Bikenh -

If the highway is part of the state highway network - SAP or SAS -
Then it is likely to have minimum pavement standards.
If I had to wager after 25 years of riding -
I'd say that moderate traffic roads with heavy trucks -
have worse surfaces than the lightly-travelled highways.
Since maintenance and paving cycles often have political components - i.e. fixed -
it's the moderate-traffic roads that are in worse shape before repaving.

I do agree that planning your own route has many rewards.
And county roads - esp. in the east - are untapped treasures.
There's usually less info out there for county roads -
And their surfaces can vary widely - but I usually don't care.

East of the Mississippi, it's usually the county roads -
that open up whole new worlds to explore.