The area past Crow Hill and Route 100 is desolate, even now it feels like the middle of nowhere even though it's not. Especially around the bend after passing Saw Mill Rd, and certainly in the middle of a storm in the 70s. There are many areas where the road is close to the water.
Guardrails wouldn't be the same (if they were even there) back then, much like the area past where 120 meets Nanny Hagen. A local who wanted to take 100 towards the IBM to stay off the perceived more dangerous 684, might indeed take it to Millwood and then come through Chappaqua up King Street (120) on to Armonk. There are parts of that stretch basically at the water's edge, if she slid out where the ice was frozen solid along the shore and then broke through further in, they wouldn't be found w/o AWP type effort.