NATIONAL  ·   August 4, 2014

Red light, green light: How traffic signals work

- The first patented electric traffic signal went into operation 100 years ago. PostTV looks into how a modern traffic signal system keeps traffic flowing—or not—in a busy city like Washington, D.C. ()

- The first patented electric traffic signal went into operation 100 years ago. PostTV looks into how a modern traffic signal system keeps traffic flowing—or not—in a busy city like Washington, D.C. ()

Red light, green light: How traffic signals work (3:20)

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