And of course the colored highway shields also became non-standard, again thanks to the MUTCD.

(More here, and incidentally, this site also has information on the Allegheny County Belt System, a local "we're too cheap to build a beltway" approach to getting people around the county. Note that the upper right corner of the Orange Belt enters Westmoreland County, briefly!)

And then there werethe pre-numbering routes, the most famous of which was the Lincoln Highway.

My county has actually posted historic Lincoln Highway signs along the roads which were the old Lincoln Highway (e.g. not the current US30 bypasses of Greensburg, Latrobe and Ligonier).