Here's the First page of the NY 218 Story-I thought I'd split it up to reduce the loading time-appropriately enough, NY 218 comes in two parts. There's the part that leaves US 9W goes thru Cornwall-on-Hudson, Nicks the NE part of West Point, and rejoins US 9W at the NY 293 junction, which is on the next page. NY 218 runs concurrently w/US 9W for a mile, and leaves again at the Stony Lonesome Junction, goes thru Highland Falls and then ends for real at yet another elevated junction with US 9W. NY 218 was probably US 9W before the dual-carriageway was built in the 40's.  Nathan Perry supplied this information:
Speaking of the Storm King Bypass section of US 9W, it was underway by 
1940. From the 1940 New York: A Guide to the Empire State (NY Writers'
Project):

"Construction is under way (1940) on a new road up the Highland ridge to 
the west, but work has been halted from time to time by injunctions 
obtained by villages which this new and almost equally scenic road is 
destined to ignore."

Apparently Cornawall-on-Hudson and Highland Falls were afraid of losing business when traffic was diverted to the current US 9W.  The fears were unfounded, since neither is a ghost town 50 years later :)

Here's the elevated junction with US 9W North, just west of Cornwall-On-Hudson.

Two snaps of the road from a layby south of Cornwall-on-Hudson. This is very mountainous looking, and just inches from the Hudson-which is about 500(!) feet down from the wall. The third snap is what you see looking toward the southeast. The straight spit of land is a railway-unfortunately it's a CSX freight line. All passenger traffic runs on the east bank of the river.

These shots are just inches from the Washington Gate at West point.

Here's a snowplow all ready to go at a NYSDOT maintenance barn just east of the NY 218-US 9W-NY 293 junction. These guys didn't see too much action in the winter of 2001-2002.