A question I have had more than one person ask is about the technology used to turn red lights green. Some folks wonder if it is true police (or Fire and Ambulance) have the technology. Others know about it but want clarification on how it works.
Traffic light preemption has been around for a few decades in one form or another. Some of the early versions had microphones attached to traffic lights listening for the noise patterns of sirens.
These systems were plagued by issues of direction. Sound echoes could cause the green to go to the wrong traffic.
The use of infrared (IR) light has become more popular than the siren based system. A strobe mounted with the standard police lights sends out an IR signal to turn traffic lights green. A sensor mounted on a traffic light watches for a specific IR flash pattern.
Light only travels in one direction, so it is unlikely to give the green light to wrong traffic. The strobes are
programmable making it hard for a citizen to get one and use.
A downfall to this system is when responders are coming at an intersection from two angles. One squad is coming straight at the call, a second car needing to turn at the intersection. The first IR beam the sensor sees will get the go ahead.
Interestingly the sensors that look for the IR signal are often times confused by people as being cameras for traffic enforcement. I have been at crash scenes when people point at the Opticom sensor and tell me I need to review the camera footage to see who was at fault for a crash.
Finally, GPS technology is starting to be added to the traffic preemption. When activated the squad car starts signaling speed and direction to the traffic grid. Then calculates when intersections on the car’s path will require green light. It can also calculate multiple vehicles approaching from multiple directions.