What does a red traffic light indicate?

A red traffic light indicates stop. Drivers and other road users must not proceed through the intersection unless a separate signal or local rule permits a movement such as turning on red.

The familiar three-color system uses red, amber, and green to organize competing traffic flows. Red is normally placed above amber and green on a vertical signal, making it easier to recognize even when the lamps are partly obscured. The first modern traffic signal was installed in December 1868 at Parliament Square in London, although it used gas lamps and was operated manually.

A common mix-up is treating red as merely a warning to slow down. That meaning belongs more closely to amber or flashing signals. A steady red requires stopping at the stop line; a flashing red generally functions like a stop sign. Rules for turning during a red phase vary by jurisdiction, so the basic meaning is universal but the permitted exceptions are not.