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Photo Radar Operation

speed how it works

Although photo radar uses the same Doppler principle as traditional radar, its unique feature is that it transmits a very narrow beam across the road, thereby eliminating the risk of tracking two vehicles at once. Conventional hand-held radar, in contrast, transmits a very wide beam at a very long distance down the road—often requiring the operator to distinguish among speeding and non-speeding vehicles—all traveling at a wide range of speeds. Also with photo radar, the offending vehicle is tracked much closer to the photo radar car, generally no more than 100 feet from the photo radar vehicle.

The radar unit may be set up on a tripod, in a fixed installation, or mounted in a vehicle positioned at the side of the road. The MPDC plans to use various types of installations. All mobile photo radar units are operated by radar-certified Metropolitan Police officers who have received additional training in photo radar. These officers work in an overtime capacity. The radar beam is transmitted at an angle of 20 degrees across the road. When a vehicle enters the radar beam at a speed higher than the threshold speed established for the program, the Camera Control Unit is triggered and a photograph of the rear of the vehicle is taken. The Camera Control Unit is set up by programming certain information such as the time, date, film magazine number, location, film type, whether traffic is approaching or receding, and the threshold speed, and connecting it to the camera. Certain other types of data, such as the weather conditions and whether children are present, are written on a plate that is inserted into the unit. The data, both programmed and written, is exposed directly onto the film negative for the violation. The 35mm camera operates as an ordinary camera, but it is controlled by the Camera Control Unit and Radar Control Unit.

After each deployment, photographs of the speeding vehicles are processed and then reviewed by specially trained technicians, as well as a sworn member of the MPDC. Based on motor vehicle registration data, a Notice of Infraction is created and mailed to the registered owner of the vehicle. Under District law, the registered owner is responsible for paying or adjudicating the citation, or identifying who was driving the vehicle at the time. Motorists who receive photo-enforced tickets have the same rights to request a hearing and contest a citation as someone who is issued a ticket by a police officer.

Technical Note: If the vehicle is traveling toward the radar unit, the returning beam will be compressed. If the vehicle is traveling away, the returning beam will be expanded. How much the frequency is increased or decreased is directly proportional to the speed of that relative motion. What is most important about the Doppler effect is that the frequency change happens only when there is relative motion between the objects. If both objects are standing still (i.e., parked vehicles) there is no relative motion, and the reflected signal has the same frequency as the transmitted signal (e.g., no change in frequency = no speed reading). Police traffic radar merely measures this change in frequency and converts it to a speed reading.

The Fixed Installation Pole

A 5° wide K-Band Radar beam using Doppler radar is projected at 20° angle across the road from the front of the stationary radar unit. The unit takes 200-300 speed calculations per second. Deployment logs are completed and validated by certified, trained police officers to ensure proper setup, testing, and operations of the unit during the deployment period. When the radar control unit detects a vehicle in the beam traveling at or greater than the speed specified for that location, it instructs the camera control unit to take two photographs of the detected vehicle. The camera will not take a photograph if the system malfunctions or if there is more than one vehicle in the radar beam (Traveling in the same direction). A copy of the certified log, photographs that show the violation and a close-up of the rear of the vehicle, and a template that shows the vehicle captured in the radar beam will be provided as evidence for any adjudication of a radar ticket.