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Sub-problem 4c - Page 4 of 4

ID# C204C04

Sub-problem 4c: Clifton Country Road 2004 PM - With vs. Without Conditions

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 Dataset 42 to see the analysis of the PM With condition plus 30% higher traffic and the poorer performance. Dataset 43 contains the analysis of the dataset in the enhanced condition.  The results from these two analysis are shown in Exhibit 2-46.

The differences derive from three sources. First, we assumed that coordination would be introduced for the eastbound flows. We changed the arrival type from 3 to 5 for the eastbound through and right.

We 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.

We 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.

We did consider increasing the cycle length beyond 108 seconds, but chose to reduce the cycle time instead. (See later text.)

The more 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.

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