What visual aid indicates the runway holding position?

The indication of the runway holding position is represented by red inset lights. These lights are strategically placed at the holding position of the runway to provide pilots and vehicle operators with a clear visual cue to stop and wait before entering the runway. The use of red lights is standardized in aviation to signify a stop condition, enhancing safety by ensuring that vehicles remain clear of active runways and allowing for safe aircraft operations.

In contrast, the other options serve different purposes in airside operations. Flashing yellow lights typically indicate caution or a warning, while green surface markings generally indicate taxiway routes or safe paths for aircraft and vehicles. Blue boundary lights are used to outline the perimeter of taxiways and runways, providing guidance for vehicle operators but not specifically indicating a holding position.

Using red inset lights to mark the holding position creates a universally understandable signal that enhances communication and safety on the airfield, which is crucial given the high stakes involved in aviation operations.