I've done a few of these in my time. But this shows some of the least busy highways, not the least busy roads. Utah, for example, because that is my old stomping grounds -- Hwy 50 is downright busy compared to the roads that go up into the old mining country, towards Topaz Mountain for example.
I've driven on at several of these. Wyoming's US 212 over Beartooth Pass and Colorado and Oklahoma on US 385, California route 139, and US 395 in Oregon