Stop bar and lead-in lights are required when the RVR is less than what?

The requirement for stop bar and lead-in lights is tied to the visibility conditions indicated by Runway Visual Range (RVR). When RVR falls below a certain threshold, reduced visibility means that additional visual aids are necessary to ensure that pilots can safely navigate during approaches and landings.

The correct choice indicates that stop bar and lead-in lights become essential when the RVR is less than 600 meters. This requirement is implemented to improve safety by providing clear visual cues to pilots during landing operations in low visibility conditions. When visibility is restricted, these lights help demarcate critical areas such as runway hold positions and provide guidance for a safe approach, thereby enhancing situational awareness for pilots.

Choosing a number lower than 600 RVR would fail to account for the need for these additional visual aids to maintain safety during conditions of significantly reduced visibility.