Why are traffic lights so fail safe? Why does it never happen that the North/South street AND the East/West street both have a green light?
In: Engineering
Because they are built that way.
Joke aside, a good portion of the control boxes for traffic lights isn’t there to make them work, but to stop any error condition. Aside from carefully planning out all allowed patterns so dangerous ones don’t happen in the first place, there are detectors for the dangerous ones that shut the whole system down.
Take an old-fashioned electromechanical control circuit, for example. There, the switch that enables green lights for one direction would also switch off the power to the switch for the green lights for the other direction (and vice versa). So unless there’s a mechanical defect, that can’t happen.
Then, if there was a mechanical defect, there’s a switch that turns on only when both direction’s green lights are on. And that switch would cut the energy to all green lights and trigger the emergency program (e.g. yellow blinking).
And then another shutoff switch for every other dangerous combination.
Even nowadays with computerised systems, there are two independent controllers. One controls the light, and the other only watches the signals and checks them for disallowed combinations.
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