CHELSEA — The high-speed cameras mounted on Sergeant Robert Griffin’s cruiser trigger a beeping alarm every time they read another license plate, automatically checking to see if each car is unregistered, uninsured, or stolen. In a single hour of near-constant beeps, Griffin runs 786 plates on parked cars without lifting a finger.

The plate-reading cameras were introduced for police use in Massachusetts in 2008, and quickly proved their worth. The one on Griffin’s Chelsea cruiser repaid its $24,000 price tag in its first 11 days on the road. “We located more uninsured vehicles in our first month . . . using [the camera] in one cruiser than the entire department did the whole year before,” said Griffin.

Now, automated license plate recognition technology’s popularity is exploding — seven Boston area police departments will add a combined 21 new license readers during the next month alone — and with that expanded use has come debate on whether the privacy of law-abiding citizens is being violated.

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These high-tech license readers, now mounted on 87 police cruisers statewide, scan literally millions of license plates in Massachusetts each year, checking not only the car and owner’s legal history, but also creating a precise record of where each vehicle was at a given moment.

The records can be enormously helpful in solving crimes — for example, Fitchburg police used the technology to catch a serial flasher — but they increasingly make privacy advocates uneasy.

Use of the technology is outstripping creation of rules to prevent abuses such as tracking the movements of private citizens, or monitoring who visits sensitive places such as strip clubs, union halls, or abortion clinics.

A survey of police departments that use automated license readers found that fewer than a third — just 17 out of 53 — have written policies, leaving the rest with no formal standards for who can see the records or how long they will be preserved.

“The worst-case scenario — vast databases with records of movements of massive numbers of people — is already happening,” warns Kade Crockford of the American Civil Liberties Union of Massachusetts, which is pushing for a state law to regulate use of license plate scanners and limit the time departments can routinely keep the electronic records to 48 hours.

But police fear that zeal to protect privacy could stifle the use of a promising law enforcement tool, especially if they are prevented from preserving and pooling license plate scans for use in detective work. Currently, all of the police departments keep their plate scans longer than two days, with data storage ranging from 14 days in Somerville and Brookline to 90 days in Boston and up to a year in Leicester, Malden, Pittsfield, and Worcester.

Sergeant Griffin, whose own department has no written policy, agrees that there should be rules to prevent abuse, but thinks that these should be set by local departments rather than at the State House. He said that rather than restrict use of the scanners, the Legislature should “trust law enforcement to do the right thing.”

The usefulness of the automated license plate reader as an investigative tool springs from the astounding number of license plates the units can scan and record. With an array of high-speed cameras mounted on police cruisers snapping pictures, these systems are designed to capture up to 1,800 plates per minute, even at high speeds and in difficult driving conditions.

“I’ve had my [license plater reader] correctly scan plates on cars parked bumper-to-bumper when I’m driving full speed,” said Griffin, who caught three scofflaws owing a combined $1,900 in parking tickets from the 786 license plates his reader checked on a recent one-hour patrol. The devices misidentify plates often enough that scans have to be confirmed by an officer on the scene before writing a ticket. In this case, after confirming the parking tickets, and the money owed, police initiated the collection process. Griffin called headquarters to confirm that the vehicles still had unpaid tickets, and then arranged for them to be towed.

Boston’s four scanner-equipped cars do 3,500 scans a day and more than 1 million per year, according to police data. Even smaller departments such as Fitchburg scan 30,000 plates per month with just one license-reading system, easily 10 times more than an officer could manually check.

Most of the departments that deploy license plate readers use them primarily for traffic enforcement. But the scanners — sometimes called by the acronym ALPR — are also used for missing persons, AMBER alerts, active warrants, and open cases.

“Every once in a while our detectives will use the ALPR database for retrospective searches,” said Griffin, adding that the technology has proved useful to scan vehicles in neighborhoods surrounding crime scenes.

Griffin’s counterpart in Fitchburg, Officer Paul McNamara, said license scanner data played a crucial role in solving a string of indecent exposure incidents at Fitchburg State University in April 2011. At the request of the university police, McNamara entered the alleged flasher’s plate into his license-scan database. The system indicated that a suspect’s vehicle had passed the scanner just 10 minutes earlier, leading to a suspect’s arrest and later guilty plea to charges of indecent exposure and lewdness.

