Under the Vienna Convention, red is placed at the top of a vertical traffic signal.
The rule is part of the convention’s international standard for traffic lights, which also assigns meanings to green, amber, red, and combined red-and-amber signals. Red means traffic must stop, while green permits movement and amber warns drivers to stop if it is safe to do so.
The vertical arrangement makes the signal’s meaning easier to recognize consistently, including for drivers who may have difficulty distinguishing colors. When signals are arranged horizontally, the convention specifies that red should be on the side closest to oncoming traffic rather than above the other lights.
This is an international convention, not a universal domestic traffic code. Countries can differ in signal housings, mounting styles, and detailed operating rules, but the red-top convention is a standard feature of the Vienna system. The treaty was signed in Vienna on 8 November 1968 and entered into force in 1978.