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How Red-light Cameras Work
by Tom Harris

These days, even traffic lights are keeping an eye on you.
According to the Insurance Institute for Highway Safety, 22 percent of all traffic accidents in the United States are caused by drivers running red lights. Every year, these accidents kill some 800 people and rack up an estimated $7 billion dollars in property damage, medical bills, lost productivity and insurance hikes. And this sort of traffic violation seems to be on the rise. In many areas, red-light violations have increased by 10 percent or more since the 1980s.

To curb this trend, more and more cities are installing red-light cameras. These fully automated devices collect all of the evidence authorities need to prosecute light-runners. If a camera catches you speeding through the intersection, you can expect a ticket (along with a photograph of the violation) to arrive in your mailbox a month or two later. In this edition of HowStuffWorks, we'll look at the basic elements in these systems to find out how they catch drivers red-handed.

The System
Red-light systems rely on some sophisticated technology, but conceptually they are very simple. The system includes only three essential elements:

  • One or more cameras
  • One or more triggers
  • A computer

In a typical system, cameras are positioned at the corners of an intersection, on poles a few yards high. The cameras point inward, so they can photograph cars driving through the intersection. Generally, a red-light system has cameras at all four corners of an intersection, to photograph cars going in different directions and get pictures from different angles. Some systems use film cameras, but most newer systems use digital cameras.

Multiple cameras are mounted high above the intersection to get a full view of any traffic violators.

There are a number of trigger technologies, but they all serve the same purpose: They detect when a car has moved past a particular point in the road. Red-light systems typically have two induction-loop triggers positioned under the road near the stop line (more on this later).

The computer is the brains behind the operation. It is wired to the cameras, the triggers and the traffic-light circuit itself. The computer constantly monitors the traffic signal and the triggers. If a car sets off a trigger when the light is red, the computer takes two pictures to document the violation. The first picture shows the car just on the edge of the intersection and the second picture shows the car in the middle of the intersection.

Most modern red-light-camera systems use digital cameras. Older ones use 35-mm cameras, in which case the film has to be collected for development periodically.

In some states, a ticket is issued to the car's owner, no matter who's actually driving. In these states, the red-light camera only needs to photograph the car from behind, since the authorities only need a clear view of the rear license plate. In other states, the actual driver is responsible for paying the ticket. In this case, the system needs a second camera in front of the car, in order to get a shot of the driver's face. The ticket is still sent to the car's owner, but the authorities have the information available if there is any disagreement down the line.

In the next couple of sections, we'll look at the major components of this system in greater detail.

Early Detection
As we saw in the last section, the main trigger technology used in red-light systems is the induction loop. An induction-loop trigger is a length of electrical wire buried just under the asphalt. Usually, the wire is laid out in a couple of rectangular loops resting on top of each other (see diagram below).

This wire is hooked up to an electrical power source and a meter. If you've read How Electromagnets Work, you know that when you send electrical current through a wire, it generates a magnetic field. Positioning the wire in concentric loops, as in any electromagnet, amplifies this field.

When a car drives over an induction loop, it disturbs the loop's electromagnetic field. This changes the total inductance of the loop circuit.

This sort of field affects not only objects around the loop, but also the loop itself. The magnetic field induces an electrical voltage in the wire that is counter to the voltage of the circuit as a whole. This significantly alters the flow of current through the circuit.

The intensity of this induction depends on the structure and composition of the loop; changing the layout of the wires or using a different conductive material (metal) will change the loop's inductance. You can also change the inductance by introducing additional conductive materials into the loop's magnetic field. This is what happens when a car pulls up to the intersection. The huge mass of metal that makes up your car alters the magnetic field around the loop, changing its inductance.

Construction crews cut into the asphalt to install loop sensors. You can see where a loop was installed at this intersection.

The meter in the system constantly monitors the total inductance level of the circuit. When the inductance changes significantly, the computer recognizes this shift and knows that a car has passed over the loop.

This is the most common trigger mechanism, but it's not the only one in use. Some areas have had success with radar, laser or air-tube sensors.

One emerging trigger mechanism is the video loop. In this system, a computer analyzes a video feed from the intersection. As the computer receives each new video frame, it checks for substantial changes at specific points in the image. The computer is programmed to recognize the particular changes that indicate a car moving through the intersection. If the light is red and the computer recognizes this sort of change, it activates the still cameras. The main advantage of this system is you don't have to dig up the road to install it, and you can adjust the trigger areas at any time. Essentially, it is a virtual inductive-loop trigger.

The trigger mechanism isn't worth much if it isn't connected to a central brain. In the next section, we'll see how a red-light system's computer puts everything together to construct a case against any traffic violators.

Caught Red-handed
As we saw in the previous sections, a red-light-camera system is controlled by a computer. To see how these computers bring everything together, let's look at a typical intersection and a typical traffic violation.

The central control box houses the computer, the brains of the system. The computer activates the cameras based on information it receives from the traffic lights and triggers.

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.

Click on "Run the light" to see how the basic elements of a red-light-camera system work together.

To fully document the violation, the computer superimposes some extra information on these two photos. It includes:

  • The date
  • The time
  • The intersection location
  • The speed of the car
  • The elapsed time between when the light turned red and the car entered the intersection

With all this information and photos of the infraction, the police have everything they need to charge the driver. In most areas, the police, or a private firm hired to maintain the system, simply look up the license plate and send the ticket in the mail. The driver (or car owner) can pay the fine through the mail and be done with it or he or she can try to contest the ticket in court. Of course, the police send the photos along with the ticket, so most drivers end up just paying the fine.

The camera is connected to the computer via long, durable wires. The camera feeds digital pictures to the computer, which stores them in its memory.

Red-light cameras have been around for more than 40 years, but they've only gained widespread popularity in the past decade. Police forces all over the world claim that these systems are a great addition to their communities. They serve as a deterrent against traffic violations, and they help police keep track of the worst offenders. Additionally, they are a good source of government revenue. It doesn't cost much to maintain the system once it's installed, and it works 24 hours a day, seven days a week, systematically catching violators and sending out revenue-generating traffic tickets that are really hard to contest.

To find out more about this technology, as well as some of the issues surrounding its use, check out the links on the next page.

Lots More Information!

Related HowStuffWorks Articles

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