Patent US 6,980,125

A warning light system for a traffic intersection for alerting pedestrians and operators of passenger vehicles to the approach of an emergency vehicle. The warning light system is activated only by sounds in the range of frequencies which are emitted by the siren of an emergency vehicle. The warning light system has a warning light assembly having a control unit, and also has an audio sensor unit. The warning light assembly has a blue light and a white light, which flash alternately and repeatedly, after receipt of an audio signal by the audio sensor unit from the siren of an emergency vehicle, and processing of that signal by the control unit. The lights of the warning light assembly continuously flash until the sound emitted from the siren is no longer detectable by the audio sensor.