I'm surprised how many of these roads I've been on.
Alabama 5 is far from lonely in most places. It goes through the edges of Birmingham's metro area, for goodness sake! While AL 62 seems to have the lowest overall AADT, it's not really desolate. If we're looking for a sizable route length, AL 17 would be a really desolate one, passing though no towns of major significance (but a few county seats) and lots of quiet space in between.
Rhode Island 102 isn't that lonely. It just doesn't hit any major towns or cities.
Mississippi 57...really? It's one of the state's busier two-lane roads that isn't a US Route. Without choosing tiny spur routes into minuscule towns, 26 is fairly quiet. 30 is empty.
Florida 29 is a decent choice, though I think there's a few others with lower counts. It really goes though a lot of desolate places in between some small towns.