Sub-problem 4c: Separating the Conflict Points for TWSC Control

Let's consider the questions from the previous page.

Why would separating conflicts produce a more optimistic assessment of the intersections?  Separating conflicts may produce a more optimistic assessment because, as conflicting streams of traffic are removed from consideration, there may appear to be more opportunities for acceptable gaps (and fewer impedance effects from higher-ranked minor movements) than is actually the case. The relationship is exponential and depends on a number of conditions, resulting in the potential of significant overestimation of capacity. For this reason, caution must be used when separating the conflict points for an unsignalized intersection.

How are the conflict points inter-related?  The most obvious relationship between the conflict points is how the paths of vehicles overlap multiple conflict points. For example, the northbound left-turn movement must pass through two points. Thus, if the second conflict path (northbound left turn and westbound through) is currently blocked by a queue of vehicles waiting for access, the analysis may be invalid. To determine whether the conflict points at an intersection may be separated, it is necessary to estimate the queue length for the each of the entering movements. If the estimated queue lengths are greater than the available storage space, then the separation of conflict points may overestimate or produce an unrealistic assessment of the operation.

Step 2. Results

Exhibit 3-31 shows the results of this analysis. In all cases, the movement capacities were improved in comparison with Sub-problem 4b (see Exhibit 3-30), which considered all of the intersection conflicts simultaneously. This would be expected, but the important question is whether or not the queue backup would exceed the available storage space, thereby invalidating the analysis. Inspection of Exhibit 3-31 indicates that the 95^th-percentile queue lengths remained well within the storage boundaries. So, it could be concluded that it is appropriate to separate the conflict points for this intersection. While the separation of conflict points improved the operation slightly, some of the movements remain badly oversaturated—and the earlier conclusion that TWSC will result in a peak hour deficiency is reaffirmed.