Runway Entrance Lights (REL) These flush-mounted, in-pavement, unidirectional red lights run parallel to the taxiway centerline and are directed toward the pilot at the hold line. Stop at the hold line when the lights are illuminated, which means there is traffic on the runway or final approach within the activation area. Contact ATC if your clearance to proceed onto the runway conflicts with the lights. If they become illuminated when you're already on the runway, proceed according to your best judgment knowing that the runway is unsafe to enter or cross. Contact ATC at the earliest possible opportunity. Ref. AIM Para. 2-1-6-b