When my folks lived in Denver during the war, and for some years after depending on traffic, the higher elevations of almost all the highways in Colorado were either gravel or dirt. Cheaper to maintain, just run a grader over them from time to time to knock down the ruts. Freeze & thaw can do nasty things to pavement lifetimes.
When I lived in Gunnison in the 1980's there were several secondary roads that had been paved (usually blacktop) at some point in the past(1950's?) but were returning to gravel. There'd be a stretch of blacktop, then dark gravel, then lighter gravel or dirt then another stretch of blacktop. As an example, the road up to Waunita Hot Springs was like that. I always figured they were paved in some fit of enthusiasm and then the counties didn't have the $$$ to keep them up. I suppose that's changed by now although I haven't checked on it.
hank