The mapping varies considerable by state.
Federal highway legislation for the past 20 years has required multi-modal planning.
That means that states have included bicycle/pedestrian use in their DOTs.
(Sometimes that is 10% a a single person's job description - FWIW)
The results reflect as much.
Oregon's bike map is super - has been since well before the federal mandate.
Arizona's is iffy - their criteria for traffic volume is too high.
Also, states often only consider state highways -
when county roads are nearby which are far preferable.
(Except that the maps are usually free)
What federal mandate? Oklahoma (where I live) does not have a suitability map and the DOT refuses to produce one even if the grunt work is done by volunteers.