Law enforcement agencies across the U.S. have amassed millions of digital records on the location and movement of every vehicle with a licence plate, according to a study by a prominent civil rights organisation.

A rapidly growing network of police cameras is capturing, storing and sharing data on licence plates, making it possible to stitch together people’s movements whether they are stuck in a commute, making tracks to the beach or up to no good.

For the first time, the number of licence tag captures has reached the millions, according to the study published on Wednesday by the American Civil Liberties Union based on information from hundreds of law enforcement agencies. Departments keep the records for weeks or years, sometimes indefinitely, saying they can be crucial in tracking suspicious cars, aiding drug busts, finding abducted children and more.

Attached to police cars, bridges or buildings — and sometimes merely as an app on a police officer’s smartphone — automated scanners capture images of passing or parked vehicles and pinpoint their locations, uploading that information into police databases.

Over time, it’s unlikely that many vehicles in a covered area escape notice. And with some of the information going into regional databases encompassing multiple jurisdictions, it’s becoming easier to build a record of where someone has been and when, over a large area.

While the Supreme Court ruled in 2012 that a judge’s approval is needed to use GPS to track a car, networks of plate scanners allow police effectively to track a driver’s location, sometimes several times every day, with few legal restrictions. The ACLU says the scanners are assembling a “single, high-resolution image of our lives”.

— AP