US law enforcement agencies are using license plate scanners designed to track down criminals to build databases detailing the whereabouts of millions of US drivers, the American Civil Liberties Union (ACLU) said in a report released on Wednesday.

The ACLU's new report summarised the advocacy group's 2012 investigation into the way law enforcement agencies collect and store data from license plate readers, which are typically installed alongside roads or on police cars.

The license plate scanner systems quickly photograph passing cars and analyze their license numbers to check against lists of cars sought by law enforcement in ongoing investigations.

ACLU's review of documents from 38 states and Washington, DC found that the systems are also often used to log databases of information - photographs, plate numbers, time and location - gathered by the cameras over months or even years from all the passing cars, not just select ones.

"I think (people) fail to appreciate the tremendous scope of tracking, which can occur using license-plate readers," said Catherine Crump, the main author of the report.

"We've never before lived in a society where you couldn't go out the door without the government knowing where you went."

The report, based on documents ACLU affiliates received from local police departments through 587 freedom of information requests, gives new fodder to the growing debate over the scope of the US government's surveillance.

Law enforcement authorities say gathering data on the comings and goings of US drivers is a valuable resource for speedier investigations, including future ones.

The ACLU said it does not oppose the use of license plate readers in fighting crime, but worries about the massive systems of location tracking being too broad and ripe for abuse, be it personal by individual officers, or political, by institutions.

"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," the group said in its report.

According to the report, "only a fraction of 1 per cent" of license plate scans done by the readers are hits for cars of interest to law enforcement, and fewer lead to an arrest.

How long law enforcement authorities keep the records from license plate scanners varies. Some agencies, such as the Ohio State Highway Patrol or the Minnesota State Patrol, quickly delete data on regular passers by, the report said.

In other jurisdictions, such as Tiburon, California or Burbank, Illinois, the data is kept for less than a month. In New Jersey, law requires storage for five years. In some jurisdictions, such as Yonkers in New York and Mesquite in Texas, the retention is indefinite, according to the report. - Reuters