Rumor has it that Groundspeak is working on a solution to caches along a route, but for now check out GeoTruckers.com.
Created for truckers and RVers who drive/tow large vehicles, they are recording caches along America's highways and describing big-rig parking and access to them.
If you are on a trip and want to know what's available along, say, I-40, this is a great site for anyone.
They're new, so not a lot of caches listed so far, but you can contribute by describing caches you have found close to highways - before long we'll have a good database of caches along any highway we might be traveling!
Ed