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Here is how it works:
At every busy intersection you place a minimum of twelve lights, four groups of three, a group facing each of the cardinal directions and so screened that each group can be seen only from its direction. Each group has one red light, one green light, one amber light. These lights are served by electrical power and each shines brightly enough to be seen at a distance of a mile, more or less, even in bright sunlight. These are not arc lights; these are very powerful Edison lamps - this is important because these lights must be turned on and off every few moments and must function without fail hours on end, even days on end, twenty-four hours a day.
These lights are placed up high on telegraph poles, or suspended over intersections, so that they may be seen by teamsters or drivers or cyclists from a distance. When the green lights shine, let us say, north and south, the red lights shine east and west - traffic may flow north and south, while east and west traffic is required to stand and wait exactly as if a police officer had blown his whistle and held up his hands, motioning traffic to move north and south while restraining traffic from moving east and west.
Is that clear? The lights replace the policeman's hand signals.
The amber lights replace the policeman's whistle; they warn of an imminent change in the situation.
But what is the advantage? - since someone, presumably a policeman, must switch the lights on and off, as needed
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