Patent US 5,287,411

A siren detection system controls the lights at an intersection to direct traffic and permit an emergency vehicle to travel through the intersection unimpeded. The system determines the frequency of the sound emanating from a siren carried by the emergency vehicle by counting pulses that indicate the frequency of the sound, by determining the elapsed time necessary to count a selected number of pulses, and by utilizing the elapsed time and number of pulses counted to determine the frequency of sound emitted by the siren.