Sub-problem 4b: Two-way Stop Control with a Normal Urban Intersection Treatment

Exhibit 3-30 explains why the capacity lines shown in Exhibit 3-27 are in different places. The formula for computing the capacity of a movement that must yield to an opposing movement is given in HCM equation 17-3. This equation contains two parameters:

1. The critical gap, which indicates the length of a gap in the opposing flow required to accommodate the first queued vehicle trying to cross the opposing flow.

2. The follow-up time, which indicates the additional gap length required to accommodate each subsequent vehicle entering the same gap in the opposing traffic.

Larger values for each of these parameters will lower the capacity for the entering movement. The values shown in Exhibit 3-30 indicate lower values for the left turn crossing the opposing traffic than for the minor street entry movements. This indicates that drivers making left turns from the major street are able to accept shorter gaps in the opposing traffic than drivers who are entering the major street from a minor street approach. The result is a higher capacity for the WBL movement compared to the NBL movement.

One of the objectives of this exercise was to judge whether it is appropriate to consider the intersection in the context of a normal urban intersection with TWSC control. This task can be accomplished best by comparing the results in Exhibit 3-30 with the corresponding results obtained by treating each of the conflict points separately. We will examine the separation of conflict points in the next sub-problem.