Under the Vienna Convention on Road Signs and Signals, prohibition signs commonly use red as their distinguishing color.
The convention establishes internationally agreed principles for road signs, signals, and markings so that drivers can interpret important instructions across national borders. Red is used to attract attention to restrictions, such as no entry, maximum speed, no overtaking, or limits on particular vehicle movements.
Many prohibition signs are circular with a white or light-colored center and a red border. Specific national systems can vary in symbols, wording, and details, but the red border remains a strong visual cue that an action or movement is forbidden or restricted.
Red should not be confused with every red sign function. Red is also prominent on stop signs and warning borders, while blue circular signs generally communicate mandatory actions in the convention’s system. The sign’s shape and symbol must therefore be read together with its color.