Well, this could help you detect malfunctioning/hacked unit. But I doubt it's installed on every traffic light. AFAIK the traffic lights are controlled by PLCs which can be easily reprogrammed if you gain access to them.

On the other hand, I've had an assignment a few weeks ago to make a circuit to control some traffic lights for crosswalk, and I'm pretty sure it was simple enough that the only way to hack it was to have direct access to it with a soldering iron.