Police say it's a high-tech tool to help cops stay one step ahead of the bad guys.

But a mobile license-plate reader used by Woodland's police force since early November has attracted critics who say the technology trumps privacy rights.

One of Woodland's patrol cars is equipped with the system, which uses cameras to scan license plates within its field of vision, matches the scanned images against national crime databases and alerts the officer.

"We wanted a vehicle driving around to scan those license plates, hit on stolen vehicles and make arrests," said Woodland Police Lt. Anthony Cucci. "It's another crime-fighting tool."

Woodland paid for its system with $18,000 in grants and another $3,000 from retailer Target Corp., which has donated to other local law enforcement efforts, Cucci said.

Woodland police hope the new technology can help them get a handle on a mounting problem: car thefts.

"Property crime for us is up. Stolen vehicles are up," Cucci said.

As of October, 149 cars were reported stolen within the Woodland city limits this year, up from 96 reports in the same period a year ago, Cucci said.

In October alone, 20 cars were reported stolen, compared with seven in October 2011, he said.

The system, designed by Livermore firm Vigilant Solutions, is being used by several Bay Area agencies, including the Marin County Sheriff's Department and, for the past four months, the Novato Police Department.

In western Nevada, Sparks police last week received more than $61,000 in grants to pay for the technology, according to news reports. The city plans to team with Reno police and Washoe County to share the system.

"It helps us collect a lot of information that we otherwise wouldn't have. It scans continuously," said Novato Police Sgt. Jay Demski.

Demski said the plate-scanning system, embedded in three Novato cruisers, has helped his officers recover four stolen vehicles and hit on a stolen license plate that led to the arrest of two people suspected of drug-related crimes.

"It's a tremendous assistance to us," he said.

So the technology seems to work. But at what cost?

The American Civil Liberties Union of Massachusetts, on its Privacy SOS website, calls license-plate recognition "a very dangerous technology absent proper oversight and data controls."

Chris Conway, an attorney with the ACLU in San Francisco, is troubled not only by the technology's speed and range but by how the data collected might be used.

"Our greater concern is that it records information on every single person that drives by – people going about their daily business," Conway said.

Cucci, of the Woodland Police Department, said his department holds onto the data, saved in a secure server for two weeks, and then purges it.

No personally identifiable information, such as Social Security or medical data, is contained in a license-plate capture, says Vigilant Solutions in information on its website.

The ACLU, Conway said, is requesting public records nationwide to determine how widespread law enforcement's use of license-plate recognition is and the practices surrounding its use.

"Information is being collected on innocent drivers who are doing nothing wrong, with no justification," Conway said. "If they're collecting (data), how long are they keeping it? Is it being shared with other agencies? Is that information being respected and protected?"

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