https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0ahUKEwiIy9i245HVAhUMFz4KHR8fCw4QFgg6MAU&url=https%3A%2F%2Fwww.newspapers.com%2Fnewspage%2F185336089%2F&usg=AFQjCNHN-lgwyXZ4eSkKEMmmZwZnKYD26A
This newspaper article gives good insight on when the first traffic signal mast-arms were installed, but I need help with farther investigating on when the first mast-arms were up. So far, we have 1975 for Philly.