I wouldn't get your hopes up. Large government organizations usually don't have procedures in place for "weird member of the public wants to save a piece of obsolete tech" so they handle it badly. Some DOTs may even have some sort of by-weight accounting out of an attempt to keep employees from skimming off part of the scrap metal for their own profit.
I'd love to be wrong about this, though. Can't really hurt to ask, so long as you're happy with accepting "no" as the final word should that be the decision.
Now, can you get to the sign with a screwdriver faster than PennDOT can? Probably . . .