**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.