Nearly all state departments of transportation have traffic volume information online.
It is usually referred to as AADT - average annual daily traffic.
Some states now have interactive map apps.
Here's the state map for Kansas - with all state and US highways -
https://www.ksdot.org/Assets/wwwksdo...untmap2018.pdf
Kansas is unusual in that it also has county road AADT numbers. Maybe 25% of states do this.
The map is color-coded for levels of traffic.
Traffic tends to be lighter in western Kansas with less population.
My general rule of thumb -
Less than 500 - Magical
500 - 999 - Comfortable
1000 - 1999 - Some caution
2000 - 3999 - Getting busy, shoulders nice
4000 or more - Busy, shoulders needed
I prefer empty roads without shoulders to busy roads with shoulders.
Also, even when busy roads have wide shoulders, these can disappear on bridges and overpasses.
Which makes for a pretty dicey situation.
Some states have shoulder information - but this is less trustworthy.
The Montana bicycling map is an example -
https://www.mdt.mt.gov/travinfo/docs/bike_map.pdf
Note the shoulder width colors and the indication of rumble strips.
If the rumble strips are in the middle of a moderate shoulder,
then the shoulder is larger useless - - BUT - -
drivers think you should be over on the shoulder.
So it is actually more dangerous than no shoulders.
You can use Google Streetview, too, to see if there are shoulders
and to ascertain whether rumble strips are nasty or not.