Problem 2: Discussion

There are several issues that we should consider as part of the analysis we have just completed.

First, while we have completed an analysis of the major components of the I-87/Alternate Route 7 interchange, we don't have results that we can say are completely reliable. For two of the weaving sections and for the ramp junction, the model boundaries were exceeded, thus putting some uncertainty on the results produced by the HCM analysis.

Second, while we have looked at the components of the interchange, we are still conducting this analysis in some isolation. How does the operation of the interchange affect the mainline section of I-87, particularly in the vicinity of the various ramp junctions? How does the interchange affect the operation of Alternate Route 7?

These questions and issues dictate the need to consider two additional problems in this case study. In Problem 4, we will use the Freeway Systems methodology from Chapter 22 of the HCM to consider Alternate Route 7 as a system, from the I-87 interchange to the I-787 interchange. This system perspective, looking at how the individual components operate together, is important for several reasons. One of these is that drivers consider this perspective every time they use (or judge) the facility. Drivers do not generally think about the performance characteristics of individual pieces of the freeway system, and then use this to judge the overall quality of their driving experience. Rather, their perception of the quality of service they have received develops over time, and they would typically describe the freeway they drove on as a single entity as opposed to a sequence of individual elements.

In Problem 5, we will use a micro-simulation model to help us study the performance of the segments of Alternate Route 7 that could not be adequately analyzed by the HCM, particularly those segments in which demand exceeded capacity.