How It Works: Red-Light Cameras

We have the science behind those eyes in the sky.

Confession: I’ve been caught by a red-light camera. It’s pretty much the only smudge on an otherwise spotless driving record (although I’m not sure if that’s a testament to my skills behind the wheel or just sheer luck). Either way, receiving a ticket in the mail from the city of Chicago with a close-up shot of my frazzled, late-to-an-interview face really stunk. Especially when it cost me $100 and I didn’t even get the job.

While the effectiveness of red-light cameras is often up for debate, red-light running is one of the top causes of accidents, injuring over 165,000 people per year, killing hundreds, and costing about $7 billion in property damage, medical bills, and insurance increases.

A 2011 study by the Insurance Institute for Highway Safety found that large cities with red-light cameras saw a 24 percent decrease in fatalities caused by red-light runners. The fact that running red lights still accounts for 22 percent of traffic accidents makes the demise of red-light cameras seem pretty unlikely.

So, if red-light cameras are here to stay, let’s take a look at how they actually work. What cues the cameras to snap that oh-so-flattering headshot of the driver, followed by a close-up of their license plate? Are they just endlessly snapping, hoping to catch rule-breakers in the act? Believe it or not, there’s actually a lot of science behind those eyes in the sky.

The triggers

Although we’ve all noticed the ominous cameras keeping watch over our intersections, the process actually starts underground. While trigger technology varies, most red-light systems use induction loops buried in the asphalt. Induction loops are made by layering 2 electrical wires on top of each other in rectangular loops. The wires are connected to a power source that supplies electricity and a meter that measures the current running through the loops.

If you had the pleasure of sitting through physics class in your formative years, this should all sound pretty familiar. But in case you “missed that day” (or opted to take botany, ahem), the power source creates a magnetic field, sending a constant, measureable flow of electricity through the wires.

When the light turns red, the system turns on. The meter monitors the current in the wires, and if a massive hunk of metal rolls over those wires, the meter will detect the shift in electric flow. In order for the meter to send a signal to the computer operating the camera, the car must be traveling at a particular speed to avoid ticketing drivers who are simply stopped on top of the loop.

Caught on camera

Typically, red-light systems have cameras at all 4 corners of an intersection, ready to catch traffic violators at different angles. If the trigger sends a signal to the computer that a car has passed through the induction loop, the camera automatically starts snapping.

The first photo will show the car entering the intersection, while the second shot catches the driver passing through the crossroads, focusing on their license plate. The computer then records the date, time, intersection, speed, and elapsed time from when the light first turned red and attaches it to the image.

With the specifics of your slipup detailed on the photo, the police have all the evidence they need to look up your address, drop your photo and fine in an envelope, and send the little surprise your way.

“But I wasn’t driving!”

So what happens if you receive a red-light camera ticket, but it’s a case of mistaken identity (say, your good friend Adam was breaking the rules in your ride)? Like any other ticket, you have the right to contest the charges in court. Unfortunately, in some states, red-light cameras only capture the license plate. While you’re still able to contest the ticket, you’ll likely have a harder time proving it wasn’t you.

If you can prove that the person behind the wheel wasn’t you, a judge can decide to throw the ticket out, effectively saving you the fine, any points issued, and a possible jump in your insurance rate. (And if you’re an Esurance policyholder, you can check out our handy What If® Calculator to see how a ticket or accident-related claim could impact your rates.)

The aftermath

Since paying my debt to the city of Chicago, I’ve been much more cognizant of the amount of time I need to make it through an intersection. The reality is I got off easy. Causing an accident and injuring myself or others would have been infinitely worse. Being punctual is great, but it’s better to be a few minutes late and arrive in one piece.

Share your story

Have you ever been caught running a red light on camera? Did the experience teach you a lesson in taking your time? Tell us about it in the comments below.

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4 Responses to “How It Works: Red-Light Cameras”

  1. Avatar for Jennifer Wood
    James C. Walker
    August 5, 2013 #

    Ms. Hall has captured the mechanics of how the cameras work.

