My understanding has it that it's a comparatively simple piece of hardware, and isn't networked. It certainly ever happens that they're wired up wrong, but aren't something you can remotely hack.
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.
Mind you, none of this is first hand.