PHOENIX - Officer David Callister was about to drive past the 1991 Nissan sedan when an alert sounded inside his cruiser and an image of a license plate flashed on his laptop. It was a signal that the run-of-the-mill clunker was stolen.
The alert came from a $20,000 device that uses small infrared cameras mounted on the police car to automatically scan license plates and match the numbers against databases of stolen vehicles and people wanted for crimes.
"That car wasn't even on my radar screen," Callister said. The Arizona Department of Public Safety officer was focused on other vehicles as he searched for stolen cars in an apartment complex parking lot. "The plate reader would get you stuff you wouldn't normally be thinking about."
An estimated 400 of the nation's 18,000 police agencies own at least one license plate scanner, and police officials expect the readers to become more common in the coming years as the price of the devices falls.
The readers let officers scan about 75 times more plates during an eight-hour shift than the traditional method: writing down numbers and running them past a dispatcher.
Even though scanner-equipped cars represent a small part of a given agency's fleet, the devices are helping police recover stolen cars, find people wanted on criminal warrants and respond to urgent situations, such as robbers on the run.
For civil libertarians, however, the scanners raise the troubling question about whether the government will expand its use of the technology to track people's private lives.
"That's a lesson in history: Whenever the government collects data, sooner or later they will misuse it," said Jeff Gamso, legal director for the American Civil Liberties Union of Ohio.
But as police were quick to point out, anyone can jot down license plate numbers on a street corner, and that's what the scanners do, only more efficiently.
"What privacy?" asked Patrick Camden, a spokesman for the Chicago Police Department, which uses a small number of the scanners. "You're driving on a public way. There is no privacy about driving a car on a public way."
In central and southern Arizona, a dozen Department of Public Safety vehicles with scanners are trying to catch stolen cars headed to Mexico and disrupt smugglers using ripped-off vehicles to move drugs and illegal immigrants.
The California Highway Patrol is trying to combat the state's high auto-theft rate by scattering its 16 vehicles with scanners on urban freeways and rural highways and putting a handful of stationary readers on the ground.
In Long Beach, Calif., where seven vehicles with scanners were used to catch car thieves, officials were testing a plan to link a database of parking-ticket scofflaws to the scanners.
No studies have documented the effectiveness of scanners on a large scale. But some police agencies credit the devices with raising their vehicle-recovery and arrest numbers.
The appeal of the scanners is simple: The more plate numbers that are run, the better the chances of spotting stolen cars and wanted criminals. And much of the work is done automatically, allowing officers to focus more on the road.
After the two to four cameras mounted on the light bars or bumpers of cars spot a license plate, the numbers are fed to a computer processor in the trunk where the information is matched against databases.
Images of license plates appear on the laptop when an officer is driving freeways and busy streets, beeping each time a car is scanned. Then, the officer must verify, through a call to a dispatcher, that the numbers captured by the device match the information in the database.
The scanners can sometimes help police direct urgent searches.
Earlier this year, as authorities in southern Arizona sought a suspicious car on Interstate 10, an officer who entered information about the vehicle into his scanner discovered the suspected drug smugglers had passed him 20 minutes earlier. With an estimate of the speed and direction of the vehicle, officers located it, made arrests and seized a large amount of methamphetamine and $80,000 in cash.
Privacy advocates worry that authorities will use the readers to track the movements of law-abiding people, a risk they said will grow as the devices drop in price and proliferate.
People may drive to abortion clinics, substance-abuse counseling meetings, racetracks or other lawful gatherings but might prefer to keep that private, the advocates said.
"I shouldn't have to take extra precautions to prevent the government from seeing what I am doing every Thursday night," said Lee Tien, an attorney for the Electronic Frontier Foundation, a group concerned about privacy rights in the digital age.