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Home > Intersection Safety > Red-Light Running

Red Light Camera Enforcement

Camera Technology

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 cars enter the intersection. Photographs are taken of the rear of the car or both the front and rear ends. (If both the front and rear of a violating vehicle is to be photographed, two cameras will be used.) If large commercial vehicles are present on the road, front photographs are essential for identifying the owner of the truck. Normally, the camera records the date, time of day, time elapsed since beginning of 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.

Passetti 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
• Components that are environmentally friendly.

Three camera types are generally used for red light enforcement:

• Wet film/35-mm
  Industrial quality 35-mm camera (wet film) technology is the most common type used for photographing red light runners. Most automated enforcement systems equipped with 35-mm cameras produce black and white photographs, but some systems may produce color photographs. Although black and white photographs are less expensive than color photographs, it is often difficult to tell which light is illuminated on the traffic signal. Color photography can be used to eliminate any doubt as to whether the traffic signal is actually red.

  Cameras are located in a special unit to protect them from the elements and vandalism and placed atop poles. Poles may be either hinged or contain specially designed "elevator" systems to allow access to the cameras. A notable quality of wet film systems is the need to have personnel visit every camera location, often on a daily basis, to retrieve exposed film and reload. The film is then transported for processing, developed, sent to a facility for review and then converted to a digital image.

  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 signal 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. A common grace period is 3/10 of a second (though an international standard of 0.5 seconds exists) and a minimum speed necessary to activate the system ranges from 15 to 20 miles per hour.

  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 the 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 of 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 piezoe 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 passed the stop bar to be photographed.

• 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. Photographs produced by digital cameras may be in color or black and white. The configuration of digital camera applications is very similar to the one described for applications using 35-mm cameras. As with 35-mm cameras, digital cameras are placed in protective housings atop poles. Sensors are placed in the pavement in the same manner as for 35-mm applications, with two sets of sensors per lane to detect vehicle presence and speeds. The cameras are wired to the signal controller and the loop sensors so when the signal turns red, the system becomes active. When a vehicle traveling over the allowed range of 15 to 20 miles per hour crosses the sensors, two pictures will be taken. Again, the first picture will be before the entrance to the intersection, usually the cross-walk or the stop bar, and the second picture will be a preset time later, usually 0.5 to 0.9 second later, with the vehicle in the intersection.

  A major expected benefit of digital cameras is in easing the photo collection and accelerating the processing and distribution of notices of violation (tickets). This benefit is brought about because 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 such things as the film, processing, and the personnel required for daily film handling.

  Hansen introduces a variety of issues associated with digital cameras. Very importantly, he questions how the courts will view digital violation images. Specifically, he points out 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. Another issue is that some digital cameras are out of service while capturing an image. This could result in an inability to capture multiple violators i.e. the second or third violators going through the red signal.

• 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. Video images allow close-ups of both the front and rear license plates. Newer video cameras are digital which allows real-time transmission of images and, like digital still cameras, reduced transport, handling and reproduction costs. Full video sequences can increase the number of detected violations for subsequent ticketing.

  An 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 consequences of the violation. Additionally, video cameras can be used for non-enforcement activities such as traffic monitoring and surveillance, incident response, and crash reconstruction. If digital video cameras are used, the same concerns, i.e., lack of negatives and other non-tamperproof forms of evidence, etc. apply as for the digital still cameras.

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Program Contact

Ed Rice
ed.rice@dot.gov
202-366-9064

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