License-plate readers give law enforcement a location tracking tool that may violate constitutional privacy rights, the American Civil Liberties Union argues in a new report released today.

The new attack on LPR technology comes after an ACLU review of agency policies about how long data is stored and how it's used in investigations. Agency policies governing those areas vary widely, according to the group.

"The spread of these scanners is creating what are, in effect, government location tracking systems recording the movements of many millions of innocent Americans in huge databases," said ACLU Staff Attorney Catherine Crump, the report's lead author. "We don't object to the use of these systems to flag cars that are stolen or belong to fugitives, but these documents show a dire need for rules to make sure that this technology isn't used for unbridled government surveillance."

The Los Angeles Police Protective League that represents LAPD officers refuted the argument that LPR data violates privacy in a guest blog post ("LPR Protects Officers and the Public") on PoliceMag.com. LPR technology does "nothing more than what officers have been doing manually since the creation of the license place: writing down license plate numbers, or radioing license plates in for checks against criminal databases," the LAPPL wrote in the post.

For its study, the ACLU collected data from agencies in 38 states that revealed a range of records management policies. Some departments delete records within days or weeks, some keep them for years, while others have no deletion policy.

"The fact that some jurisdictions delete the records quickly shows that it is a completely reasonable and workable policy," said Allie Bohm, ACLU advocacy and policy strategist. "The police should not be storing data about people who are not even suspected of doing anything wrong."

By Paul Clinton