The license-plate readers, which police typically mount along major roadways or on the backs of cruisers, can identify vehicles almost instantly and compare them against "hot lists" of cars 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 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 attorney 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.
Such details, say police and law enforcement experts, 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 Roberts, senior program manager for the Technology Center of the International Association of Chiefs of Police.
But 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 in 2012.
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 around 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, which 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 they are collected, 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 Washington and the Virginia suburbs, where they are mounted on many of the major roadways entering and exiting the city. A District of Columbia police spokeswoman did not immediately comment on the ACLU report.
Private companies also are using license-plate-reading technology to build databases, typically to help in repossessing cars.
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