Regular US Route.
US Alternate Route.
If this is too confusing, just use the previous and suffix it with 'A'.
US Business Loop.
US Route on Tollway.
:^) Purple can be used instead of Prussian Blue.
The first sign above could then be used on freeways.
US Truck Route.
US Historic Route - The Mother Road.
US Scenic Route (2di).
Road through the Cumberland Gap.
US Scenic Route (3di).
I'd extend it to Canada using David Sturm's idea, also endorsed by R.V. Droz.
Tomorrow I'll throw up some other ideas how to treat US Routes on Expressways (Freeways and Tollways).