Sub-problem 6b: Consideration of a Grade-Separated Flyover

Step 2: Results

The flyover eliminates all of the critical conflict points and leaves us with only three conflict points as illustrated in Exhibit 3-44. We will consider these conflict points separately:

1. EB through vs. WB Left: This conflict was examined as a separate conflict point with TWSC operation in Sub-problem 4c (see Exhibit 3-31). It was concluded that the TWSC operation would accommodate the fairly low WB left turning volume (120 vph) with no difficulty. The v/c ratio was 0.56 and the estimated delay was 41.7 sec/veh, corresponding to LOS E.

2. EB through vs. NB right: This conflict point was examined as a freeway merge in Sub-problem 4d (see Exhibit 3-34), using the procedures from HCM Chapter 25. The resulting density (i.e. the service measure for freeway merging) was 17.7 pcpmpl, corresponding to LOS B.

3. NB left vs. WB through: This is a new merging conflict point created by the flyover as a substitution for the previous crossing conflict. If the flyover was designed for a freeway-type merge with an adequate acceleration lane, it is suggested that the HCM chapter 25 procedure would also be the best choice for analysis of this conflict point. Assuming the same operating parameters as the eastbound merge, with a 300 ft acceleration lane from the flyover, the resulting density is estimated at 8.5 pcpmpl, corresponding to LOS A.

So, we can conclude that the construction of a northbound flyover at this intersection would eliminate the critical conflicts and leave an acceptable operation at the remaining conflict points. The actual decision to construct a flyover would, of course, require consideration of many other factors that are beyond the scope of this analysis.