Detroit traffic officer William Potts created this three-color, four-way traffic signal in 1920. Previous illuminated signals used only red and green lights.

The addition of an amber "caution" light made driving safer and the three-color signal became the standard by the mid-1930s.

 This first traffic light was installed at Woodward Avenue and Fort Street in Detroit, Michigan. The automated lights cost 10% of what it cost to man a Street Semaphore crow’s nest.

Prior to that, Detroit had police towers with a red and green, like this at West Grand Blvd

Traffic lights weren’t the only traffic control device invented in Detroit.

In 1911, the city was the first to implement one-way streets as a way of improving traffic flow and making commercial deliveries easier.

The first stop sign in the United States was installed in Detroit a century ago in 1915.

■Detroit was among the first to have a police squad dedicated to traffic control, and second to New York City in creating a judicial court for traffic violations.

■The city drew national attention for using a tennis court line painting device to mark pedestrian crossing areas, safety zones and parking spaces.

Drivers education classes didn't begin until the 1930s

James Couzens was the key figure who brought order to Detroit's streets, enacting many new traffic management and safety ideas, first as commissioner of street railways, later as police commissioner, then mayor. Couzens would go on to serve as a U.S. senator.

James Couzens had been the business manager of the Ford Motor Company from its founding until he got sick and tired of working for Henry Ford in 1913. For a $10,000 investment in FoMoCo, Couzens was eventually paid $38 million.

https://www.reddit.com/r/Detroit/comments/8w3mk6/a_traffic_controllers_crows_nest_on_w_grand_blvd/
https://www.thehenryford.org/collections-and-research/digital-collections/artifact/227457/
https://www.thetruthaboutcars.com/2015/04/detroit-invented-traffic-cops-traffic-lights-no-parking-zones-towing-car/