There's a good chance that a local law enforcement agency or a private company has all the information it needs to track your general whereabouts over the course of weeks, months, or even years. This detailed surveillance is made possible by automatic license plate readers that can capture and record the details on every single vehicle that passes by them. They've existed for years mounted to bridges and police cars, but their numbers have been accumulating throughout the US — and in a report released Wednesday, the American Civil Liberties Union (ACLU) has detailed just how ubiquitous the technology has become, and how it could be abused.

Over 99 percent of scanned plates belong to innocent drivers

According to the ACLU, nearly three-quarters of police agencies were using plate scanners as of 2011. The ACLU doesn't take issue with their outright use — but it's concerned with how their findings could be mishandled. For the most part, vehicles' location data ends up in a database specific to the local law enforcement agency. If a vehicle has been placed on police watch list, it may trigger an alert for the officers. But the vast majority of the plates belong to innocent drivers. The ACLU found that less than one percent of plates matched up with a watch list, and even fewer led to an arrest.

That leaves "hundreds of millions of data points" on innocent drivers stored inside of law enforcement computers. "Anyone with access to these systems could track his boss, his ex-wife," the ACLU notes. "An agent could target the owners of vehicles parked at political meetings, gay bars, gun stores, or abortion clinics." Many agencies prohibit officers from looking into the data for personal reasons, but as one New York police department put it, the use of this data "is only limited by the officer’s imagination."

Because of the potential for abuse, the ACLU believes that the surveillance data should be deleted within a matter of days or weeks. In its investigation of 293 state and local law enforcement agencies, the ACLU found that a number of agencies were deleting the scanner data within a short time frame, but others held onto it for years or never deleted it at all. However, the ACLU did not fully detail how many agencies fell into either camp.

Some surveillance data is never deleted

Private companies are getting in on the action too. Parking garages and repossession agencies — which capture the license plate data for their own use — hang onto their data as well, and sometimes sell it to law enforcement groups. While the scope of a private company's collection may seem to be limited, the ACLU reports that one repossession company claims to be in possession of photographs and location data on "a large majority" of registered vehicles in the US.

The growing presence of plate readers has its benefits though: it makes finding a stolen vehicle or a vehicle associated with a crime much easier. So far five states have enacted laws to manage the scanners' use, and the ACLU hopes that others will soon move to govern the widespread collection of American citizens' travel details.