A traffic light is a signaling device that controls the flow of vehicles and pedestrians at intersections by displaying colored lights: red for stop, yellow for caution, and green for go. The lights change in a timed sequence so cars from different directions can take turns moving through the intersection safely.

Before traffic lights were invented in the early 1900s, busy intersections were chaotic and dangerous. Police officers had to stand in the middle of streets directing traffic by hand, which was exhausting and risky. The first electric traffic lights transformed city streets, making them much safer and allowing cities to grow larger and more complex.

Modern traffic lights use sensors to detect waiting cars and adjust their timing. Some intersections have buttons for pedestrians to press, which triggers a walk signal. In many cities, traffic lights are synchronized along major roads so drivers can catch multiple green lights in a row if they maintain a steady speed.

When people say something is getting a green light, they mean it's been approved to proceed, borrowing from the traffic light's most welcome signal. And if someone says a project got a red light, it means it was stopped or rejected.