HOW IT WORKS; Zeroing In on the Suspicious Number Above the State Motto
CAR washes use them to cut down on nonpaying customers. Parking garages use them to prevent customers from swapping tickets and paying lower fees. But while digital license plate readers, which read and verify a vehicle's license plate, have primarily been used in the United States as a means of ensuring payment, they are now being sized up as security tools.

Digital license plate readers use a camera with a light source to take an image of the front or the rear of a vehicle as far as 250 feet away. Optical character-recognition software converts the plate image to text that is then sent to a database for verification purposes.

Although digital license plate readers have their limits, some industry experts suggest that they could be used to monitor traffic for possible security threats.

Joe Wenzl, director of technical services at Federal APD, a company based in Novi, Mich., that manufactures automated parking equipment, said that the systems could be used to create records of vehicles' movements around airports. ''Recent events have caused us to look at the ticketing systems to see how we can better secure parking areas, and I think the cameras are an absolute way to do that,'' he said.

Alan Sefton, president of PIPS, a manufacturer of digital license plate readers based in Knoxville, Tenn., said that the systems could keep a log of vehicles that enter airports and detect potentially suspicious activity. ''It could create a database of employees and taxis who are authorized to be in the area, yet also have records of vehicles that pass through three times in one week,'' he said.

Some experts say that to be of use in high-security areas, however, digital license plate readers need to overcome some serious shortcomings, like difficulty in reading different kinds of plates and even a state's name, which is often in smaller or cursive type. ''They're not quite ready for prime time, particularly when it comes to differentiating between states' license plates,'' said Michael Kolb, a consultant with Traffic Technologies of Rhinebeck, N.Y., which designs electronic toll systems for the transportation industry.

The systems can also have trouble reading license plates on cars moving at high speeds, said Lee Nelson, a consultant with Electro-Optical Technologies in Falls Church, Va. ''In a low-speed application, the license plate readers have time on their side,'' he said. ''But if the system is being used in motion, there's more of a challenge to capture an image that can be read by a machine.''

Mr. Nelson said that he knew of more than 50 companies that specialize in plate recognition but that many people in the security industry were still unfamiliar with the technology. ''It's not perfect, and it never will be, but I think there are a lot of problems that can be solved with license plate readers,'' he said.

Although digital license plate readers have generally been slow to catch on with law enforcement agencies in the United States, they are being used by American customs officials at some border crossings, including those with Mexico.

As a car pulls up to the customs inspector, two cameras record the license plate in front and two record the one in back. The information is run through a database that is linked to motor vehicle registration records and the National Crime Information Center, so that the inspector can receive information about the vehicle's registration and any criminal activity by the owner.

''It's pretty effective for what we need it for,'' said Dennis Murphy, a spokesman for the United States Customs Service. ''Weather issues make it hard to read in some locations, and some plates are pretty beaten up, which makes them hard to read. But we feel good about the effort that they help us save.''

The cameras are also used as cars exit the border control area, recording departure time. ''This way, when they return to customs, if the drivers are giving misinformation about when they left, you can tell,'' Mr. Murphy said.

Some companies, including PIPS and ViewSystems, a Baltimore-based manufacturer of remote access digital and wireless surveillance products, are developing mobile license plate readers with cameras that allow highway patrol officers to record images while on the move. Typically, officers now have to enter the plate numbers themselves by typing them into a terminal in their patrol cars that is connected to a remote database.

Using a camera system would allow the police officer to drive without having to look at a keyboard. With a zoom lens, the officer would also be able to get an image of plates that are too far away for the eye to see, said Gunther Than, president and chief executive of ViewSystems.

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