Despite the fact that no federal license plate legislation has been proposed, the International Association of Chiefs of Police (IACP) has sent a pre-emptive letter to top Congressional lawmakers, warning them against any future restrictions of automated license plate readers. The IACP claims to be the "world's oldest and largest association of law enforcement executives."

As the letter, which was published last week, states:

We are deeply concerned about efforts to portray automated license plate recognition (ALPR) technology as a national real-time tracking capability for law enforcement. The fact is that this technology and the data it generates is not used to track people in real time. ALPR is used every day to generate investigative leads that help law enforcement solve murders, rapes, and serial property crimes, recover abducted children, detect drug and human trafficking rings, find stolen vehicles, apprehend violent criminal alien fugitives, and support terrorism investigations.

Sarah Guy, a spokeswoman for the IACP, told Ars that current state and local restrictions have made the police lobby group concerned at the federal level.

"Last year during the appropriations process there was an amendment that would have prohibited fed funds to purchase LPRs or any camera that collects or stores license plate numbers," she said. "That didn't pass but we think that something like this could be tried again."

We are the 99.8 percent

The cameras scan at an extremely high rate, usually around 60 plates per second. Law enforcement policies vary widely concerning how long that information can be retained. Different agencies keep that data anywhere from a few weeks to indefinitely. Some cities have even mounted such cameras at their city borders, monitoring who comes in and out.

In December 2013, the Boston Police Department indefinitely halted its use of LPRs following an investigation into their use by the investigative journalism organization MuckRock and the Boston Globe. The devices have also been banned in New Hampshire since 2007, and have a 21-day retention limit in Maine. In nearby Vermont, the retention limit is 18 months under a law set to expire this summer.

Meanwhile, in 2012, the Drug Enforcement Administration decided that it would reduce its retention period from two years to six months. In California, the wealthy Silicon Valley city of Menlo Park (home to Facebook) retains data for just 30 days, while the Los Angeles Police Department (LAPD) retains data for two years.

The organization failed to mention that it is often the case that nearly all collected ALPR data is innocuous.

For example, in Oakland, California, the police informed to the city council in April 2008 (after using four LPR units for 20 months), that it had read 793,273 plates, with hits on just 2,012 of them—a "hit rate" of just 0.2 percent. Despite this, in that same report, then-Oakland Police Department Deputy Chief Dave Kozicki (who has since retired) dubbed the ALPR setup an "overwhelming success."

Semantic games

Unsurprisingly, civil libertarians want there to be more national-level restrictions on the use of ALPR, including a more unified retention policy.

"The fact is, ALPR systems collect sensitive location information on millions of innocent Americans and, in many cases, retain that data for years or even indefinitely," Jennifer Lynch, an attorney with the Electronic Frontier Foundation, told Ars by e-mail. "99.8 percent of the vehicles recorded by license plate readers are never involved in criminal activity or even vehicle registration issues. It is fully appropriate for Congress and state legislatures to investigate and place limits on this privacy-invasive technology."

Mike Katz-Lacabe, a privacy activist in San Leandro, California, who famously shared photos that his city's police had taken of him and his daughters exiting their own car on their own driveway, told Ars by e-mail that the letter is a bit disingenuous.

"While it is technically correct that license plate readers do not track people in real time, it does track vehicles," he wrote. "Most of the time, that means you are tracking the person to whom the car is registered. It’s the equivalent of stating that the stingray isn’t used to track people, it’s used to track a specific phone."

"The data is described as anonymous, but it’s trivial to take the data from a license plate reader and associate it with a person," he continued. "In fact, in some photos, you can identify the driver—as with a photo of me that was captured by a license plate reader as I exited my car in my driveway. And the longer this data is kept, the more this data reveals about individuals and the intimate details of their lives."

Similarly, Kade Crockford, of the American Civil Liberties Union of Massachusetts, who has studied the use of ALPRs for years, concurred.

"Right now, police and private companies across the nation are compiling massive databases containing the driving location histories of millions of people accused of no crime," she said by e-mail. "This is the problem congress needs to regulate, and it's therefore not surprising that the police fail to mention it."

"Worse still, the officials represent to Congress that there are no examples of license plate tracking abuse. That's factually incorrect. But again this claim obscures the real problem: We would likely know of many more cases of police abuse of this data if legislators ensured the systems were appropriately audited, and if there was necessary transparency around how the systems are used and abused. A cloak of secrecy largely surrounds law enforcement use of plate tracking databases. In the context of this secrecy, these claims about a supposed lack of evidence of abuse are particularly difficult to swallow."