Choose the sentence expressing the main idea of the text.
In 1920, a Detroit policeman named William L.Potts worked out an electric light system that allowed him to control three street intersections from one tower. He picked the colours red, yellow and green because railroads used them.
The origin of the railroad lights
W.L.Potts’s promotion in his work
The first street traffic lights
Policemen’s life in Detroit
The choice of colours for railroad lights
