What characteristic defines mandatory airport signs?

Mandatory airport signs are characterized by a red background with white numbers or letters. This distinct color scheme is specifically designed to indicate to pilots and vehicle operators that a certain action must be taken. The use of red is universally recognized as a signal for 'stop' or 'mandatory,' which aids in enhancing safety and compliance with airport regulations.

The red background reinforces the urgency and importance of adhering to the instructions conveyed by these signs. The white numbers or letters provide clear visibility and easy readability, even from a distance or in challenging conditions. This design helps prevent confusion and ensures that critical instructions are communicated effectively.

In contrast, other sign color schemes serve different purposes. For example, a white background with black letters typically indicates informational signs, while a blue background with yellow letters may signify services available at the airport. Similarly, green backgrounds with white symbols often indicate directional information. Each color scheme serves a specific function within the airport environment, but the red and white design is specifically reserved for mandatory signs to emphasize their critical nature.