Experience.
YMMV - but people have different measures of what entails a good cycling route - services, shoulders, scenery, traffic levels. I have toured more than 100,000 miles and regularly include unpaved sections into my inineraries. Why? If you have a 40-mile paved road from Farmville to New City vs one that is 32 miles of pavement and 10 miles of dirt - 98% of the traffic will be on the paved road. Furthermore, the speed will usually be much slower on the latter route. Also, older county roads will rarely have the same level of engineering as state/federal highways - thus they will have lower speed limits and, generally, lower speeds. They will have more curves and steeper grades. That's the trade-off.
Unless you Google Streetview every mile - and some routes still have no Streetview possible - you can't be sure how much shoulder you have to use. Then there's the issue of rumble strips on the shoulder - which can sometimes not be seen in Streetview but make the shoulder largely unusable for cyclists. Nevada is the worst offender on this one. Such a situation is even more dangerous than no shoulder at all since drivers will expect cyclists to be on the shoulder. Technically speaking, I would concur that an Interstate shoulder is safer than a rural highway with low traffic but no shoulder. But I do not tour to listen to 18-wheelers all day and smell diesel fumes.
Most state DOT websites have AADT maps for state/federal highways. Some state such as Kansas & Iowa even have county road AADT maps. State highways usually have 55-65 mph speed limits. County highways are usually 45-55 mph. In Virginia, county roads are usually numbered in the 600s and 700s - although technically state highways. In North Carolina they have four-digit numbers such as 1701.
I'll give you an example: Nebraska AADT Map
http://www.dor.state.ne.us/maps/Statewide%20Traffic%20Flow%20Maps/2010-Statewide-Traffic-Flow-Map.pdfYou could ride across Nebraska on the Old Lincoln Highway - US 30. Plenty of towns and services. Usually a shoulder. Moderate to moderate-plus traffic.
Google Streetview Section: Between Hershey and North Platte
AADT - 3225
Or you could use Hwy 2 which is quite scenic in the Sandhills. Lower traffic in the west - much higher in the east.
Google Streetview Section: Mullen and Thedford
AADT - 715
Or you could choose Hwy 92 in combination with county roads for a very quiet, stunning ride.
Google Streetview Section: Between Tryon and Stapleton
(Check out the fab Round Valley Road between Broken Bow and Sargent.)
AADT - 130
AADT rought rule of thumb for rural highways -
Under 500 - Heavenly
500-1000 - Quite nice
1000-2000 - OK, but enough traffic to require caution
2000-4000 - Moderate to moderate-plus traffic, shoulder really needed
4000+ - Heavy traffic, shoulder essential - risky without shoulder for short essential connection
Best - J
