Which city had the first electric traffic light?

Cleveland, Ohio, in August 1914 - a red and green pair at a single junction, operated by a policeman in a booth. Traffic control before that was entirely a matter of a person standing in the road with their arms out, which worked until the volume of cars made it lethal.

There was an earlier attempt, and it went badly. A gas-lit semaphore signal was installed outside the Houses of Parliament in London in 1868 to protect pedestrians, and it worked until it exploded about a month later, injuring the constable operating it. The idea was abandoned in Britain for over fifty years.

The amber light arrived in Detroit in 1920, invented by a policeman called William Potts who also built the first three-colour signal. Two conventions that look natural are not: red for stop was borrowed from the railways, which had used it since the 1840s, and green originally meant caution while white meant go - until a driver mistook a star for a white signal and caused a fatal crash, after which white was removed entirely.