Early cars indeed had no turn signals; drivers had to use hand gestures to indicate their turning intentions until 1938, when Buick first introduced a relay-controlled turn signal system. Modern turn signals use yellow lights, as they have the strongest penetration in the light spectrum and can improve visibility in rainy or foggy weather. From a technical evolution perspective, turn signals have evolved from mechanical relays to today's LED and dynamic turn signals, which not only consume less energy but also feature adaptive lighting functions-for example, some high-end models adjust the light angle according to the steering wheel's turning angle. The "clicking" prompt sound of turn signals originated from the switching sound of early mechanical relays. Although it is now replaced by electronic simulation, the sound feedback is retained to remind the driver of the operating status. U.S.-spec cars allow red turn signals, but locally registered vehicles must follow the yellow standard, which is a unified regulation based on traffic safety and visual warning effects.
