Don't let the term motorway fool you, this is a primitve road built by the conservation corps and hasn't been maintained since.  This primitive road follows the historic route of Lewis and Clark, and was used by the Nez Perce before them.

http://www.fs.fed.us/r1/clearwater/LewisClark/Assets/driving_lolom.pdf

This route is north of Hwy 12 in Idaho.  The Macgruder Trail in my other post is South of Hwy 12.  This would make a  great loop trip.

Craig