i just planned a 5 day trip from DC to Western Maryland via VA, WVA, and PA. I used google maps, and the "avoid highways" feature. From there, you can drag and drop onto smaller more rural sounding roads. I'll post back to let you know if I get run down by rush hour traffic on "Old Furnace Road" or "Ferry Landing Drive."

edited: there is also a seemingly sizable movement afoot to get google to add a 'bike there' feature. The petition got a 150 signatures that I know of at the U of Maryland. I think the idea is that google maps will recognize bike lanes, paths, and unpaved routes. (think C&O canal or your local rail --> trail)