To maintain safe separation of aircraft on the airport surface, air traffic controllers issue verbal clearances to pilots to sequence aircraft arrivals, departures, and runway crossings. Although controllers and pilots work together successfully most of the time, mistakes do occasionally happen, causing several hundred runway incursions a year and, less frequently, near misses and collisions in the United States. With this rate of incursions, it is imperative to have an independent warning system as a backup to the current system. Runway status lights, a system of automated, surveillance-driven stoplights, have been designed to provide this backup function. The lights are installed at runway-taxiway intersections and at departure points along the runways. They provide a clear signal to pilots crossing or departing from a runway, warning them of potential conflicts with traffic already on the runway. Existing FAA-installed radar surveillance is coupled with Lincoln Laboratory-developed algorithms to generate the light commands. To be compatible with operations at the busiest airports, the algorithms must drive the lights such that during normal operations pilots will almost never encounter a red light when it is safe to cross or depart from a runway. A minimal error rate must be maintained even in the face of inevitable imperfections in the surveillance system used to drive the safety logic. A prototype runway status light system has been designed at Lincoln Laboratory and installed at the Dallas/Fort Worth International Airport, where Laboratory personnel have worked with the FAA to complete an operational evaluation of the system, demonstrating the feasibility of runway status lights in the challenging, complex environment of one of the world's busiest airports.