You roll through a yellow light that turns red a half-second later, and two weeks later, a ticket lands in your mailbox with a photo of your plate attached. It can feel like something out of a spy movie. In reality, it's a chain of sensors, math and paperwork.
What Sets Off the Camera in the First Place
Traffic cameras don't just snap photos at random. They're wired into the intersection, and most systems use one of two ways to spot cars:
Wires under the road. Coils buried near the stop line carry a small current. A car passing over disturbs it, telling the system where the car is and how fast it's going.
Cameras and radar overhead. Newer setups skip the buried wires. A radar unit or pole-mounted camera just watches traffic and tracks cars as they come in.
Either way, the job is the same: pinpoint a car's position and speed in real time, dozens of times a second.
The Split-Second Decision Behind Every Flash
That data goes straight to a computer at the intersection, which already knows when the light turned red and how long the yellow lasted. It compares that against the car's speed and position.
If the car is still coming toward the line after the light's gone red, and it's moving too fast to stop in time, the computer tells the camera to fire. All of it happens in under a second, before the car's bumper reaches the crosswalk.
Speed cameras skip the intersection timing. A radar or lidar unit just clocks how fast the car is going, and once that crosses the limit by a set amount, often 10 or 11 mph depending on the state, the camera fires on its own.
Two Photos, Timed to Prove the Violation
Once triggered, the camera doesn't just take one snapshot. A typical red-light violation produces at least two images, timestamped and synced to the sensor data:
One shot as the car crosses the stop line after the light has turned red
A second showing the car still inside, or exiting, the intersection while the light remains red
Many systems also capture a short video clip and a close-up of the plate. Multiple images matter because they rule out drivers already committed to the intersection, like someone stuck mid-turn when the light changed.
A Person Checks the Footage Before You Get a Ticket
Here's the part people skip: a machine doesn't issue the ticket. A vendor or police staff review every flagged image first. If it confirms a real violation, the ticket arrives by mail within a few weeks. If not, no ticket gets sent.
Do These Cameras Actually Save Lives?
Short answer: yes, when they're used right. IIHS research comparing cities with camera programs to cities without them found cameras cut the fatal red-light-running crash rate by 21%, and cut all types of fatal crashes at signalized intersections by 14%.
That matters to anyone driving through a monitored intersection, not just the people who get ticketed. In New York City, cameras cut red-light running by 73% and cut T-bone crashes, the kind most likely when someone blows through a red, by 65%, according to a 2024 city Department of Transportation report.
The danger they're aimed at is still very real. IIHS reports 1,119 people were killed in red-light-running crashes in 2024, with more than 141,000 hurt. About half weren't even the driver who ran the light. They were pedestrians, cyclists or people in other cars.
There's also a fix underway for one of the technology's oldest complaints, that cameras cause more rear-end crashes from drivers braking hard to dodge a ticket. As automatic emergency braking becomes standard equipment, IIHS researchers found it's shrinking that rear-end risk.
Who It's Really For
The flash in your mirror isn't really the point. The point is the driver who cruised through that same intersection an hour earlier and didn't get T-boned by someone running the light. All those wires under the road, the radar overhead, the person squinting at two photos before a letter goes out, that's what it's actually for.
