WASHINGTON (AP) -- Chances are, your local or state police departments have photographs of your car in their files, noting where you were driving on a particular day, even if you never did anything wrong. Using automated scanners, law enforcement agencies across the country have amassed millions of digital records on the location and movement of every vehicle with a license plate, according to a study published Wednesday by the American Civil Liberties Union. Affixed to police cars, bridges or buildings, the scanners capture images of passing or parked vehicles and note their location, uploading that information into police databases. Departments keep the records for weeks or years, sometimes indefinitely. As the technology becomes cheaper and more ubiquitous, and federal grants focus on aiding local terrorist detection, even small police agencies are able to deploy more sophisticated surveillance systems. While the Supreme Court ruled in 2012 that a judge's approval is needed to track a car with GPS, 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 assemble what it calls a "single, high-resolution image of our lives." "There's just a fundamental question of whether we're going to live in a society where these dragnet surveillance systems become routine," said Catherine Crump, a staff attorney with the ACLU. The civil rights group is proposing that police departments immediately delete any records of cars not linked to a crime. Law enforcement officials said the scanners can be crucial to tracking suspicious cars, aiding drug busts and finding abducted children. License plate scanners also can be efficient. The state of Maryland told the ACLU that troopers could "maintain a normal patrol stance" while capturing up to 7,000 license plate images in a single eight hour shift. "At a time of fiscal and budget constraints, we need better assistance for law enforcement," said Harvey Eisenberg, chief of the national security section and assistant U.S. attorney in Maryland. Law enforcement officials also point out that the technology is legal in most cases, automating a practice that's been done for years. The ACLU found that only five states have laws governing license plate readers. New Hampshire, for example, bans the technology except in narrow circumstances, while Maine and Arkansas limit how long plate information can be stored. "There's no expectation of privacy" for a vehicle driving on a public road or parked in a public place, said Lt. Bill Hedgpeth, a spokesman for the Mesquite Police Department in Texas, which has records stretching back to 2008, although the city plans next month to begin deleting files older than two years. "It's just a vehicle. It's just a license plate."