Vigilant Solutions, the Livermore-based company that provided Portola Valley's cameras and stores its data, has been in the news recently in connection with a contract to allow the U.S. Immigration and Customs Enforcement agency access to Vigilant's data. (Vigilant declined to confirm whether it has a contract with ICE, according to news reports.)

Portola Valley's third entrance, on Alpine Road at the border with the unincorporated community of Ladera, remains unmonitored. San Mateo County has plans to install a license-plate-reading camera farther east on Alpine that will track all traffic into and out of Ladera, which would also capture all traffic into and out of Portola Valley. That camera is likely to be operational in late spring, Mr. Dennis said.

The cameras are up and running at the town's border with Woodside on Portola Road and on Arastradero Road near the intersection with Alpine Road, according to a Jan. 31 emailed announcement from Town Manager Jeremy Dennis.

Two of the three avenues into and out of Portola Valley are now under round-the-clock surveillance by cameras that capture images of the license plates of every passing vehicle.

The council at the time acknowledged an exception to the 12-month rule, allowing NCRIC to retain images if the vehicle is of interest to law enforcement authorities.

Portola Valley's data will be stored for one year, after which it will be "permanently destroyed," according to an ordinance approved by the Town Council in April 2017.

In his statement, Mr. Dennis noted that "the Town controls access to all of its data and has not authorized Vigilant to make the data available to ICE."

The camera data belongs to the towns and cities that authorize its collection, Mr. Sena said. "We make sure that we respect how people would like to share their data and who has access to it," he said.

The Northern California Regional Intelligence Center (NCRIC), an agency within the U.S. Department of Homeland Security, grants law enforcement agencies access to license-plate-image databases, but only for investigations of criminal cases, according to Mike Sena, the director of NCRIC and a captain in the San Mateo County Sheriff's Office.

Asked of instances of unauthorized access to license plate data, Mr. Sena said there have been none. "I'd like to think it's because we spend a lot of money on infrastructure," he said when asked about the agency's perfect record. "There is a huge effort on everyone's part. ... If people can't trust where the information is (being held), we wouldn't be holding it."

The data cannot be used to enforce traffic violations; to harass, intimidate or discriminate against any individual or group; or to invade someone's privacy "where a reasonable expectation of privacy exists," the ordinance says.

Legitimate reasons to view Portola Valley's license plate data include assisting in an investigation of a crime, locating stolen vehicles, locating missing persons and wanted persons, searching the area around the scene of a crime, and "any other purpose deemed appropriate by a majority of the Town Council upon the request of law enforcement," the ordinance says.

Once a year, the town manager will report to the council on how many times the data has been accessed, how many times the cameras captured a license plate of interest to law enforcement, the number of subsequent inquiries by law enforcement, the reasons for those inquiries, and whether and why NCRIC retained any data beyond the 12-month expiration date.

Portola Valley: 24-hour surveillance begins with license-plate readers