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Sub-problem 1c - Page 5 of 9

ID# C101C05

Sub-problem 1c: Analysis of Future Conditions

For TWSC (see Exhibit 1-12):


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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