Classmates.com has added scans of yearbooks from all across the country. There are lots of localized pictures in these yearbooks, and there are bound to be signs and bridges and other cool things in the backgrounds of some of these pictures. If somebody had the time to go through and search these books they would probably find some cool old signs. You have to have a membership but I am pretty sure you can sign up for free. Use a fake name if you want. If you find anything interesting, be sure to post the pictures here or if you find interesting signs, post them to the Historic Road Signs group on Facebook (https://www.facebook.com/groups/1692760040977607/)
The site is at http://www.classmates.com/yearbooks/
Happy hunting!
I didn't realize Classmates.com was still around. I guess this is a good way for them to stay relevant.