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A unit such as this[1] monitors the state of the lights and will not allow an "illegal" output such as "all green".

[1] http://www.orangetraffic.com/en/products/edi-mmu-16e-malfunc...

What if the monitor unit malfunctions or is misconfigured?

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.

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.

It's my understanding that it is quite specifically installed on every traffic light (in the sense that it's both the purpose of the device and that it's legally required). Note that this does not control the lights, it simply monitors and switches to flashing-yellow (until a manual reset) if there's an error.

Mind you, none of this is first hand.

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