An Amber Caution Zone

 

A Detroit, Michigan traffic officer, William Potts, created the three-color, four-way traffic signal in 1920 which was first installed at Woodward Avenue and Fort Street in Detroit, Michigan. It became the standard by the mid-1930s. The prior red and green-only signals resulted in numerous fender-benders: when the signal turned green, vehicles would immediately proceed through the intersection, expecting others to have stopped before crossing into the intersection. The middle amber/yellow signal allowed for a "caution zone" to proceed through the intersection if one could not safely stop when the signal turned red. Likewise, vehicles already stopped would need to wait until they received their green light to proceed. The design was adopted in future traffic light designs across the world. Today there are well over 310,000 traffic signals in the US alone. 

Note: The first traffic lights were invented in London in the 1860s, but they were not effective. Adapted from the railroad signal system, only red gas lights were used at night to signal stops, and green gas lights during the daytime. The life of the first traffic light signal was short-lived when a gas leak caused one of the signals to explode not long after being installed. It took the United States nearly forty years to adopt the system but with higher safety standards.