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Originally Posted by HowStuffWorks "Running the Red Light"
For simplicity's sake, we'll only consider traffic moving in one direction through this intersection. When the light is green or yellow for incoming traffic, the computer ignores the triggers and does not activate the cameras. The system doesn't "turn on" until it receives a signal that the light is red. If you're already in the middle of the intersection when the light turns red, the system will not activate the cameras (this is not a traffic violation in most areas). Some systems wait a fraction of a second after the light turns red, to give drivers a "grace period."
In most systems, the computer will not activate the cameras if a car is just sitting over the induction loops. To trigger the cameras, you have to move over the loops at a particular speed. In most systems, there are two loop triggers for each lane of traffic. When the triggers are both activated in quick succession, the computer knows a car has moved into the intersection at high speed. If there is more of a delay, the computer knows the car is moving more slowly. If the car activates only the first trigger, the computer knows it is stopped at the edge of the intersection.
When a car activates both triggers after the light is red, the computer automatically takes a picture. This first shot shows the car just as it is entering the intersection. The computer then hesitates briefly and takes another shot. This catches the car in the middle of the intersection. The computer calculates the length of the delay based on the measured speed of the car. It's important to get two pictures of the car to show that it entered the intersection when the light was red and then proceeded through the intersection.
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