Minnesota, for at least one state, has a bicyclists map which indicates high, medium, and low volume roads, bike trails, and which roads have at least 6 foot shoulders - also roadways, like interstates, on which cycling is not allowed. This is downloadable from the internet. Worked great in planning last summer's tour.

Google earth is a very useful resource, esp. aerial photos.

Generally secondary highways and farm to market roads work the best in my experience.