The only highway still labeled 101 is along the edge of the Pacific Coast, but obviously Memphis and New York City are in the east. It could be that a highway was renamed when it was turned into an Interstate route.

I seem to remember a description of the highway out of New York City toward the lower Midwest in Kerouac's On The Road. I'll have to look into that (lit, not song, but it is a clue.) The state highways are the ones he would have taken, or federal ones that weren't freeways yet.