Auto safety questions: how red-light cameras work

In response to the growing numbers of traffic accidents at intersections, red light cameras have been installed to catch violators.

It is a scenario drivers are becoming all too familiar with: they think they have gotten away with running a red light only to receive a ticket in the mail months later. Police officers are not available to patrol every intersection anymore, both because there are too few of them and because they are needed on high-priority calls. Still, it is necessary for the police department to keep an eye on the traffic situation at all times. Too many accidents occur every year because motorist run red lights, speed, and ignore other traffic laws. For this reason, many jurisdictions have installed red light traffic cameras to catch offenders when they think nobody is watching. If you have ever been at the receiving end of a ticket because of these so-called "camera cops," it can be helpful to understand how they work.

One day Joe Driver receives an upsetting piece of mail. There is a note about a traffic violation that occurred weeks or months ago along with a fine and a picture of his car at the time of the incident. He has been caught in the act of performing a driving faux pas, even though he never saw a cop. It occurs to Joe that that box he noticed on the traffic light pole was not the crosswalk sign for which he had taken it. Instead, it was part of a camera designed to keep an eye on the intersection. Still, he wonders how this machine connected him to the picture. Was someone watching the camera at the far-off police station just waiting for him to drive by? No, the technology used in these cameras is actually far more sophisticated than that. Here is what happened the day that Joe ran the red light.

There is a traffic camera mounted at the intersection that Joe passes on his way home. Since he is running late and there is no one else there, he decides not to stop when he sees that the light is red. Instead, he rolls through at 35 mph. Buried in the asphalt of his lane, there are two loops of wires through which electricity run called induction loop triggers. Every time the traffic light to which these wire are connected turns red, the camera and computer in the system are activated. One loop in the set detects when Joe's car approaches the intersection, while the other one can tell if he crosses the stop line. This happens because the metal of Joe's car disrupts the normal electromagnetic levels of the charged wires. Any shift in the levels serves as a trigger, hence their name. Once triggered, the wires send messages to the traffic computer.

The computer is manned when Joe's car crosses the first trigger, at which point it waits to see if he will cross the second while the light is red. Once this happens, it sends a message to the camera to turn on. The camera, which is generally digital, although older models use film cameras, takes a picture just as Joe crosses the stop line and rolls into the intersection. This picture captures the car's license plate number. After a short pause, the camera takes a second picture, which catches Joe inside the intersection. Beyond providing double proof of the traffic violation, this second picture is also used to gauge Joe's driving speed. The computer calculates how far the car has traveled in between shots (which occur at set times) to judge if he was speeding on top of running a red light. The computer adds the day and time of the violation, as well as where he was and how fast he was going. Finally, it notes how much time passed between the light turning red and the car's entrance into the intersection.

Each violation photo is sent to the computer's memory bank, where it is stored until collection. The police station accesses the photos and has only to match license plates to their citizens. They then print out a ticket and a copy of the photos, which are mailed to the car's owner. If the state in which the violation occurs does not hold the owner responsible if another driver committed the crime, the owner will still receive the ticket. However, the camera will also take a picture of the front end of the car to identify who was at the wheel. In this way, the owner can prove his innocence in court if necessary. Police departments are very fond of these cameras because they provide indisputable proof of wrongdoing and work all day every day. Drivers, however, are not so fond of what some consider a sneaky way to catch people. They argue that there is no room for leniency, as a camera cannot give a warning under special circumstances. Like any other ticket, these violations can be disputed in court, but most people simply mail in the fee they have been charged. This is another perk for law enforcement, as they can catch everyone who runs red lights. This generates more money for the department on top of protecting thousands of lives every year.

In conclusion, the red light traffic camera offers a deterrent to reckless drivers, reminding them that someone -- or something -- is always watching them on the road. New designs are being implemented in different cities that use lasers, radars, and even video sensors in exchange for buried trigger wires, which require tearing up portions of the road. They cost little to maintain and are very reliable, all of which are important factors in a time when police departments are undermanned and under-funded. It is law enforcement's hope that the widespread use of these cameras will curb the rising numbers of accidents and deaths that result from running traffic lights.