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> Sub-problem 6c - page 3 of 4 Sub-problem 6c: Planning Analysis at Sweet Avenue For this example, the Northbound left turn is protected, the eastbound left turn is not opposed, and the other two left turns do not exist. The analysis results are presented in Exhibit 1-52. Note that the minimum cycle length of 60 seconds will accommodate the projected 20-year traffic volumes with no apparent operational problems.
Notice also that, even though this is a T-intersection with no westbound approach, the phasing designation for east west traffic is EWT, indicating the presence of westbound vehicles. This is an important principle that describes the way that traffic movements are represented in the planning level model. The principle is that, for the quick estimation method, all movements must exist in the model. Those that don’t exist on the street are simply assigned zero volume and capacity. This is an important distinction between the quick estimation method and more detailed traffic models. As indicated above, the HCM quick estimation method distinguishes between protected and protected-plus-permitted left-turn treatments. It is important to note, however, that the mathematical treatment of the permitted phase differs from the full operational procedure. The full operational procedure computes a capacity for vehicles turning left on the permitted phase, based on advance knowledge of the signal timing plan. The quick-estimation procedure does not require this information, and cannot, therefore, perform the computations at the same level of detail. This is an essential feature of the tradeoff between the reduced data requirement for planning level applications and the amount of detail that can be expected in the results. |
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