McNamara said that there is no formal process when another police department requests a licence inquiry of this kind into his unit’s database.

“It can’t be a fishing expedition, though,” he said. “We look at it as a form of mutual aid, so it has to be a serious criminal matter for us to share data.”

While law enforcement officials are enthusiastic, critics can point to alleged abuses:

 In 2004, police tracked Canadian reporter Kerry Diotte via automated license scans after he wrote articles critical of the local traffic division. A senior officer admitted to inappropriately searching for the reporter’s vehicle in a license scan database in an attempt to catch Diotte driving drunk.

 Plainclothes NYPD officers used license readers to scan license plates of worshipers at a mosque in 2006 and 2007, the Associated Press reported, under a program that was partially funded by a federal drug enforcement grant.

 In December, the Minneapolis Police Department released a USB thumb drive with 2.1 million license plate scans and GPS vehicle location tags in response to a public records request, raising fears that such releases might help stalkers follow their victims. A few days later, the Minneapolis mayor asked the state to classify license scan data as nonpublic.

ACLU attorney Fritz Mulhauser warned last summer that, within a few years, police will be able to use license scan records to determine whether a particular vehicle “has been spotted at a specific church, union hall, bar, political party headquarters, abortion clinic, strip club, or any number of other locations a driver might wish to keep private.”

But many law enforcement officials say they are just starting to tap the potential of license plate scanners.

“If anything, we’re not using ALPR enough,” said Medford’s Chief Leo Sacco, who would like to deploy the scanners 24 hours a day on all of his cruisers.

Massachusetts public safety officials are trying to create a central repository of license scans similar to a system in Maryland where all 262 scanner-equipped cruisers feed data to the state. In 2011, the Executive Office of Public Safety and Security handed out $750,000 in federal grants for 43 police departments to buy scanners with the understanding that all scan results would be shared.

The bill introduced on Beacon Hill by Senator Cynthia Creem and Representative Jonathan Hecht would allow police to share scan results for law enforcement purposes, but it would require every agency to develop formal policies that protect privacy. It would also set statewide standards for preserving camera scanner data and require regular reports to the state on how departments are using their scanners.

Currently, even the state Executive Office of Public Safety lacks a formal policy governing the use of its planned database, while 36 police departments out of the 53 using automated license readers have no written policies, the survey found. The Massachusetts State Police are currently developing a policy for the department’s 20 camera scanners.

Even departments that do have formal license-reading policies differ widely on specifics such as whether the collected data must be released to the public in response to a written request; Wakefield and Revere say no; many others use vague language that leaves it unclear.

Likewise, the police differ widely on how long they can retain license scans not connected to an ongoing investigation or law enforcement action, ranging from as little as 14 days in Somerville and Brookline to indefinitely in Milford.

The town of Brookline, police, and selectmen have worked with the ACLU to develop perhaps the most detailed policies in the state.

Brookline’s policy prohibits using camera scanners to intimidate or harass, to infringe free speech, or to conduct discriminatory surveillance based on race, gender, sexual orientation, disability, or other protected characteristics. The Brookline policy also requires civilian oversight and biannual audits for the town’s camera scanner system, which will be online within the next two months, according to Police Chief Daniel O’Leary.

Though police have concerns about provisions in the Creem-Hecht bill — particularly the proposed 48-hour limit on saving license scans — some law enforcement officials say they can see a role for the state in setting standards on the technology.

“Some departments are keeping the data forever, others seem to be dumping it every 20 minutes,” said Peabody Police Chief Robert Champagne, who said his department has yet to set a limit on retaining its license scans.

Hecht, who represents parts of Cambridge and Watertown, said the important thing is to get people talking about the potential pitfalls of the new technology — and avoid them.

“Technology is rapidly moving ahead in terms of our ability to gather information about people,” said Hecht. “We need to have a conversation about how to balance legitimate uses . . . with protecting people’s legitimate expectation of privacy.”