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The Four Flavors of Automated License Plate Reader Technology


The Four Flavors of Automated License Plate Reader Technology

Automated License Plate Readers (ALPRs) may be the most common mass surveillance technology in use by local law enforcement around the country—but they're not always used in the same way.

Typically, ALPR systems are comprised of high-speed cameras connected to computers that photograph every license plate that passes. The photo is converted to letters and numbers, which are attached to a time and location stamp, then uploaded to a central server.  This allows police to identify and record the locations of vehicles in real time and also identify where those vehicles have been in the past. Using this information, police could establish driving patterns for individual cars.

The type of data ALPRs collect, analyze, and access often depends on what kind of systems they use and how they combine the data. 

Whether you're a policymaker, journalist, or a citizen watchdog, it is important to note the specifics about how these technologies are used.

1) Stationary ALPR Cameras

Many law enforcement agencies install ALPR cameras in a fixed location, such as permanently affixing the cameras to traffic lights, telephone polls, or at the entrances of facilities. The city of Paradise Valley, Ariz. even disguises ALPRs at cacti. Often a city or county will attach these on freeway exit ramps to capture the plates of every vehicle entering or leaving. With stationary cameras, law enforcement are able to capture only vehicles passing through that specific location. If cameras are pointed opposite each other, or can be repositioned remotely, law enforcement can know which direction a driver is traveling.

2) Semi-Stationary ALPR Cameras

Some law enforcement agencies acquire truck trailers or special surveillance vans outfitted with ALPR systems that they will tow and place at strategic locations. When parked, they function much like stationary cameras, capturing the plates of moving vehicles that pass within view.  For example, law enforcement agencies have placed these vehicles at fairgrounds during high-traffic events like gun shows and political rallies to capture information on attendees and to screen them against existing databases. The big difference is that semi-stationary ALPR cameras can easily be moved to different locations as police feel their surveillance needs change.

3) Mobile ALPR Cameras

Police patrol cars can also be outfitted with ALPR cameras, allowing law enforcement officers to capture and screen plates as they drive along their normal beat or from crime scene to crime scene.  Mobile ALPR cameras are also more effective at capturing the license plates of parked cars than stationary or semi-stationary cameras. With mobile ALPRs, officers can drive around a mall parking lot and pick up the plates of everyone shopping at that moment.  Of more concern to civil libertarians is the ability for law enforcement to target sensitive places, such as centers of religious worship, health facilities, immigration clinics, union halls, political headquarters, and gun shops. Only two patrol cars in Oakland, for example, were able to cover most of the city in a week of driving around, with a disproportionate amount of coverage in Black and Hispanic neighborhoods.