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Long before there was a Route 66, in fact before there were any numbered highways in America, there was the first ever tans-continental route, the Lincoln Highway. And that starts in San Francisco, goes through both Reno and Salt Lake City then "straight across as much as possible", and includes some scenic and historic sections such as the old Oregon Trail route along the Platte River in Nebraska. You might also want to consider using US-36 across northern Missouri (as the old Lincoln Highway stays north towards Chicago) and come through Hannibal MO and Lincoln country in Illinois before finally dropping south into Kentucky.

AZBuck