In 1858, mechanical wrench-type traffic lights using red and blue gas as a light source were installed on major streets in London, England, to direct horse-drawn carriages. These were the world's first traffic lights. In 1868, British mechanical engineer John Neuter installed the world's first gas-powered traffic light in the square in front of Westminster Palace in London. It consisted of rotating square glass lanterns with red and green lights; red meant "stop" and green meant "caution." On its 23rd day of operation, the gas lamp suddenly exploded, killing a policeman on duty, and it was subsequently discontinued.
In 1914, electrically operated traffic lights appeared in the United States. These lights consisted of circular red, green, and yellow projectors and were installed on a tower on Fifth Avenue in New York City. A red light meant "stop," and a green light meant "go."
In 1918, controlled traffic lights and infrared traffic lights appeared. Traffic lights with controls come in two types: one uses pressure detectors installed underground, turning the light green as a vehicle approaches; the other uses a loudspeaker to activate the light, turning it green when the driver sounds a horn at a red light. Infrared traffic lights detect pedestrians stepping onto pressure-sensitive surfaces, extending the red light period to delay traffic and prevent accidents.
Traffic lights have enabled effective traffic control, significantly improving traffic flow, road capacity, and reducing accidents. In 1968, the United Nations Convention on Road Traffic and Road Signs and Signals defined the meanings of various traffic lights. A green light is a go signal; vehicles facing a green light may proceed straight, turn left, or turn right, unless another sign prohibits that turn. Vehicles turning left or right must yield to vehicles legally proceeding within the intersection and pedestrians crossing at crosswalks. A red light is a stop signal; vehicles facing a red light must stop behind the stop line at the intersection. The yellow light is a warning signal; vehicles facing a yellow light must not cross the stop line, but may enter the intersection if they are very close to the stop line and cannot stop safely. This rule subsequently became universally accepted worldwide.
