A traffic light is a road signal for directing vehicular traffic by means of colored lights, typically red for stop, green for go, and yellow for proceed with caution.

The world's first traffic lights was invented by J P Knight who was the rail-road signal engineer and were installed near London's House of Commons in 1868. It had two semaphore arms which, when extended horizontally, meant "stop"; and when drooped at a 45-degree angle, meant "caution." At night, red and green gas lights accompanied the "stop" and "caution" positions.

• Earnest Sirrine patented the first automatic street traffic system in 1910.
• Sirrine's system used the non illuminated words "stop" and "proceed".
• Lester Wire invented an electric traffic light in 1912 that used red and green lights. The first hand-made model was a wooden box with a slanted roof so rain and snow would fall off. The lights were colored with red and green dye. The box was mounted on a pole, and the wires were attached to the overhead trolley and light wires. It was operated by a policeman.
• James Hoge patented manually controlled traffic lights in 1913, which were installed in Cleveland, Ohio a year later by the American Traffic Signal Company.
• Hoge's electric-powered lights used the illuminated words "stop" and "move".
• William Ghiglieri of San Francisco, California patented the first automatic traffic signal using red and green colored lights in 1917. Ghiglieri's traffic signal had the option of being either manual or automatic.
• William Potts in 1920, invented automatic electric traffic light systems including an overhanging four-way, red, green, and yellow light system. The first to use a yellow light.
• Garrett Morgan produced manual traffic signal in 1923 with 'Don't Walk' Signs. The Morgan traffic signal was a T-shaped pole unit that featured three positions: Stop, Go and an all-directional stop position. This "third position" halted traffic in all directions to allow pedestrians to cross streets more safely.
• On February 5, 1952, the first Don't Walk' automatic signs were installed in New York City.

Benefits of Signals

• Interrupt extremely heavy flows to permit the crossing of small movements that could not otherwise move safely through an intersection.
• Increase the traffic handling capacity of an intersection.
• Reduce certain types of accidents, most notably right-angle collisions.