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.
I assume you are wanting a site that does routing with an option to avoid interstates. You will be better off to purchase Delorme's TopoUSA or Street Atlas. Either of these software packages will let you specify routing preferences for different classes of roads. The main difference between the two programs, from a cycling perspective, is that TopoUSA wil give you elevation profiles of your routes.