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License plate readers have proliferated across the country, from the Hudson River Valley to San Francisco Bay. But cities and states are all over the map on how long they hold information they collect and with whom they share it. That is according to a new report from the American Civil Liberties Union, which collated license plate reader policies from nearly 300 law enforcement agencies around the country.

Minnesota State Patrol deletes the data after 48 hours. New Jersey requires its police departments to hold the data for five years. Grapevine, Tex., doesn’t specify, which means the city could keep the data for as long as it wants. Some police agencies are allowed to use the information picked up by license plate readers for any criminal investigation. Other agencies also share the information with so-called fusion centers, where data from various government sources are kept.

The A.C.L.U. says you should care because the license readers are another form of location tracking. It warns that “enormous databases of motorists’ location information are being created” that could lead law enforcement authorities “to assemble the individual puzzle pieces of where we have been over time into a single, high-resolution image of our lives.”

License plate readers are often attached to bridges, street lights, and police patrol cars. They snap pictures of the license plate, record the date and time, and corresponding software turns it into readable data that can be stored and analyzed.

The readers present a classic case of how new technology bewilders the law. There is nothing that prohibits cameras in public places from collecting images. But the collection and analysis of location data has stumped judges, with some noting what a powerful window it can be into a person’s private life.

The mayor of Minneapolis learned from a public records request filed by The Star Tribune last year that license plate readers in his city had picked up his car 41 times in the previous year.

Only a handful of states have enacted laws on how the data can be kept and used, the A.C.L.U. report said.

American Civil Liberties Union