Sub-problem 3c: The Southwestern Quadrant

Discussion
This sub-problem shows that we can use engineering judgment in combination with the HCM capacities for single and multi-lane ramp sections to determine where problem spots might exist in the interchange. It also shows us how we can look at a given spot from a number of different perspectives to determine how the facility is performing. When we analyzed the section of I-787 south of the merge as a three-lane basic freeway section, we predicted LOS C. When we assumed it was a weaving section, we predicted LOS F. Since the worst case governs, the conclusion we have to reach is that the weaving section is LOS F.

We also see the attention to detail that is required to identify bottlenecks. We looked at each of the ramp roadway sections individually to determine that the queue on Route 7 eastbound is caused by the restrictive capacity of the short single lane ramp section beyond the Route 7 exit. It has a v/c ratio of 1.01. There’s another short section of ramp, between the diverge from the collector-distributor and the merge with the semi-direct ramp, where the v/c ratio is again nearly 1.0. We determined that the cascading impacts of these two constrictions, in terms of flow dynamics, produce the queues on Route 7.

In summary, there is more than one way to view a given situation. Different views are possible, producing different results. Our responsibility as traffic engineers is to identify these views, study the system from each, and portray the results clearly and concisely to decide what recommendations to make regarding facility enhancements.