The characters combine in a perfectly literal way: 红 (red) + 绿 (green) + 灯 (light) = 红绿灯 (traffic light). Note that the yellow light (黄灯, huángdēng) is understood to be part of the unit, even though it's not in the name.
While a traffic light is a universal concept, its interaction with daily life in China has a unique flavor. The foundational rule, taught to every child, is the rhyme: “红灯停，绿灯行，黄灯亮了等一等” (hóng dēng tíng, lǜ dēng xíng, huáng dēng liàng le děng yi děng), which means “Red light stop, green light go, when the yellow light is on, wait a moment.” However, learners will quickly encounter the real-world phenomenon of “中国式过马路” (Zhōngguóshì guò mǎlù), or “Chinese-style street crossing.” This refers to the tendency for a group of pedestrians to cross the street against a red light, operating on the principle of safety in numbers—the idea being that drivers will be forced to stop for a large enough crowd. This contrasts with the generally stricter, individual-based adherence to traffic signals in many Western countries. While this practice is becoming less common in major cities due to increased traffic enforcement and public safety campaigns, observing it provides a fascinating window into the dynamics of collective versus individual behavior in public spaces. The 红绿灯 thus becomes not just a traffic tool, but a stage for observing social norms in action.
红绿灯 is a high-frequency word used in everyday, neutral contexts.
The term is informal enough for daily conversation but also the standard, formal term. There is no other common word for a traffic light.