WASHINGTON — You can drive, but you can’t hide.

A rapidly growing network of police cameras is capturing, storing and sharing data on license plates, making it possible to stitch together people’s movements whether they are stuck in a commute, heading to the beach or up to no good.

For the first time, the number of license tag captures has reached the millions, according to a study published Wednesday by the American Civil Liberties Union based on information from hundreds of law enforcement agencies. Departments keep the records for weeks or years, sometimes indefinitely, saying they can be crucial in tracking suspicious cars, aiding drug busts, finding abducted children and more.

Attached to police cars, bridges or buildings — and sometimes merely as an app on a police officer’s smartphone — scanners capture images of passing or parked vehicles and pinpoint their locations, uploading that information into police databases.

While the Supreme Court ruled in 2012 that a judge’s approval is needed to use GPS to track a car, networks of plate scanners allow police effectively to track a driver’s location, sometimes several times every day, with few legal restrictions. The ACLU says the scanners are assembling a “single, high-resolution image of our lives.”

“There’s just a fundamental question of whether we’re going to live in a society where these dragnet surveillance systems become routine,” said Catherine Crump, a staff attorney with the organization.

Local cameras

Locally, Edisto Beach Town Council, in a 4-1 vote, authorized $60,000 in accommodations-tax revenue for eight 24-hour cameras, including ones at the entrance to town that provide a record of license plates of people as they travel on and off the island.

The town said the technology increases public safety but some residents said they see the cameras as an invasion of privacy.

Other municipalities have backed off from using surveillance cameras because of privacy concerns. And questions have arisen about how long the information is stored and who has access to it.

The city of Charleston uses a video-monitoring system based on the same sort of technology. More than 40 cameras keep tabs on areas such as the Port of Charleston, The Market and East Side housing, such as Gadsden Green, officials said.

Mount Pleasant and North Charleston officials said those cities do not use surveillance cameras. North Charleston has four automated license-plate-reader systems scattered throughout the city to detect stolen license plates.

Isle of Palms police have surveillance cameras at the Recreation Center.

Eyes on you

Although less thorough than GPS tracking, plate readers can produce some of the same information, the ACLU says.

In Minneapolis, for example, eight mobile and two fixed cameras captured data on 4.9 million license plates from January to August 2012, the Star Tribune reported. Among those whose movements were recorded: Mayor R.T. Rybak, whose city-owned cars were tracked at 41 locations in a year.

A Star Tribune reporter’s vehicle was tracked seven times in a year, placing him at a friend’s house three times late at night, other times going to and from work — forming a picture of the dates, times and coordinates of his daily routine. Until the city temporarily classified such data late last year, anyone could ask police for a list of when and where a car had been spotted.

As the technology becomes cheaper and more widespread, even small police agencies are able to deploy more sophisticated surveillance systems. The federal government has been a willing partner, offering grants to help equip departments, in part as a tool against terrorism.

Law enforcement officials say the scanners are strikingly efficient. The state of Maryland told the ACLU that troopers could “maintain a normal patrol stance” while capturing up to 7,000 license plate images in a single eight-hour shift.

“At a time of fiscal and budget constraints, we need better assistance for law enforcement,” said Harvey Eisenberg, assistant U.S. attorney in Maryland.

Law enforcement officials say the technology automates a practice that’s been around for years. The ACLU found that only five states have laws governing license plate readers. “There’s no expectation of privacy” for a vehicle driving on a public road or parked in a public place, said Lt. Bill Hedgpeth, a spokesman for the Mesquite Police Department in Texas.

In Yonkers, N.Y., police said retaining the information helps detectives solve future crimes.

The ACLU study, based on 26,000 pages of responses from 293 police departments and state agencies across the country, found that license plate scanners produced a small fraction of “hits,” or alerts that a suspicious vehicle had been found.

In Maryland, for example, the state reported reading about 29 million plates between January and May of last year. Of that number, about 60,000 — or roughly 1 in every 500 license plates — were suspicious. The main offenses: a suspended or revoked registration, or a violation of the state’s emissions inspection program, altogether accounting for 97 percent of alerts.

Even so, Eisenberg, the assistant U.S. attorney, said the program has helped authorities track 132 wanted suspects.

Post and Courier writer Prentiss Findlay contributed to this story.