How do automated license plate readers work?

By: - July 8, 2019 3:04 pm
ALPR camera

An ALPR camera in Paradise Valley. Photo by Jerod MacDonald-Evoy | Arizona Mirror

Automated license plate readers are high-speed cameras that take photographs of license plate numbers, capturing their geolocation along with the time and date. These cameras can be stationary, in which case they’re often attached to fixed locations such as exit ramps, telephone poles, traffic lights – or, in the case of Paradise Valley, fake roadside cacti. 

The license plate readers can also be mobile, such as those attached to police squad cars to capture license plate data of vehicles on public roads. Some ALPR cameras work hand-in-hand with automated red-light cameras or speed enforcement systems.


ALPRs can collect data on thousands of plates every minute. The information is uploaded to a central server. Some police departments, such as Phoenix and Mesa, share license plate data with hundreds of other agencies across the country. Others, such as Scottsdale and Tempe, only share with each other. 

While some cities have policies to only retain license plate data for 6 or 12 months, some police departments don’t appear to have retention policies at all.

Vigilant Solutions (acquired by Motorola Solutions in January 2019), ELSAG, L3 Technologies, and Neology, Inc. are the largest ALPR vendors. (ALPR technology originally developed by PIPS was sold to 3M and then acquired by Neology, Inc.) Data is often maintained by private companies rather than the police departments themselves. 

Vigilant Solutions subsidiary Digital Recognition Network and the Illinois-based MVTrac LLC are two companies that hire contractors to collect ALPR data. This commercially collected data is shared not only with law enforcement but also with insurance companies, banks, auto recovery (“repo”) companies, and credit reporting agencies – private entities that are free from retention limits, public records requests or public oversight.

Although ALPR data has been used to determine whether vehicles were at the scene of a crime, for example, or to track people down to fine them for minor traffic violations, that data can also be used to surveil and even target drivers who attend political events, go to gun shows, visit mosques, or drive or park near other sensitive places – whether or not the people with these specific license plate numbers have ever been accused of a crime.

When license plate numbers have been associated with a crime or an outstanding warrant, the system can send alerts to agencies or police officers to let them know if camera has scanned a plate on this hotlist. If information isn’t updated, whether by a law enforcement agency or a rental car company, it can lead to innocent people being pulled over at gunpoint.

Additionally, police officers could, for example, track various locations where a plate number has been photographed, or find out which license plate numbers were parked near the scene of a crime – whether or not the individuals were involved. ALPR data can reveal historical travel patterns and even make predictions about future driving behavior.

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