OAKLAND — City leaders are trying to figure out how to handle a massive database police have collected from automated licence plate readers attached to 35 patrol cars.

The Privacy Advisory Committee is developing guidelines for using specific surveillance devices, such as those on Tuesday’s City Council agenda concerning “Stingray” technology that simulates cellphone towers to locate cellphone-carrying suspects.

As part of that effort, on Feb. 2 police representatives and two experts described to the panel the extent of local collection and possible uses of license-plate-reading cameras.

Oakland has been using license plate readers since 2006 and storing the captured information, which is well into the millions of records logging where individual cars were and when. In 2015, the data exceeded the city’s ability to store it, and some was dumped. The city now keeps its data for six months.

But there are no standards for how the data is managed at the local, state or national level, nor with whom it is shared. The committee is drafting an ordinance to establish some rules.

The city’s license plate readers check for anything matching a Department of Justice list of stolen cars, stolen license plates and felony warrants, police Capt. Paul Figueroa said.

In Oakland, the readers are mounted at 45-degree angles on either side of the vehicle, enabling them to take images of parked cars or those moving in either direction.

The readers each have a pair of lenses and are capable, at the rate of 60 frames per second, of capturing a license plate, an image of the vehicle, and data on its exact location and the hour at which it was photographed. In some situations, the cameras also can record the vehicle’s speed.

If the reader gets a hit, the officer tries to verify the vehicle is stolen, Figueroa said. “It really just alerts us to check our database,” he said.

He said it’s possible, for instance, that a car reported stolen had since been recovered and the plate reader has spotted its owner driving home.

One of the experts who spoke, Cyrus Farivar, has been studying Oakland’s use of license plate readers for years. Using a public records request, he obtained Oakland police’s dataset for Dec. 23, 2010, to May 31, 2014. It contained 4.6 million records of more than a million different cars’ movements.

Oakland’s data is added to the Contra Costa County Sheriff’s Automated Regional Information Exchange System, which can potentially be accessed by other government agencies.

Mike Katz-Lacabe, director of research for the Center for Human Rights and Privacy, became interested in the cameras when he spotted one atop a San Leandro police car and made a public records request.

Katz Lacabe found that his own car had been photographed 112 times in a little more than two years, and that a single police car in that time had obtained more than 2 million images around San Leandro. Among the ones he found were a photograph of him and his two young daughters getting out of their car in the driveway of their San Leandro home, he told the committee.

Key issues that interested the committee were how long the data is stored, who has access to it and the risk accumulating the data poses to civil liberties and privacy.

Farivar, using information that emerged when he plugged City Councilman Dan Kalb’s license plate into the three-plus years of data he had received, told the committee how he was able to deduce where Kalb lived.

Similarly, patterns of someone’s plates showing up in the database could reveal more about them, such as where they worship, get health care and socialize, he said.

Every database studied showed no more than a 0.03 percent of hits on stolen cars. More than 99.5 percent were of seemingly innocent activity. And the extent of their utility in helping combat crime at this point is “strictly anecdotal,” Katz Lacabe said. But law enforcement has typically resisted efforts to allow public access to license plate records.

And the readers are not foolproof. San Francisco in 2015 paid $495,000 to settle a lawsuit from a woman because of a one-digit error by a plate reader that officers did not check before arresting her for allegedly driving a stolen Lexus.

The error rate is about one in 10 digits, depending on the brand of reader used, according to speakers last week.

The data can be useful in checking suspects’ alibis, police said.

Especially for an understaffed agency, there is a value in having the database available long after it is acquired, police said. Because of the backlog of cases and too few officers, “it may be seven to eight months before we begin to investigate a burglary,” said Timothy Birch, Oakland police research and planning manager.

“You never know when that break about a particular street comes to an investigation. It might be the very thing that breaks that case wide open,” Bureau of Investigation Deputy Chief John Lois said. Oakland often dispatches its cars with license plate readers to “hot spots,” officers said.

“We want to keep it as short as we can,” Lois said about how long to retain the data. “But in terms of crime reduction, 12 months” might be better, he said.

The Privacy Advisory Commission took no vote but asked police for more data on how useful the technology has been and recommendations on how long to retain it.

Contact Mark Hedin at 510-293-2452, 408-759-2132 or mhedin@bayareanewsgroup.com.