After the coming of the automobile, the first traffic controllers were foot patrolmen, directing traffic by hand. Then they were given hand-operated traffic lights. It was not until the early 1920's that automatic traffic lights were first used.
But these lights left an important problem unsolved. The amount of traffic passing through an intersection changes at different times of the day.
In 1927 two men patented "traffic-actuated" controllers. These were traffic lights designed to adjust to the amount of traffic passing through an intersection at a given time. One of these lights, invented by Harry Haugh of Yale University, was first installed in New Haven, Connecticut, in April 1928.
This device worked by means of pressure detectors in the road pavement. A car passing over a detector signaled the "call box" on the light pole, which caused the light to turn green for the approaching vehicle. This type of traffic light, with some changes, is still widely used today.
Charles Adler, also in 1928, invented a traffic light that used a microphone to activate the call box. When a motorist, facing a red light, blew his horn, the microphone transmitted the sound to the call box, which caused the light to change. Today, there are other types of traffic controllers that use sound to change the light.
