I knew those inop lights would not be a detriment to our safe flight, but I took note of the NOTAM. While holding short #2, the aircraft in front of us asked ATC "Where are those red lights I usually see when holding short?" He didn't even know what those lights were called, let alone know that they were NOTAMed inop! I realized that if an airline pilot doesn't know how these lights really work, there could be others out there are as well who don't know.
Runway Status Lights (RWSLs) were developed by the FAA to decrease runway incursions and aid in runway situational awareness. The system automatically monitors the location of conflicting traffic, whether it be an aircraft or ground vehicle. RWSLs consist of embedded (in-pavement) red lights and show only the status of a runway- not a clearance for takeoff or a clearance to cross a runway. RWSLs consist of Runway Entrance Lights (RELs) and Takeoff Hold Lights (THLs).
Runway Entrance Lights (RELs) are located near the Hold Short bars on the runway side, and will be installed where a taxiway intersects a runway. These lights will illuminate when an aircraft is taking-off or landing. In this picture, you can see the lights to the left of the taxiway centerline, leading to the runway.
These red lights will illuminate when it is NOT safe to enter the runway and especially not safe for takeoff. Keep in mind, they have nothing to do with your clearance. If ATC issues you a takeoff clearance and these lights are illuminated, there could be conflicting traffic, so advise ATC of the light illumination and continue to hold short until the conflict is resolved.
Takeoff Hold Lights (THLs) are located on the runway and run parallel to the centerline stripes.
These lights are not installed at every airport in the US, just a few of the busiest. But if you fly to an airport with these installed, remember what the 'red lights' are and what they mean!
To find current NOTAMs you can visit https://pilotweb.nas.faa.gov/PilotWeb/ for a reference.
For more information on RWSLs, you can visit http://www.faa.gov/air_traffic/technology/rwsl/. The photos used in this post are from this website.