traffic. We were relatively formal about that, doing the performance assessment with and without the site-generated traffic, looking at the resulting changes in delay, and deciding that the impact was insignificant. Sometimes, for expedience, analysts make a decision based on the percentage increase in intersecting traffic that results from the site-generated traffic.

Do you routinely perform sensitivity analyses on the volumes? Planners often use high, low, and base case predictions of population and traffic growth to bracket possible future conditions. Do you ever do operational analyses for such conditions or design for such conditions and consider the incremental impacts?

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