CHICAGO (CBS) — Civil libertarians in Illinois have raised privacy concerns about a new tool being used by some police departments.

Automated license plate readers (ALPRs) consist of a camera used to scan the license plate of every passing car. That number then is recorded and stored indefinitely.

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For example, ALPRs are used on red light cameras to catch drivers running red lights, and are used by the Illinois Tollway to catch motorists who don’t pay tolls.

Adam Schwartz, an attorney with the American Civil Liberties Union of Illinois, said he doesn’t object to certain uses of ALPRs, such as watching for a vehicle involved in a kidnapping.

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“Of course, we don’t object to running the plates of passing cars, and seeing if the car going by is the AMBER Alert car everyone’s looking for,” he said.

However, the ACLU objects to unlimited use of the devices.

“What this is is a database of the comings and goings of millions of innocent Americans,” he said.

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The ACLU has supported proposed legislation in Springfield that would restrict how and when ALPRs could be used, and how long the data could be stored.