Sub-problem 4c: Peak Operational Analysis of Alternate Route 7

The main point that we can learn from Exhibit 4-73, which shows results from time period 2, is that we have a bottleneck, a point along the freeway facility that limits or constrains the demand. This bottleneck shows up in section 6, where the volume/capacity ratio equals 1.0.

What is the cause of this constraint? If we review the line drawings showing the geometric information for the eastbound portion of Alternate Route 7, we see that this is where the mainline drops from three lanes to two lanes. At this point, the demand exceeds the capacity of the two lane section and a queue begins to build, traveling upstream from this location. At the end of this 15-minute period (time period 2), the queue extends the entire length of section 5 (1,890 feet). It also reaches section 4 nine minutes after the beginning of time period 2 and extends 745 feet through this section by the end of the 15-minute time period.

Both sections operate at level of service F, even though the demand/capacity ratios for these sections are well below 1.0. Why? These sections are in the congested regions of the speed/flow diagram (see Exhibit 4-12), as shown by the very low speeds (below 30 mph) that exist in these sections.