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 vehicle was stolen, according to the Web site www.kvoa.com. 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.

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 report said. 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.

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.

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. A burst of license plate images appear on the laptop when an officer is driving freeways and 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 prospect of catching more suspected felons prompted police in Springdale, Ohio, to get a card reader for one of their 13 cruisers, according to www.kvoa.com.