Traffic light

A traffic light is a signal that controls traffic at road junctions using coloured lights. The first traffic light was installed on 10 December 1868 outside the Houses of Parliament in London, to help pedestrians cross the busy road. It was designed by a railway engineer, John Peake Knight, used semaphore arms by day and red and green gas lamps at night, and was turned by hand by a police officer.^[1] It predates the motor car, and was therefore built to stop horses.

The first one

In January 1869 a gas leak caused one of the lamps to explode, injuring the police officer operating it. The signal was removed soon afterwards, and London did not get traffic lights again for more than fifty years.^[2]

Electric traffic lights appeared in the United States in the 1910s. The familiar three-colour light, with an amber or yellow light added between red and green, followed in the 1920s.

Colours

Red for stop and green for go were borrowed from railway signals. Amber was added to warn that the light was about to change, a warning that drivers have been interpreting in two opposite ways ever since. It is the only colour that means both “slow down” and “hurry up”, depending on who is asked.

• Red: stop.
• Green: go.
• Amber: disputed.

See also

• Red – stop
• Green – go
• Lamppost – a quieter relative

References

1. Semaphore, S. (1868/2003). Stop the Horses. Fictional reprint.
2. Junction, J. (2010). “Fifty Years Without”. Road History Quarterly 1: 1.
3. Amber, A. (1999). Slow Down or Hurry Up? Highway Books.