You purchase the Redlight Camera List POI file from www.redlightcameralist.com and transfer it to your GPS. You will be emailed updates to this file.
When you have a GPS with POI's (Points of Interest) set to alert you, your GPS keeps a certain perimeter around your vehicle. You can in most cases adjust the perimeter to different distances if needed.
When you drive close enough to a POI (red light camera) and it comes inside this distance based perimeter, your GPS will alert you with either simple beeps or voice commands. This depends on your model of GPS.
This gives the driver an advanced warning that lets them slow down long before they are at the POI (red light camera intersection). This can be very useful in bad driving conditions as any driver always needs to avoid accidentally sliding through a red light camera intersection.
Over time you will drive more safely, and not stop quickly to avoid going through a red light camera intersection. You will have already begun to slow down far in advance, avoiding rear end collisions with other drivers who stop suddenly when they see a red light camera.
You will also be made aware in advance of the intersection's high risk. Just because you stopped at this red light camera doesn't mean everyone does. You will have extra caution while at these intersections, which may help you avoid a collision with another vehicle who is running the red light.