Both Detroit and Cleveland claim to be the home of the first traffic light.
We do know that both cities installed traffic lights in 1920, apparently independently of each other, and both inspired by the red, yellow, and green safety-light combo used by railroads.
However, London beats them both: In 1868, long before motorcars were even invented, the city government decided to bring some order to the chaotic procession of horse-drawn buggies, wagons, and pedestrians near the Houses of Parliament.
It installed a revolving gas lantern with red and green signals on its sides, manually operated by a bobby who turned the globe with a lever at its base.
Unfortunately, the light proved about as dangerous as the traffic.
On January 2, 1869, it exploded, injuring the police officer on duty and sending traffic-light technology back to the drawing board for another half a century.