Police can track your movements with automatic license readers

via Josh Kellogg on flickr

via Josh Kellogg on flickr

Last month I attended Cambridge’s first-ever Police Innovation Conference, a two-day event that mixed law enforcement with technology experts.

During a session on wearable technology, the conversation turned toward privacy concerns. Some attendees worried aloud about whether always-on video cameras would be used to impinge on citizens’ rights. A representative from Mutualink pointed out that many police are already inadvertently surveilling ordinary citizens with automatic license plate readers.

Those readers, he said, are going to be the subject of our next big data debate.


“Ordinary people going about their daily lives have every right to expect that their movements will not be logged into massive government databases.”


Automatic license plate readers use high-speed cameras to identify passing plates, along with the current location and the time. Police can check this data against “hot lists” of stolen cars or wanted felons. The readers are commonly found on bridges, overpasses, or attached to the top of patrol cars, and they work very quickly. According to the Electronic Frontier Foundation, some can scan up to 1,800 plates per minute.

Recently, the American Civil Liberties Union released a study on automatic license plate readers. (The full report can be read here.) They collected records from 293 police departments and agencies on their readers. The ACLU found  that the departments’ data practices varied widely. Boston, for example, only retains its reader data for 90 days. But some cities keep the information on file for years, and still others – such as Mesquite, TX and Yonkers, NY – never throw it away. Altogether, the records add up to millions of stored plates, as well as some car occupants and pedestrians unintentionally included in the shot.

“While it is legitimate to use license plate readers to identify those who are alleged to have committed crimes, the overwhelming majority of people whose movements are monitored and recorded by these machines are innocent, and there is no reason for the police to be keeping records on their movements,” read 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.”

With enough data, it would be possible to track a single person. An article in Scientific Reports out earlier this year crunched mobility data on 1.5 million people. The researchers found that “even coarse data sets provide little anonymity” — only four data points were needed to identify 95% of individuals.

In the meantime, the EFF and ACLU have teamed up to sue two LA law enforcement agencies that failed to turn over their reader records.


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