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Red-Light Camera Technology

Red-Light camera - Photos by Brian Chandler (used with permission) Usually, a photo detection system is composed of electromagnetic loops buried in the pavement, a terminal block that houses a microprocessor, and a camera (wet film, digital or video) atop a 15+/- foot pole. When the signal turns red, the system becomes active and the camera takes pictures when a vehicle enters the intersection. Photographs are taken of the rear of the vehicle or both the front and rear (if utilizing two cameras). The camera records the date, time of day, time elapsed since beginning of the red signal, and the speed of the vehicle. Upon review of the photographs and depending on State or local law requirements, tickets are issued by mail.

Suggested Use Requirements

In his report titled Use of Automated Enforcement in Transportation, Karl A. Passetti of Texas A&M University cited the following 10 requirements that automated enforcement systems should include:

The ability to capture, transmit, process, store, and recover captured images so that data may be managed in an efficient manner;
Sufficient resolution to satisfy court standards for the image-reading of vehicle license plates, clear detail of the vehicle, and identification of the vehicle operator (if necessary);
The capability to prevent the spreading of overexposed portions of an image (anti-blooming) that may result from vehicle headlights or sunlight from highly reflective surfaces;
Adequate differentiation of light to dark areas within an image to provide necessary details (also referred to as contrast latitude);
The ability to provide blur-free images of moving vehicles;
The ability to detect at varying levels of light;
Image enhancement circuitry to eliminate major sensor defects such as bright or dark columns which detract from the visible presentation of an image;
Continuous read-out of images to support monitoring along with single frame capture capability for recognizing several successive vehicles committing a violation;
The ability to be moved to different locations or to be mounted into a permanent position; and
Use of environmentally-friendly components.

The Photo Enforcement Process

Cameras are located in a special unit to protect them from the elements and vandalism and are placed atop poles. Poles may be either hinged or contain specially designed "elevator" systems to allow access to the cameras for maintenance purposes.

Although vendors of automated enforcement technology will often claim that a single camera can enforce four through-travel lanes, experience in New York and other areas has shown that reliable, accurate enforcement can only be performed on the first three travel lanes next to the red-light camera. By having the loop detectors used only for the automated enforcement system, interference and conflicts with other detectors used for the traffic control system can be avoided.

When the traffic signal switches to the red phase, the camera used by the automated enforcement system becomes active (ready to take photographs). Vehicles traveling over the detectors while the camera is active communicate to the system to photograph the vehicle. A small period of time, referred to as a grace period, and a preset speed necessary to activate the system are usually allowed in order to differentiate between vehicles attempting to stop or turn right on red and vehicles that clearly are running the red-light.

When the system is activated by a vehicle running a red light, at least two pictures are taken by the camera. The first picture shows that the front of the vehicle is not in the intersection when the traffic signal is red. This picture must show the pavement marking defining the intersection (usually the stop bar or crosswalk), the traffic signal displaying a red-light, and the vehicle in question. The second picture then shows the vehicle in the intersection a short time later (0.5 to 1.5 seconds). If driver identification is necessary, a third picture, showing the driver, may be taken. From the pictures taken, the license plate will be magnified to allow for identification.

The ability of a system to photograph only red-light violators is important in limiting the costs associated with each picture and the amount of resources needed to reduce the data collected by the cameras. The placement of traffic loops or other sensors will often determine how many pictures will be taken by the system and the capability of the system to differentiate between vehicles accelerating to run the traffic signal and vehicles attempting to stop or turn right. In Pasadena, California, for example, several problems were experienced with a red-light violation automated enforcement system. Ninety-five percent of the photographs taken by the system were of non-violating vehicles. The high rate of photographs was attributed to the improper placement of the loops, which caused left-turning vehicles "trapped" in the intersection making turns after the onset of a red signal and vehicles that would creep forward past the stop bar to be photographed.

Camera Types

Industrial quality 35-mm camera (wet film) technology was originally the most common type used for photographing red-light runners, but has since been replaced with digital photography.

The following two camera types are used for red-light enforcement:

Digital

Digital cameras have the capability to produce higher resolution, more sharply detailed images of vehicles and are equipped to prevent reflections or headlights from smearing the image. The configuration of digital camera applications is very similar to the one described for applications using 35-mm cameras.

A major expected benefit of digital cameras is in easing the photo collection and accelerating the processing and distribution of violation notices. The captured image can be electronically transmitted directly to the review facility and immediately incorporated into a citation. In addition, digital cameras eliminate costs of film, processing, and the personnel required for daily film handling.

There are a variety of issues associated with digital cameras, including how courts will view digital violation images given the ease with which digital images can be tampered. In comparison with a wet film system, an original 35-mm slide and photo can be produced in court to support the veracity of the evidence. This back-up plan does not exist with digital images. The following suggestions are offered:
- When a digital image is transferred to a review facility, store a duplicate image at the camera site using a "tamper proof" data storage device.
- The storage media should, when full, be handled as evidence and viewed only in instances when the original is questioned.
- Maintain a documented chain of custody so that the court can be shown an image that has not been viewed by human eyes.
- Other issues with digital cameras include the large file sizes for high resolution photos. This in turn brings about slower and more costly file transfers. This could be especially cumbersome with multi-camera systems.
Video

The use of video cameras and video processing technologies is receiving more attention for red-light enforcement activities. Video cameras can be used to determine a vehicle's speed as it approaches the intersection, predict whether or not the vehicle will stop for the red-light, and then track the vehicle through the intersection and record a brief video sequence of the violation. Digital video cameras allow real-time transmission of images and reduced transport, handling, and reproduction costs. Full video sequences can increase the number of detected violations for subsequent ticketing. Furthermore, some automated enforcement systems allow the violator to access their violation video on a secure website. After viewing the video, violators who may have fought the ticket may be more likely to submit payment.

An additional advantage of a video system may be its ability to detect vehicle speed and predict whether or not a red-light running violation will take place. With this prediction, it is possible to preempt the normal signal changes to create an all-red signal to prevent crossing traffic from entering the intersection when a collision is possible. Though this does not prevent the violation, it can help to mitigate the potential for a crash resulting from a red-light violation. Additionally, video cameras can be used for non-enforcement activities such as traffic monitoring and surveillance, incident response, and crash reconstruction.