Police know where you drove last summer

The recent proliferation of license plate readers has allowed police departments to track millions of Americans and amass enormous databases detailing their movements, according to an American Civil Liberties Union report released Wednesday.

License plate readers, which are often mounted on major roadways, highway overpasses, or police cruisers, automatically photograph the plates of passing vehicles to create a log of traffic at a given place and time. The devices are intended to be crime-fighting tools, since they can almost instantly compare the scanned plates to so-called "hot lists" of stolen or wanted vehicles.

Yet the ACLU, after combing through some 26,000 pages of police documents obtained via Freedom of Information Act requests, found that millions of presumably innocent drivers have had their information logged and stored, sometimes indefinitely. The report, ominously titled "You are Being Tracked," warned that such databases were a dangerous encroachment on personal privacy — much like the federal government's controversial NSA programs.

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"License plate readers are just one example of a disturbing phenomenon: the government is increasingly using new technology to collect information about all of us, all the time, and to store it forever — providing a complete record of our lives for it to access at will," the ACLU said in a statement.

Though the devices have aided police, the hit rate on vehicles linked to crimes is extremely low, the report found. For every one million plates read in Maryland, only 47 were linked to serious crimes. And even then, the matches didn't necessarily help the police catch the wanted vehicles, further calling into question the effectiveness of the cameras, especially given the privacy concerns they pose.

The policies for storing information captured by the cameras also varied widely between departments. While Ohio's state police immediately deleted all non-hit plates, police in Yonkers, N.Y., kept their records indefinitely.

The ACLU said it was not opposed to the idea of plate readers entirely, but rather to the way the information they obtain is stored.

"Law enforcement agencies should not be storing data about people who have done nothing wrong," Catherine Crump, a privacy lawyer with the ACLU, wrote in the report. "Ordinary people going about their daily lives have every right to expect that their movements will not be logged into massive government databases."

However, some law enforcement officials said there was a compelling reason to keep everyone's data. Police may not be on the lookout for a given vehicle until well after a crime is committed, so keeping the entire database intact for weeks or months — or even years — could aid a future investigation.

"We'd like to be able to keep the data as long as possible, because it does provide a rich and enduring data set for investigations down the line," David Roberts, a senior program manager for the Technology Center of the International Association of Chiefs of Police, told the Washington Post.

The ACLU put out this handy explainer on license plate trackers:

And if you want to know if you're being tracked in your home town, the ACLU released this interactive map showing where such cameras are used.