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Garrett Morgan patented the first three-position traffic signal in 1923.
Prior to Morgan's invention, traffic signals only had two positions: "stop" and "go." This lack of an intermediate signal led to many collisions, as drivers had no time to react when the light changed. Morgan's innovation introduced a third position, a "caution" signal, which gave drivers time to clear the intersection before oncoming traffic was allowed to proceed. This "caution" signal is the precursor to today's yellow light.