Problem 2: Operational analysis of the I-87/Alternate Route 7 Interchange

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This problem focuses on the interchange complex on the western end of Alternate Route 7. Three interchanges are intertwined: I-87 and Alternate Route 7, I-87 and State Route 2, and Alternate Route 7 and U.S. 9. Click the thumbnails of Exhibit 4-17 and Exhibit 4-18 for a visual introduction to this problem. Before proceeding with this problem, we first provide more details on the interchange geometry below.

The interchange between I-87 and Alternate Route 7 is a classic trumpet with the semi-direct ramp linking Alternate Route 7 west to I-87 south. Locally, it’s called Exit 7, as noted in Exhibit 4-18. The one nuance worth noting is that the right-hand ramp from Alternate Route 7 west to I-87 north leaves Alternate Route 7 east of U.S. 9 and follows a fairly long path on its way to I-87 north.

The Alternate Route 7/U.S. 9 interchange is a partial-cloverleaf. The connections to Alternate Route 7 east are on the eastern side of U.S. 9 while the connections to Alternate Route 7 west are on the western side. An extra ramp is needed to provide the connections, because the right-hand ramp from Alternate Route 7 to I-87 starts east of the bridge under U.S. 9, before the ramps from U.S. 9 connect to Alternate Route 7. Consequently, to provide connection from U.S. 9 via Alternate Route 7 to I-87, an extra ramp diverges from the U.S. 9 on-ramp north of NYS-7 and connects directly to the right-hand ramp from Alternate Route 7 west to I-87 north.

The interchange between I-87 and State Route 2, Exit 6, is a simple diamond. It’s called Exit 6 as labeled in the diagram. Getting to it coming southbound is a bit complex. That needs a short discussion.