A vehicle that uses a line-of-sight traffic signal preemption system is equipped with an emitter which typically sends a narrowly directed signal forward, towards traffic lights in front of the vehicle, in an attempt to obtain right-of-way through a controllable intersection prior to arrival. These line-of-sight systems will generally use an invisible infrared signal, or a visible strobe light as an emitter. In the case of a strobe light, it may also serve a dual purpose as an additional warning light. To communicate to the traffic light, the emitter transmits visible flashes of light or invisible infrared pulses at a specified frequency. Traffic lights must be equipped with a compatible traffic signal preemption receiver to respond. Once the vehicle with the active emitter has passed the intersection, the receiving device no longer senses the emitter's signal, and normal operation resumes. Some systems can be implemented with varying frequencies assigned to specific types of uses, which would then allow an intersection's preemption equipment to differentiate between a fire engine and a bus sending a signal simultaneously, and then grant priority access first to the fire engine.