TopoZone.com will let you browse areas.
My guess is that you don't have much map experience.
I'd start with an atlas next to the screen and stay in the 1:250,000 scale maps and 1:500:000 setting.
There are also DeLorme atlasses of each state which - although not perfect - are pretty good.
Many western states built their Interstate highways right ono top of the old US routes - so the post above is correct. Interstate biking is legal in these states - and, given the wide shoulders, pretty safe. But why would you want to bike on an Interstate?? Take a little time to scout out better back road options.
Remember, if it's a diagonal in the Great Plains - it will have a lot of traffic. Also, if it's the shortest route between two important points (cities, national parks, etc.) - it will probably have a lot of traffic. Be willing to do a few extra miles to catch the really nice roads. Who's in a hurry, anyway.