    What was not explained is that the cameras are used to ticket mostly safe drivers for revenue, using two techniques that do not improve safety.

    1) Cameras are placed where the yellow intervals on the lights are deliberately mis-engineered to be shorter than required for the safe stopping time and distance for at least 85% of the vehicles. Chicago is a MAJOR offender in this technique. Virtually all yellows are set a the federal minimum of 3.0 seconds which is adequate only for actual approach speeds of up to 28 mph. On the major thoroughfares where the approach speeds go up to 40-45 mph, the yellows are short by 0.7 to 1.3 seconds. This is done entirely for revenue and actually can increase the total crash rates.

    2) Cameras ticket safe slow rolling right on red turns and/or ticket drivers who do stop but just over the stop line. Federal research shows only 0.06% of all crashes with an injury or fatality involve a right on red turn, so virtually all right on red tickets are entirely for revenue.

    All told, Chicago's red light cameras have produced about $400 million in fines, the vast majority of it from safe drivers for small technical fouls that had virtually zero risk to cause crashes. If the cameras and intersections were set to ticket ONLY unsafe drivers, they would lose far too much money to ever have been used. The cameras in Chicago are about money, not safety. See our website for info.

    James C. Walker
    Life Member-National Motorists Association
    Board Member and Executive Director-National Motorists Association Foundation

  2. Avatar for Jennifer Wood
    August 6, 2013 #

    As with many things, in California the cameras work differently. So, for those who live in California or might visit, here is essential info.

    1. The tickets cost $500.00 (five hundred dollars)!

    2. Camera tickets from any city in LA County (California) are voluntary, namely, the court does not report ignored camera tickets to the DMV. Skeptical? Google red light camera voluntary.

    3. California has Snitch Tickets, fake/phishing camera tickets sent out by the police to bluff registered owners into ID'ing the actual driver of the car. Over 30 California cities use them, and in some cities the fakes are more than half of everything the city mails out. Snitch Tickets have not been filed with the court, so they don’t say “Notice to Appear,” don’t have the court’s address and phone # on them, and usually say, on the back (in small letters), “Do not contact the court about this notice” or "Courtesy Notice – This is not a ticket." Since they have not been filed with the court, they have zero legal weight. You can, and should, ignore a Snitch Ticket. Skeptical? Google: Snitch Ticket.

  3. Avatar for Jennifer Wood
    James C. Walker
    August 7, 2013 #

    The obscenely high $500 fines in California produce huge revenues for the state which gets a big cut of the total loot. This has made banning the cameras statewide impossible so far.

    BUT, the massive fines have turned the citizens in many communities totally against the money-grab cameras. Local groups, often supported by opponents of the cameras like the National Motorists Association and others, have succeeded in getting the camera scams ended in over 50 cities including LA, San Diego, Hayward, and many communities.

    One city at a time the scams can be stopped, in California and everywhere else. It takes dedicated local people to lead the fight and cause the cameras to be in so much disfavor that the politicians agree to end the scams.

    Even in totally corrupt Chicago where red light cameras have taken in about $400 million and the city officials are addicted to the revenue stream — the scams could be stopped if enough people object.

    James C. Walker, Life Member-National Motorists Association

  4. Avatar for Jennifer Wood
    February 25, 2015 #

    Jackson, MS installed and then finally took out its red light system after numerous complaints. However, after about six months after the system was removed, my wife was stopped by a motorcycle policeman for running the light. He observed her after approaching from her right at the intersection. She said the light turned yellow as she entered the intersection and was able to go safely through it before the light turned red. The policeman disagreed and she was fined $300. I investigated and found two things 1) The yellow light timing was set way too low, and 2) the timing on the other side of the light (where the policeman was coming) was different from her side. The first problem had been reported by others as a means for red light companies to generate more revenue by the fast timing setting. The second problem would have caused the policeman to believe she was going through a red vs yellow light. I recorded this data using my digital movie camera. However, with an otherwise safe driving and ticketless driving record, we decided not make an issue of the event in court and paid the fine.

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