Back in 1915 Detroit was a city of about 600,000 residents and 22,000 automobiles. To more effectively deal with the growing traffic problem the police department installed a semaphore system (see the below depiction) throughout the city.
The policemen controlled the signal on the semaphore by turning the word “Go” towards traffic to allow it to move or turning the word “Stop” towards traffic to stop the traffic. This method was so successful that other police departments throughout the country implemented the same practice.