Police call license plate scanners critical, but do they violate your privacy?

(CBS News) ROCKVILLE, Md. - On Maryland Highway 200, Officer Yancey Anthony patrols with two extra sets of eyes. Cameras, mounted on his cruiser's trunk, scan and photograph the license tags of passing cars -- sounding alarms when possible violators are spotted.

The license readers instantly check plates and the names of vehicle owners against registration records, fugitive warrants, and criminal data bases -- giving police critical information.

The camera alerts Officer Anthony to a silver Toyota. He tells the driver that his tags are coming back suspended.

The license readers can scan 1,800 plates a minute -- on cars going as fast as 150 mph.

On a busy road, the scanners are reading every tag that goes by, giving officers an instantaneous read on whether there might be some violation.

The data are also funneled to Maryland's Intelligence Center, which connects and monitors 367 license plate readers around the state. Assistant U.S. Attorney Harvey Eisenberg oversees the network.

"Say, we want to find a homicide suspect, a rape suspect, or whatever. You put that into the systems -- that's a vehicle you want to pay attention to," he said, and officers out on the street are getting critical information in real time.

"Someone who's out there with a weapon, who's a homicide suspect and known to be armed -- you're going to want to approach that vehicle differently if you are a trooper," Eisenberg added.

Eisenberg says the readers have helped capture dangerous fugitives. But they are also used beyond the freeways. During President Obama's inauguration, the cameras helped scan for tags of specific vehicles connected to possible threats. And some 500 readers on the U.S. border feed data into terror watch lists.

Tax departments use them as well. Arlington, Va. relies on scanners to locate the cars of delinquent taxpayers. For the worst offenders, officials remove the tags.

Privacy advocates complain the cameras allow governments to track people's travels. But Eisenberg says that, for law enforcement, the scanners are critical.

"We have nothing to hide," he said of the privacy concerns. "There's nothing villainous about this. This is simply law enforcement trying to do its job within the law."

New Hampshire, so far, is the only state to ban license plate readers. In Maryland, authorities are looking to expand the program.