How Do Red Light Cameras Work? Complete Explainer

Red light cameras use sensors to detect vehicles entering the intersection. The most common detection methods are inductive loop sensors, which are wire loops embedded in the road surface at the stop line. When a vehicle passes over the loop, the change in electromagnetic field triggers the system. Some newer systems use above-ground radar or video analytics instead of in-road sensors. The sensors are connected to the traffic signal controller, which tells the camera system exactly when the light turns red. The system only activates when the signal is red and a vehicle crosses the detection zone.

When the system detects a vehicle entering the intersection on a red signal, it captures a sequence of images. Typically, the first photo shows the vehicle at or before the stop line with the red signal visible. The second photo shows the vehicle in the intersection, proving it continued through. Some systems capture a third image showing the vehicle exiting the intersection. Additionally, many modern systems record a short video clip (usually 10 to 15 seconds) of the violation for additional evidence. The images are timestamped and include data overlays showing the date, time, signal phase, and elapsed time since the signal turned red.

After the images are captured, the system uses Automatic License Plate Recognition (ALPR) technology to read the license plate number. The plate number is cross-referenced with the state DMV database to identify the registered owner of the vehicle. If the ALPR system cannot read the plate clearly, many jurisdictions have human reviewers who manually enter the plate number. The quality of the plate image is critical because if the plate is not clearly readable, the ticket may not be issued.

Contrary to what many people believe, red light camera violations are not fully automated. In most jurisdictions, every potential violation is reviewed by a trained technician or law enforcement officer before a citation is issued. The reviewer checks that the images clearly show the violation, verifies the vehicle and plate are identifiable, confirms the signal was red when the vehicle entered the intersection, and ensures there are no extenuating circumstances visible in the footage. This review process is one reason you may not receive a ticket even if you saw the camera flash. A significant percentage of captured events are rejected during review.

After the violation is confirmed, a citation is mailed to the registered owner of the vehicle. The citation typically arrives within 14 to 30 days of the violation and includes the date, time, and location of the violation, multiple photographs showing the violation, a citation number and instructions for viewing additional evidence online, the fine amount and payment deadline, and instructions for contesting the ticket. Some jurisdictions require the citation to be mailed within a specific timeframe (for example, within 14 days in some states), and failure to meet this deadline can invalidate the ticket.