Traffic light timing is normally governed by time of day and traffic levels. But when an emergency vehicle respond to a 911 call, its need to arrive quickly at the scene of an incident is critical. To achieve that, traffic signals in many communities, including Cranberry, can be pre-empted by approaching fire trucks, ambulances and police cruisers.
When a sensor mounted at an intersection detects the unique frequencies of the approaching vehicle’s siren, it overrides the normal timing cycle, and either turns green or extends a green light in the direction that the emergency vehicle is heading.
Once the vehicle clears the intersection, the signal sequence moves incrementally toward restoration of its normal cycle. Cranberry’s standard practice in emergency response is to dispatch multiple vehicles from different public safety services, and each of them can trigger a separate signal override. As a result, the normal signal sequence can be derailed for extended periods, sometimes taking as long as an hour to become fully restored.
Protests from motorists who arrive minutes after an emergency vehicle has pre-empted a signal are among the Township’s most frequent traffic complaints. If they didn’t see the fire truck passing, they conclude that the signal system has failed – not than that it has worked according to design by giving priority to emergency responders.
In 1995, Cranberry Township was the first community in Pennsylvania to incorporate sound-based signal pre-emption. An earlier technology, Opticom, used light frequencies radiated by special emitters mounted on the top of an emergency vehicle to trigger the pre-emption. But the system had a poor track record and emergency responders didn’t like it.
The sound-based system responds to audio frequencies which are required by state law for emergency sirens. Every vehicle outfitted with a state-approved siren is able to operate the sensors.