Sub-problem 5: I-87 Interchange - Stop and Yield Controlled Junctions

Southbound-to-Westbound Off-Ramp
One of the busiest ramps in the interchange is the southbound-to-westbound off-ramp. With the expansion in shopping centers and the development of more office space to the west of the interchange, traffic on this off-ramp has steadily increased.

The ramp is busy enough that there is a constant queue during the PM peak. The ramp is at or just beyond capacity. The AM peak is also busy, but a constant queue does not form. We can model this ramp as an unsignalized intersection. It is a single lane right turn onto a four-lane arterial.

Exhibit 2-62 shows the predicted performance of the ramp under the AM and PM peak hour conditions (Datasets 66-73). The ramp is heavily loaded during both time periods. We adjusted the critical gap downward from 6.2 seconds, which is the default, to 5.3 seconds, to get a PM Peak hour performance that matched the queue length observed in the field. All of the analyses (except the modified ones) were done with that value. The predicted AM peak delays range from 29.6 to 74.3 seconds and the PM peaks range from 86.4 to 243 seconds. The AM Existing and PM Existing values are consistent with delays observed in the field. The higher values in the Without and With conditions are unacceptable. One potential solution is an auxiliary lane on Route 146 running several hundred feet west from the ramp terminal. This would create a merge condition instead of a yield, and there may be adequate space to do this. Another possibility is to improve the sight distances at the end of the ramp to see if the motorists can use a shorter critical gap. We determined a 4.1 second gap would achieve tolerable operating conditions during both the AM and PM with condition. A gap that short yields delays in the PM With condition that are as good as or better than those predicted for the AM Existing and PM Existing conditions.

   with Sub-problem 5c