In an interesting article in last Sunday's Post-Gazette, Harrisburg Bureau Chief Tom Barnes wrote that, based on a trial in Philadelphia, red-light cameras soon could come to the city of Pittsburgh.

The cameras, which photograph the license plate of a car that runs a red light, are seen as promoting traffic safety.

Reducing such violations is important. According to the Insurance Institute for Highway Safety, 22 percent of traffic accidents in the United States are caused by drivers running red lights. These accidents kill some 800 people a year and rack up an estimated $7 billio­n dollars in property damage, medical bills, lost productivity and insurance increases.

The cameras also raise money from traffic fines as the photos, along with a ticket, are mailed to the owner of the vehicle. If you are caught on camera running a red light, you will get an unwelcome surprise in the mail, probably amounting to more than $100.

When TechMan hears about plans like this, he always has two questions: What about privacy? And how do these systems work?

Privacy is not as big a concern as it might be because in Pennsylvania the owner of the car can be held responsible for the violation. Cameras will take a photo of only license plates.

Some other states specify that the driver is responsible, and so a photo is taken of the driver. That is much more of a privacy concern.

But even if a photo is taken of the license plate, it is time stamped. It provides another entry in a database of where your car was at a specific time and likely where you were -- just like the E-ZPass system used on the Turnpike.

This data should be used only for what it is intended. But E-ZPass information has been sought in other legal cases, and presumably, so could red-light camera information.

As to how these systems work, the theory is simple, but the technology is complicated.

A red-light camera system has three parts -- one or more digital cameras, a computer and one or more triggers.

The cameras take the pictures, obviously. They are mounted high over the intersection to get the needed angle.

The triggers tell the computer when a car enters the intersection while the light is red -- a violation has occurred. The trigger will not go off if the car is already in the intersection when the light turns red.

But some argue that red-light cameras cause more rear-end collisions because people unexpectedly stop on a yellow light for fear of getting a ticket.

The computer receives the signal from the trigger, directs the cameras to take the pictures and stores the pictures after stamping with date, time, intersection location, speed of the car and elapsed time between when the light turned red and the car entered the intersection.

Usually two pictures are taken -- one when the car enters the intersection and one when the car is in the middle of the intersection. The spacing of the pictures is determined by the speed of the car reported by the triggers.

The type of trigger that has been most commonly used is an induction loop. It consists of looped wires buried under the intersection and attached to a source of electricity and an electric meter.

This creates an electromagnetic field. When a big hunk of conducting material, such as your car, goes through the field, the meter measures that there have been changes in the field -- a car has gone through. Most systems have two of these loops, so by timing the interval between the changes in the loops, the vehicle's speed can be determined.

Systems use other types of triggers also, such as radar, laser or air tube. Currently emerging is video loop technology. In this trigger, a computer analyzes video of the intersection frame-by-frame. The computer has been programmed to recognize changes that indicate a car passing. Video loops have the advantage of not having to dig up the intersection to install them.

So here is the scenario: You decide you are in too much of a hurry to wait until the light turns green. Because the light is red, the system is on.

As you pull out, the sensor loop tells the computer your car has begun to pull into the intersection. The second sensor loop confirms that you have not stopped and helps calculate your speed.

Bingo, the camera goes off, ticket through the mail.

So remember, in the future, if you are tempted to run a red light, Big Camera may be watching.

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.