Office Dennis Vafier of the Alexandria Police Dept., uses a laptop in his squad car to scan vehicle license plates during his patrols of the area, Tuesday, July 16, in Alexandria, Va. Local police departments across the country have amassed millions of digital records on the location and movements of every car truck with a license plate using automated scanners. (Pablo Martinez Monsivais/AP)

The spread of cheap, powerful cameras capable of reading license plates has allowed police to build databases on the movements of millions of Americans over months or even years, according to an American Civil Liberties Union report released Wednesday.

The license-plate readers, which authorities typically mount along major roadways or on the backs of cruisers and government vehicles, can identify cars almost instantly and compare them against “hot lists” of vehicles that have been stolen or involved in crimes.

But the systems collect records on every license plate they encounter — whether or not they are on hot lists — meaning that time and location data are gathered in databases that can be searched by police. Some departments purge information after a few weeks, some after a few months and some never, said the report, which warns that such data could be abused by authorities and chill freedom of speech and association.

“Using them to develop vast troves of information on where Americans travel is not an appropriate use,” said Catherine Crump, a staff lawyer at the ACLU and one of the authors of the report, “You Are Being Tracked: How License Plate Readers Are Being Used to Record Americans’ Movements.”

The use of license-plate readers is common in the Washington area, where concerns about terrorism have fueled major investments in the equipment, with much of the money coming from federal grants. Agreements among departments and jurisdictions allow sharing of the location information, with data typically retained for at least a year.

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Such details, police and law enforcement experts say, can help investigators reconstruct suspects’ movements before and after armed robberies, auto thefts and other crimes. Departments typically require that information be used only for law enforcement purposes and require audits designed to detect abuse.

“We’d like to be able to keep the data as long as possible, because it does provide a rich and enduring data set for investigations down the line,” said David J. Roberts, senior program manager for the Technology Center of the International Association of Chiefs of Police.

There is no definitive count on the numbers of license-plate readers in the Washington area, but the boxy cameras have become common sights in the region. They typically are mounted in pairs on government vehicles so that the lenses can gaze the adjacent lanes. Some cameras also are mounted on poles or overpasses, much as red-light cameras and speed cameras are.

The ACLU argues that data collection by most police departments is unnecessarily broad. In an analysis of data collected in Maryland, the report found that license-plate readers recorded the locations of vehicle plates 85 million times last year.

Based on a partial-year analysis of that data, the ACLU found that about one in 500 plates registered hits. In the overwhelming majority of cases, it said, the alleged offenses were minor, involving lapsed registrations or failures to comply with the state’s emission-control program.

For each million plates read in Maryland, 47 were associated with serious crimes, such as a stolen vehicle or a wanted person, the report said. Statistics collected by the ACLU in several other jurisdictions across the country also found hit rates far below 1 percent of license plates read.

Maryland officials have defended their program, which collects data from departments across the state in a fusion center that shares intelligence among federal, state and local agencies. In a recent three-month period, state officials said, license-plate readers contributed to 860 serious traffic citations and the apprehension of 180 people for crimes including stolen autos or license plates.

The center deletes the data one year after collection, in what officials said was a compromise between investigative needs and privacy rights.

“We don’t want to retain more information . . . than is necessary,” said Harvey Eisenberg, an assistant U.S. attorney who oversees Maryland’s Anti-Terrorism Advisory Council. “You strike the balance because people are legitimately concerned.”

The license-plate readers are also widely used in Northern Virginia and the District, which mounts them on many major roadways entering and exiting the city.

A D.C. police spokeswoman declined to comment on the ACLU report, but a general order posted on the department’s Web site said that data from license-plate readers are kept for two years before being destroyed, though the information can be kept for longer if it is needed for ongoing litigation.

Fairfax County police spokeswoman Lucy Caldwell said the department uses the technology but was unable to provide details about the program. The ACLU report does not have information on readers in Northern Virginia.