Ding! Ding! Ding! Ding! I'm riding in the backseat of an LAPD cruiser, and it sounds like the inside of a giant slot machine. As officer Christine Labriola speeds down the side streets of LA's notorious Rampart district, a Dell laptop mounted on the Crown Victoria's dashboard chimes intermittently. A picture of each car we pass is displayed on the screen until the next car appears.

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We're hunting for stolen vehicles using the LAPD's new tech toy: a digital license plate reader. The $20,000 system consists of two sets of cameras mounted on the squad car's roof, two more pointing out the rear window, a processing system in the trunk, and the Dell on the dash. Each Ding! means the LPR has processed a plate, checking it against the department's hot list of 123,000 stolen autos and outstanding warrants. As fast as Labriola can gun it through the littered streets, the Dell keeps pace. We pass an idling Chevy Impala. Ding! It appears on the screen. An ancient jalopy stripped to its axles sits near the curb. Ding! And up it pops.

"In the old way of doing business," Labriola says, gesturing out the window, "I'd see a suspect car like this one, an older import, and say to Ryan, 'Four-Zebra-Zebra-Nancy-Seven-Two-Five.'" Riding shotgun, officer Ryan Nguyen continues the old-school demonstration, tapping 4ZZN725 into a mobile data terminal mounted near his left knee. The device dials headquarters and checks the list. Nada. "We call it 'playing the stolen-car lotto,'" Nguyen says.

Los Angeles is the auto-theft capital of the US: 73,071 cars were reported stolen in 2004. Still, the chances of a random car on the street turning out to be hot – or driven by someone with an outstanding warrant – are boringly low.

"If you played the lotto all shift, you might type in 100 plates and get nothing," Labriola says. But their electronics-laden cruiser typically processes 12 plates a minute. (It can handle up to 240 plates a minute, but the officers never encounter that many cars in such a short time.) As a result, Labriola and Nguyen are finding an average of three stolen cars a day, leading to roughly 12 arrests a month (their personal record is six cars and two busts in a day). They're essentially doing the work of 10 pre-LPR patrol cops.

We take a right near MacArthur Park, and the Dell suddenly changes its tune. A red Toyota freezes on the screen, and a robotic voice chirps, "Stolen vehicle. Stolen vehicle."

Four two-person teams are testing digital license plate recognition setups for the LAPD. The systems were supplied by four different companies – AutoVu, Civica, PIPS Technology, and Remington-Elsag – each of which hopes to sign the department to an exclusive contract. Trials have been running since early 2005, and the results are striking: So far, the readers have spotted 200 stolen cars and led to 60 arrests. In its first few days on the job, a handheld LPR that officers were using in a Topanga mall parking lot before Christmas identified 17 stolen cars, resulting in six arrests. That's a lot: Patrol officers receive an award if they nab six car thieves in a single year.

The numbers are so impressive that the department has decided to spend $250,000 in a bulk deal to outfit 19 cruisers by summer (it hasn't picked a supplier yet). It's a tiny portion of the LAPD's 1,500-vehicle fleet, but officials say that's only the beginning. "This is a big piece of the force's push into the 21st century," says commander Charlie Beck, a 28-year veteran and assistant director of the Office of Operations. "It will have as dramatic an effect on police work as the radio did in the '40s and '50s."

LA isn't the only place toying with machine vision-based traffic enforcement. Similar trial programs are under way elsewhere in California, as well as at city and state levels in Florida, Connecticut, New York, Ohio, Alabama, and Arizona. In Miami, as in LA, the systems are helping beat cops find hot cars. On Alabama's highways, stationary cameras are being paired with radar systems that automatically write speeding tickets as violators zip by. And in Sacramento, readers have tripled parking violation revenue by letting officers quickly spot autos with outstanding unpaid tickets (which they then boot).

Europe provides a glimpse of what even bigger deployments can do. In France, 1,000 mobile and stationary plate-reading cameras have doubled speeding ticket revenue and halved speeding-related deaths in just two years. In the UK, 200 cameras policing London's Downtown Congestion Charge Zone generated 13,000 arrests in one year. British law enforcement loves the technology so much that the government has plans for a $43 million campaign to install enough cameras to monitor every motorist on the country's highways, major roads, and bigger intersections, digitally reading some 35 million plates per day. This could catch not just every stolen car but nearly every moving violation as it occurs.

Back in the US, police agencies have been reluctant to spend money on plate readers (trial systems are generally supplied for free by manufacturers hoping for an order). Price and technical problems have been cited as the main barriers. The cost of a plate-reading system just two years ago was as high as $200,000 per car. And early versions came with painfully slow software and cryptic interfaces. For Los Angeles, at least, falling prices and improved performance are finally making the systems worth the money. Other agencies seem to agree: Recently, Mesa, Arizona, purchased two readers, Sacramento got five, and Miami-Dade County bought three. "You're going to see more and more of these systems," says Lieutenant Greg Terp, commander of the Miami-Dade multi-agency auto theft task force. "An officer looks for stolen car indicators that inform his decision to run a plate. This just runs everything."

The idea of cameras monitoring every highway, boulevard, and alley might strike some Americans as Orwellian. But even the American Civil Liberties Union acknowledges that the public has no right to license plate privacy on public streets. After all, cops can enter plate numbers by hand, so why not by camera? "There's absolutely no bar on collecting plates in public," says Barry Steinhardt, director of the ACLU's technology and liberty program. "There haven't been any legal challenges, because it's not illegal."

"Stolen vehicle. Stolen vehicle." Back in the cruiser, Nguyen turns to look through the rear window for the Toyota displayed on the screen. "That one!" he says, pointing to a rotting, red '84 Celica with parking tickets sticking out of the hood. The Celica is a classic mark, renowned among thieves because it can be started with a screwdriver.

It looks abandoned. Labriola steers her Crown Vic around the corner, where we can watch the vehicle unobtrusively. Now we wait, hoping the crook will return to the car. Once, a guy who walked into a hot-car stake�-out turned out to be a heroin addict responsible for 25 open theft cases. Then there was the woman driving a rental car she had kept for more than a year. But most cases are more routine. "Truth is, most people steal cars to save gas on the drive to work, then dump them," Nguyen says.

After 10 minutes, they tag the car for towing, and we slip off down the alley, hunting for better targets.

An hour later, the cameras catch another old-model import that appears to be abandoned. A year ago, two hot cars in a single day would have earned the officers a pat on the back at the following week's roll call. Now it's a letdown.

"I'm to the point where I need my fix," Labriola says with a little laugh. "My daily felony fix."

As sunset ignites the downtown skyscrapers, we head back to the station. Labriola takes the long way home – hoping for one final score – and the Dell chimes away like we just hit the jackpot.

David Downs (david_downs@wiredmag.com) wrote about print-on-demand services in issue 13.12.