You may not like what I have to offer since it does take some time. But the resources are far better than when I started touring 25 years ago. Since I don't have an odometer, I can only guess how many miles I have on my touring bike - more than 100,000 - most of which is from actual tours. And the majority of these were my own routes.
There are two factors that are of greatest importance in choosing a route - traffic volume and shoulder width. I think traffic volume is more important of the two. If you have a road with almost no traffic, it hardly matters if it doesn't have shoulders - the ideal riding environment. If you have a road with a lot of traffic, shoulders are absolutely essential for safety - but the riding with be filled with traffic noise. Other factors to consider are parks, camping, river crossings.
In the old days you could order "Traffic Volume" maps from state DOTs. Nowadays, they are generally available on line. Some states have online bike maps as well. Iowa's is excellent because it gives traffic volume in numbers that are important to cyclists. Arizona's is only so-so because it is too general and the shoulder info is mixed. Getting accurate info on shoulders is tough. Of course, if there are Google Maps "Street Views" you can always verify.
Mississippi, apparently, does not have a statewide Traffic Volume Map - only county maps.
http://www.gomdot.com/Divisions/Inte...olumeMaps.aspx
For example - LaFayette County / Oxford
http://www.gomdot.com/Divisions/Inte...ayette_adt.pdf
Notice that Hwy 334 has far less traffic than Hwy 6.
In fact, until you get close to Oxford, it is ideal.
Speaking of numbers - here's a breakdown.
Less than 500 vehicles per day - Heavenly
500 to 999 - Quite nice, no need for shoulder
1000 to 1999 - O.K. but you have to use a little caution
2000 to 3999 - Doable. A shoulder really helps.
4000 to 7999 - Unpleasant, even dangerous without shoulder.
8000 Plus - Nasty, noisy, and stinky. Insane without shoulder.
Here's Georgia's statewide Traffic Volume Map -
http://www.gomdot.com/Divisions/Inte...ayette_adt.pdf
The nice thing about it is you can spot the low traffic highways by color.
As for highways - outside of the West and High Plains, US Highways tend to have fairly high traffic, high speeds, and a lot of trucks. Rural US Highways that parallel Interstates are a 50/50 shot. Sometimes they are alright. Other times they have a lot of development - thus traffic. Same goes for state Primary Highways. If they go directly between two cities - they will have a lot of traffic. As a cyclist, you have to be willing to take the long and winding road - if you want to get away from the traffic.
Similarly, most state DOT websites have county highway maps which show paved county roads. These usually have low traffic volumes and low speeds - like 45 mph. It takes a little work, but if you combine county roads with low-traffic stretches of state highways, you can put together an excellent tour - anywhere.
J
PS - ALWAYS!!! Check out bridges across major rivers. Some ban bicycles. Others are extremely narrow and are death traps. Others are new, with shoulders and fine views. I prefer to cross the big rivers on ferries - but there are very few left. St. Francisville is disappearing this year.