Petaluma motorists who dawdle while downtown may want to invest in some timers.

The city is preparing to purchase two GPS-aided, license-plate readers for its parking enforcement officers to more efficiently ticket vehicles parked longer than the two-hour limit.

The Auto-Vu license plate recognition and global positioning hardware package collects images and data on when, where and for how long a vehicle has been in a location.

Police say they will use the technology only to catch parking scofflaws, but the collection and storage of such detailed information about motorists' comings and goings raises privacy questions and concerns about potential misuse of the data.

Lt. Tim Lyons said the sole purpose of the technology is to make the jobs of the three parking officers safer and more efficient and nab drivers hogging parking spaces.

"The only time we'll even view it is if someone contests a ticket," he said. "We don't look at it or monitor the information."

Petaluma won't share the data or contribute to growing public and private license-plate databases law enforcement agencies throughout California are helping compile.

Some agencies install the technology on patrol cars, then collect readings and feed them into intelligence centers operated by local, state and federal law enforcement, according to a report by the Center for Investigative Reporting.

Given the recent disclosure of secret information gathering operations by the National Security Agency, privacy advocates have raised concerns about data collection that can track innocent people's movements.

Some agencies say the technology helps find wanted suspects, stolen cars, kidnapping victims and even homicide suspects or victims.

A San Leandro man who asked the city for an accounting of how many times his two cars had been photographed learned the city had taken 112 images of his vehicles in two years, including one of him and his children getting out of a car in their driveway, according to the San Francisco Chronicle.

License-plate reader "technology is used in a lot of different applications," said Sgt. Ken Savano, who heads Petaluma's traffic squad. "There are legitimate law enforcement purposes for it."

Those would include auto theft, habitual drunken driver investigations and more, he said.

He said Petaluma's data will be accessible only for parking enforcement and won't be easily searchable.

"That being said, should we have a legitimate investigative purpose, a homicide for example, we could have the ability to search it," he said. "It won't be in an easily searchable format."

Santa Rosa attorney Michael Li said he can see legitimate uses as well as the potential for misuse with the data.

Police must get a court order to place a GPS device on a person's car, and this type of unsupervised data tracking isn't far removed from that. But, Li said, if police investigating a burglary were to search the license-plate database to see whose cars were in the area at the time, it's no different than viewing surveillance video.

"If we have a specific person associated with a specific vehicle, it might be enough," Savano allowed. "But it has to be reasonable."

City Council members who authorized spending $84,000 to buy the cameras and other hardware offered no objections. "I don't believe there are some sort of evil, nefarious-type motives behind this, some sort of black ops," Savano said.

Easing the strain on parking enforcement officers Charlene Lavezzoli and Jan Scheele and making their jobs more efficient was the primary goal, Lyons said. The updated technology is similar to a system the city bought in 2005 that has become obsolete.

In the two years since the old system died, Lavezzoli and Scheele have returned to the old-fashioned chalk-on-a-stick method of marking tires, then circling back two hours later to ticket offenders.

"It should be very accurate," Lavezzoli said. "With the GPS, people will have to be more mindful of when they need to move."