Around my own state (Louisiana), many of the older bridges have the highway number stamped into the bridge, especially those built along major highways. This really helps when locating older highways, as the route numbers changed in 1955, so many of the route numbers are different than the highway signs say. I've attached a few photos below... is this just a Louisiana thing, or is it a national thing too?
State Route 200 (current LA 34)
State Route No. 152, current LA 19
State Route No. 13, current LA 546
And last, but definitely not least, US 80
Hopefully this isn't just Louisiana, since it makes it so much easier to find older alignments of highways.
Link to a US 90 bridge, recently refurbished - http://g.co/maps/3myr5