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
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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