Previous to pressure plates becoming the way to trigger stoplights, a simple proposal had a driver honk their horn to get the signal to turn.
He didn’t give up on trying to automate traffic safety, however. He continued to develop car safety devices, and in the late 1920s, he had minor success with a sonically actuated traffic signal. When a driver pulled up to a red light, honking the horn would make the light change. The system was intended for use at intersections where lightly traveled roads met major thoroughfares and where the traffic light needed to change only when a driver had to cross. (Source)