Who invented the 1868 London traffic signal outside the Houses of Parliament?

John Peake Knight invented the 1868 London traffic signal outside the Houses of Parliament. His design adapted railway semaphore signalling for crowded horse-drawn streets.

Installed on 9 December 1868 near Westminster Bridge, the manually operated system used semaphore arms by day and red and green gas lamps at night. A police constable changed the signals, helping pedestrians—particularly Members of Parliament crossing near Bridge Street—move more safely through traffic.

The experiment was short-lived. In January 1869, a gas leak caused the signal to explode, badly injuring the officer operating it, and the device was removed. This early failure did not end the idea of traffic control: semaphore signals continued appearing elsewhere, and electric systems eventually replaced gas-powered equipment.

A common mix-up is confusing Knight with later American inventors. Lester Wire developed an electric signal in Salt Lake City in 1912, while William Potts created the first four-way, three-color signal in Detroit in 1920.