License Plate Readers

San Diego County Deputy Sheriff Ben Chassen looks at a monitor as his vehicle reads the license plates of cars in a parking lot Wednesday, Sept. 17, 2014, in San Marcos, Calif. (AP Photo/Gregory Bull)

(Gregory Bull)

LOS ANGELES -- A rapidly expanding digital network that uses cameras mounted to traffic signals and police cruisers captures the movements of millions of vehicles across the U.S., regardless of whether the drivers are being investigated by law enforcement.

The

have multiplied across the U.S. over the last decade,

largely by Homeland Security grants, and judges recently have upheld authorities' rights to keep details from hundreds of millions of scans a secret from

.

Such decisions come as a patchwork of local laws and regulations govern the use of such technology and the distribution of the information they collect, inflaming civil liberties advocates who see this as the next battleground in the fight over high-tech surveillance.

"If I'm not being investigated for a crime, there shouldn't be a secret police file on me" that details "where I go, where I shop, where I visit," said Michael Robertson, a tech entrepreneur fighting in court for access to his own files. "That's crazy, Nazi police-type stuff."

A San Diego judge has tentatively ruled that a local government agency can deny Robertson's request for scans on his own vehicle under California's open records law because the information pertains to police investigations. Superior Court Judge Katherine Bacal heard additional arguments in the case Friday and plans to issue a final decision soon. Robertson said he plans to appeal if the tentative decision stands.

The San Diego case comes less than a month after another state judge, using the same reasoning, denied a petition by the ACLU of Southern California and the Electronic Frontier Foundation for one week of records on all vehicles collected by the Los Angeles Police Department and Los Angeles County Sheriff's Department. The ACLU says that network adds 3 million scans each week to a database shared with dozens of other agencies that now includes details from more than 455 million encounters.

About 7 in 10 law enforcement agencies used

in 2012 and an overwhelming majority planned to acquire such systems or expand their use, according to a study by the Police Executive Research Forum, a research and policy group.

Civil liberties advocates say these files need to be open to public scrutiny to prevent government overreach and unconstitutional privacy invasions.

On the other side are government and law enforcement officials who say they're not misusing the systems and that tracking and storing the data can help with criminal investigations, either to incriminate or exonerate a suspect.

"At some point, you have to trust and believe that the agencies that you utilize for law enforcement are doing what's right and what's best for the community, and they're not targeting your community," Los Angeles County Sheriff's Sgt. John Gaw said.

In San Diego's case, records are kept for up to two years, but other agencies keep them five years or more and are limited mainly by server space.

"If that information is deleted or purged too quickly, then we lost that, and we can never go back," said Lt. Karen Stubkjaer of the San Diego Sheriff's Department.

In Robertson's case against the San Diego Association of Governments, he was seeking access to a sweeping system that links the San Diego Police Department, San Diego County Sheriff's Department and eight other law enforcement agencies. The sheriff's department alone has made 9.8 million scans since the system was introduced in 2009, Stubkjaer said.

Robertson, who founded and later sold the MP3.com digital musical service, has no problem with officials using the technology for legitimate purposes like tracking down stolen cars. But he says license plate readers are ripe for abuse, and there's no reason for long-term storage of data on innocent people.

"I want a strong police force," he said. "But I also want my personal freedom."

Neither the San Diego case nor the Los Angeles ruling sets legal precedent, but they're part of a growing debate.

"License plate readers are part of a larger conversation," said Chuck Wexler, head of the Police Executive Research Forum. "Technology is changing how the police view crime, and it is raising a number of public policy issues: How long do you hold on to this information? And what part of this information should the public have access to?"