You still need to use something to specify what stretch of highway you need. For example, Highway 70 runs the full length of the U.S., and runs concurrent with Highway 64 for a good distance. If you said you wanted to take 70 to 64, there would be a LOT of possible choices for it to pick from.
I agree with the poster who uses waypoints. I find the intersection I need on the map, drop a waypoint, and have the exact position I am looking for. One time I was even able to pick the westbound lane vs the eastbound lane on the interstate.