Inventions

Traffic lights

London's gas-lit signal of 1868 came 18 years before Benz's petrol-car patent, and Cleveland switched on its electric one in 1914.

19 facts

At a glance

1868

Year of London's first signal

1914

Year of the Cleveland electric signal

1920

Year of the first three-colour signal, in Detroit

1935

Year the Manual on Uniform Traffic Control Devices made the three-colour signal the lens standard

18 years

Gap between London's signal and Benz's patent

John Peake Knight is credited with inventing the traffic light in 1868, with gas-lit signals at the Houses of Parliament in London.
By day the signal was a semaphore based on railway signalling, worked by a policeman. After dark it used red and green gas-powered lamps.
The gas lamps sometimes exploded and injured officers. A University of Nottingham page says one exploded and killed a policeman in 1869, and the lights were removed.
London's signal of 1868 came 18 years before Benz's 1886 patent for a gas-engine vehicle.
Benz's patent DRP 37435 for a vehicle with gas engine operation is dated 29 January 1886.

In 1912 Salt Lake City policeman Lester Wire built a handmade wooden box with red and green lights.
The signal installed at Euclid Avenue and East 105th Street in Cleveland, Ohio, on 5 August 1914 is widely regarded as the first electric traffic signal. The intersection is now on the Cleveland Clinic campus.
Claims to the first electric signal compete, and include Lester Wire's handmade wooden box in Salt Lake City in 1912.
It used four pairs of red and green stop-go lights, one pair on each corner post. They were wired to a manually operated switch in a control booth, which made conflicting signals impossible.
Its designer, James Hoge, received US patent 1,251,666 for a "Municipal Traffic Control System" in 1918.
Garrett Morgan's T-shaped signal was patented in 1923, nine years after the Cleveland installation.
Morgan's design was later reportedly sold to General Electric.

In the United States, the number of automobiles reached nearly 2 million vehicles by 1917.
Cleveland had the third-highest number of traffic deaths in 1917, at 19.8 per 100,000 population.

William Potts, a Detroit policeman, devised a four-way, three-colour signal in 1920. Detroit was the first city to use it, and the yellow light warned that the signal was about to change.
The 1935 Manual on Uniform Traffic Control Devices adopted the three-colour signal as the standard for signal lenses.
In 1928 Charles Adler Jr. installed a sonically actuated signal in Baltimore that changed when drivers honked their horns. WSU says it became the basis for modern traffic signals.
Countdown timers to show pedestrians whether they had enough time to cross were introduced in the 1990s.
Researchers expect connected vehicles that communicate with signals to improve intersection efficiency.