Sub-problem 2a: Analysis of the Existing TWSC Intersection Considering the Effects of the Adjacent Intersections

Let's discuss each of these issues and how each affects the operational analysis that we are about to complete.

Why and how do upstream signalized intersections affect the operation of a TWSC intersection? Traffic departs a signalized intersection in well-structured platoons. These platoons begin to disperse as they travel downstream from the signalized intersection. When they arrive at a downstream TWSC intersection, the platoon may remain somewhat intact, depending on the distance from the signalized intersection. When a TWSC intersection is relatively close to the signalized intersection, the large gaps that are present between the arrivals of each platoon are available for use by minor street vehicles. These large gaps generally have a neutral or positive effect on the TWSC intersection's minor movements. Consideration must also be given to the arrival patterns on the major street from the opposite direction (where there might not be a signalized intersection), which may have the effect of negating the platoon effects from the signalized intersection.

You will recall that we are using the procedure from Chapter 17 of the HCM. However, this model assumes that vehicles arrive randomly or independently of each other. This random distribution of headways results in a lower capacity than we would observe from the platooned arrival condition described above. We should also note that the HCM procedure does include a means for accounting for the non-random effects of upstream signalized intersections, albeit a fairly rough approximation.