We all know that feeling of dread when a police car appears out of the blue on the side of the road and we’re driving too fast. Instinctively, we slow down, hoping that the officer doesn’t pull us over and issue a ticket for speeding. If the police officer passes us by without incident, it could be luck, or it may be that the cop is using an automated license plate reader to see if there’s a more serious infraction to stop us for.

The technology is not new, but has spread like wildfire across the U.S. in recent years. From Maryland to Washington, law enforcement agencies are mounting the devices on cruisers and roadsides, analyzing license plates and giving cops the 411 on drivers.

Primarily, local and state police use the readers to help spot stolen vehicles, or identify wanted criminals or gang members. But the technology is also helpful during AMBER alerts, for calculating travel times for overhead roadway signs, or for assisting marshals in stopping people who have warrants issued for their arrest because of unpaid tickets.

The readers are typically mounted to a car’s bumper and are connected to an officer’s laptop inside the vehicle. When the reader locates something similar to a license plate, it performs optical character recognition (OCR) to determine a match. A program on the computer accesses a backend database, revealing in almost real-time whether the person who registered the car in question has anything on his or her record that requires a traffic stop.

Who’s Using Them

Maryland has hundreds of license plate readers in use, spread across both state and local law enforcement. In 2010, the state announced it was creating a special network to collect all data obtained by the devices that would be housed in the state’s fusion center.

On the other side of the country, Seattle has been using license plate cameras for a few years. But instead of focusing only on law enforcement use, the city has placed them on streets to help calculate travel times to specific locations. Seattle launched the feature with a new intelligent transportation system in October 2010.

In 2010 the Seattle Department of Transportation (SDOT) announced plans to install 130 cameras throughout the city and a total of 22 information signs (although not all will display travel times).

Iowa jumped on the license plate reader bandwagon in 2011. Police in Des Moines mounted the devices on patrol cars and uses them primarily in high-crime areas to identify those wanted by the authorities.

Last year, Piedmont, Calif., approved the purchase of 39 license plate cameras (for $679,000) for the same reason. Police Chief Rikki Goede believed putting the readers on 30 roads leading into and out of the city might help bring down the 50 percent increase in burglaries the city experienced between 2011 and 2012, according to Ars Technica, a technology media website. Piedmont, a wealthy Bay Area suburb with a population of approximately 11,000, is largely residential and surrounded by the city of Oakland.

Automated license plate readers can vary in cost from $20,000 to $30,000 depending on the vendor. But many law enforcement agencies are seeing a dramatic increase in efficiency and effectiveness.

Arizona first tested automated license plate readers in 2006, primarily to help cut the high rate of stolen vehicles in the state. The state’s Department of Public Safety discovered after a few months that the devices were capable of reading 1,500 plates each during an eight-hour shift. By comparison, officers manually ran approximately 40 during the same period.

Back in 2008, when the Los Angeles Police Department had readers installed on only 12 vehicles, Charlie Beck, now chief of police, said license plate readers helped recover “four to five times” the number of stolen vehicles an officer would be able to recover without the technology.

Pennsylvania faced a similar issue. In 1996, runaway auto thefts tallied 53,000. But after installing automated license plate readers on 13 squad cars, that number dropped to 28,000 in 2007.

While the technology seems helpful as both a deterrent and as an aid for law enforcement, not everyone likes the technology. Privacy advocates are up in arms over the data collected and stored by license plate readers.

The ACLU of Southern California and the Electronic Frontier Foundation (EFF) sued the LAPD and the Los Angeles County Sheriff’s Department last month over records collected by license plate readers over the last several years.

According to The Huffington Post, both law enforcement agencies denied requests for the data collected by the devices, citing various sections of California law that allow police to keep records of ongoing investigations private and free of public scrutiny.

“These started out as tools for stolen cars, but the departments have built up an infrastructure for intelligence where they’re monitoring the movements of law-abiding residents of Los Angeles,” Peter Bibring of ACLU Southern California told the Huff Post.

Brian Heaton  |  Senior Writer

Brian Heaton is a senior writer for Government Technology. He primarily covers technology legislation and IT policy issues. Brian started his journalism career in 1999, covering sports and fitness for two trade publications based in Long Island, N.Y. He's also a member of the Professional Bowlers Association, and competes in regional tournaments throughout Northern California and Nevada.