Courts are at least starting to rule that police need a warrant to attach a GPS tracking device to your car, even if the feds are fighting tooth and nail against that requirement. But that protection may be almost moot if the police can follow everybody's movements by tracking their license plates with roadside cameras and storing and linking the captured information. And, as I've written before, that's exactly what the police are doing, in an ad hoc, but increasingly networked and organized way. Now an Americans Civil Liberties Union report details how widespread the practice has become, and how the federal government encourages the adoption of license plate scanners as a matter of official policy.

The ACLU report, You Are Being Tracked: How License Plate Readers Are Being Used to Record Americans' Movements, includes a nice, real-world example of what license plate scanners can do, even at this early date.

The potential privacy harms discussed in this report are not merely theoretical. In August 2012, the Minneapolis Star Tribune published a map displaying the location, obtained via a public records request, of the 41 times that Mayor R.T. Rybak's car had been recorded by a license plate reader in the preceding year. The Star Tribune also reported that of the 805,000 plate scans made in June, less than one percent were hits. Yet for as long as the information was retained, the other 99 percent of scans were also vulnerable to the risk that they might be released, used by the police to track innocent people, or otherwise abused. The alarming fact that a law-abiding citizen's sensitive location history could be revealed so easily was not lost on this exposed mayor. In response to the Star Tribune's reporting, he directed the city's chief of police to recommend a new policy on data retention. We hope the findings of our report spur similar policy changes.

You can only hope that more politicians will share Mayor Rybak's concern over the ease with which the technology can track people's movements. If history is any guide, though, officials are more likely to see potential in the scenarios for abuse presented in the report. The ACLU points out that the scanners can be used to track political activists, whisteblowers and opponents to sitting politicians.

Almost three-quarters of police agencies surveyed in 2011 reported using license plate readers, and 85 percent of them planned to increase deployment of the technology. The scanners are subject to only those limitations that agencies may care to adopt, both in terms of application and the storage of gathered data, with some policies explicitly claiming, "reasonable suspicion or probable cause is not required."

In Maryland, which is rapidly becoming a true surveillance state, with conversations on buses routinely recorded, "three-quarters of Maryland's law enforcement agencies are networked into Maryland's state data fusion center, which collected more than 85 million license plate records in 2012 alone." For every one million plates read in Maryland, only 47 are associated with serious crimes.

Some agencies contacted by the ACLU do delete the data they collect after a set interval (immediately for the Ohio State Highway Patrol, if a capture isn't related to a case, 30 days for police in Tiburon, California). But Jersey City holds on to its data for five years and some agencies store captured license plates indefinitely.

Police departments have their own reasons for favoring technology that allows them to track people's movements, subject to few limitations, but the federal government actively pushes the use of license plate scanners. The Department of Homeland Security has handed out anywhere from tens of millions to "billions of dollars in grants," depending on the source, just to encourage the adoption of this technology.

Of course, federal agencies, including Customs and Border Protection and the DEA, are themselves enthusiastic users of license plate scanners.