Part of me is assuming it was to do with the first traffic lights, but then I also feel like it’s probably older than that. Does it have something to do with military battalions perhaps?
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Ships (and now aircraft) have something called _navigation light_ — a red light on the left side, a green light on the right side.
Rules of the Sea say that if two ships are crossing paths, the one coming from the right has the right of way (Edit: That is very, very simplified, there are also rule about types of ships).
That is, if you see the green light on the right side of the other ship, you can go, if you see the red light on the left side of the other ship, you must give way.
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