Modern technology has provided police the ability to track down vehicles belonging to people associated with serious crimes by scanning, storing and analyzing data from millions of license plates. At the same time, however, innocent drivers are being caught up in the surveillance.

This problem should serve to caution the Hono-lulu Police Department, which is acquiring the tools to adopt the practice. HPD and elected officials must guard against abuse of license-plate-scanning technology.

The American Civil Liberties Union, in a survey of 293 police departments and state agencies across the country, reported that only a tiny fraction of the data collected resulted in "hits," or the location of a suspicious vehicle. An even smaller fraction resulted in arrests.

For example, Maryland’s system of license plate readers had more than 29 million reads, but only 0.2 percent were associated with any crime. The information gathered is retained for a year.

The Honolulu Police Department plans to acquire license plate cameras and readers, which will be mounted on patrol cars, according to spokeswoman Michelle Yu.

HPD will also develop a policy governing the use of the technology, including how long license plate information can be stored. The department has no plans to share the data with other agencies.

The details of the policy will be important in determining whether the level of surveillance could violate the civil rights of citizens, including their reasonable expectation of privacy and rights of free speech and association.

Not only should the policy be shared with the public, but vetted by elected officials to ensure that the technology is used appropriately.

HPD should examine the use of the system in states that have laws governing license plate readers, including how the data can be kept and used.

For example, the Arkansas legislature enacted a requirement in April that requires captured license plate data that is not part of an ongoing investigation to be deleted from the record within 150 days, and prohibits all sharing unless it is evidence of an offense.

On its own, the Minnesota State Patrol deletes the data after 48 hours. The Ohio State Highway Patrol’s policy states that all "non-hit" license plate reader captures "shall be deleted immediately."

"The mass collection and retention of plate data about innocent Americans is alarming in and of itself," the ACLU points out, "but it is all the more worrying because these data are increasingly being fed into larger regional databases."

What’s the harm?

The U.S. Court of Appeals for the District of Columbia makes it clear: "A person who knows all of another’s travels can deduce whether he is a weekly church goer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups — and not just one such fact about a person but all such facts."

The International Association of Chiefs of Police, in a 2009 report on license plate readers, warned about the chilling effect of such surveillance, even if the police don’t use the data they’ve gathered.

"The risk is that individuals will become more cautious in the exercise of their protected rights of expression, protest, association, and political participation because they consider themselves under constant surveillance," the report said.

That level of shadowing law-abiding citizens should not be exercised anywhere, including Honolulu.