"There's no expectation of privacy" for a vehicle driving on a public road or parked in a public place, said Lt. Bill Hedgpeth, a spokesman for the Mesquite Police Department in Texas, which has records stretching back to 2008, although the city plans next month to begin deleting files older than two years. "It's just a vehicle. It's just a license plate."
In Yonkers, N.Y., just north of the Bronx, police said retaining the information indefinitely helps detectives solve future crimes. The department said it uses license plate readers as a "reactive investigative tool" that is accessed only if detectives are looking for a particular vehicle in connection to a crime.
"These plate readers are not intended nor used to follow the movements of members of the public," the department said in a written statement.
However, even if police say they don't want a public location tracking system, the records add up quickly. In Jersey City, N.J., for example, the population is only 250,000 but the city has more than 2 million plate images on file. Because the city keeps records for five years, the ACLU estimates that it has about 10 million on file, making it possible for police to plot the movements of most residents, depending upon the number and location of the scanners.
The ACLU study, based on 26,000 pages of responses from 293 police departments and state agencies across the country, also found that license plate scanners produced a small fraction of "hits," or alerts to police that a suspicious vehicle has been found. In Maryland, for example, the state reported reading about 29 million plates between January and May of last year. Of that amount, about 60,000 -- or roughly 1 in every 500 license plates -- were suspicious. The No. 1 crime? A suspended or revoked registration, or a violation of the state's emissions inspection program accounted for 97 percent of all alerts.
Eisenberg, the assistant U.S. Attorney, said the numbers "fail to show the real qualitative assistance to public safety and law enforcement." He pointed to the 132 wanted suspects the program helped track. They were a small fraction of the 29 million plates read, but he said tracking those suspects can be critical to keeping an area safe.
Also, he said, Maryland has rules in place restricting access to criminal investigations only. Most records are retained for one year in Maryland, and the state's privacy policies are reviewed by an independent board, Eisenberg noted.
At least in Maryland, he said, "there are checks, and there are balances."