How Does Traffic Signal Preemption work

Traffic signal preemption is a system that prioritizes traffic flow as per requirements through manipulation of normal traffic lights. The most common reason is controlling traffic signals in the path of an emergency vehicle so that normal traffic is regulated and it has right of way. Traffic signals are also preempted to help public transportation quickly through intersections or for regulating traffic at railroad crossings to avoid collisions.
 
Traffic preemption systems can be categorized as follows.





Acoustic sensors – Acoustic sensors are linked to the preemption system where a specific pattern of wails or tweets from the siren of an emergency vehicle overrides the traffic signal. The advantage here is that such a system can be easily integrated into the existing traffic signals.

Radio signals – A radio-based signal in the short-range 900MHz band uses a directional signal from an emitter to preempt traffic signals. This radio signal is not blocked by lighting, weather conditions or other visual obstructions.

Line-of-sight – An emergency vehicle that is fitted with a line-of-sight traffic signal preemption device has an emitter that sends a signal ahead to the traffic lights in front of the vehicle. This ensures that the traffic, usually at intersections is clear before the vehicle arrives so that it can go quickly through. Once the vehicle has passed by, the traffic signal no longer senses the emitter’s signal and resumes its normal functioning. The downside to this is that it is not effective in inclement weather or before obstructions.

These are some of the traffic signal preemption systems in vogue today.          

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