DFW was the first airport to get RSL and I was in on some of the testing. The system monitors traffic on the runway and on the crossing taxiways. If something is going down the runway at 30 knots or more, it will light up a string of red lights along the taxiway centerlines. If something is moving on the taxiway at a speed the system thinks will carry it into the runway, it will light up a string of red lights along the runway centerline.
NOTICES TO AIRMEN