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
There’s a box nearby that tells the traffic light what to do. It literally cannot send the green signal to the lights in both directions. Since all the instructions come from the same place, there’s never desynced signals.
Also, if a traffic light fails (power outage) it becomes a 4 way stop. So even if it fails, the intersection is still safe (provided the drivers follow the rules)
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.
Well, it’s not quite *never*. Malfunctions like that aren’t impossible, just unlikely.
The electricity that makes the traffic signals light up, comes out of a box containing some control circuits. The different lights facing different ways at the same intersection, they all get their electricity from the same control box, and the box is designed so that when the circuit powering one green light is closed, the circuit powering the opposite green light, *must* be broken, and vice versa.
(Or alternatively the lights are driven from different boxes, which are communicating with each other to the same effect.)
Electronics engineers have many kinds of switching components to choose from, and some of these components – notably, a common type of device called “relays” – are designed as magnetically-controlled “three way switches”. In a three-way switch, a central terminal B can make contact with either terminal A *or* terminal C, but not both. Components like this are ideal for setting up “mutually exclusive” circuits, where either one of a pair of wires can be receiving power, but never both at once.
N.B.: I’m kinda answering as if it were 50 years ago. Relays are generally considered the ‘oldschool’ way of building reliable control electronics. They’ve been used in things like streetlights and elevators going waaay back to the early days of electricity. More modern traffic systems undoubtedly use more digital and solid-state control and switching components, but I’m not so up-to-date on how that works these days.
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