CBS did a story this morning on automatic license plate readers (ALPR) in California, highlighting a technology that is spreading like wildfire across police departments and even federal agencies nationwide. It's a tool that will have profound implications for the privacy of every person who drives, and yet many people have never even heard of it.

License plate reader cameras capture an image of each license plate they encounter, convert the plate number to machine readable text, and run it against a host of databases to look for hits pertaining to violations like auto theft, outstanding warrants, expired registrations, or even terrorist watchlist alerts. The machines are capable of processing thousands of plates per minute.

But that's only part of what the technology does. The systems also create a record for each plate they read, documenting not only photographs of the car and license plate, but also precise time, date and GPS location information showing when and where the machines encountered each car. 

There are therefore two fundamentally different ways license plate readers can be useful to the police. On the one hand, plate readers help officers identify people against whom some infraction or crime has been alleged. On the other, they create databases containing detailed records of where millions of drivers go every day in their cars, and when. 

The first use is a totally acceptable means of deploying a new technology to make police work more efficient. The second is an unacceptable workaround of the Fourth Amendment, enabling mass, retroactive and warrantless tracking of everyone who drives.

It sounds like a complicated problem, but the solution is actually quite simple and within reach: we need data retention regulation enshrined in law.

The sky will not fall if we pass data retention laws

You get the sense from watching the clip above that the police don't want to talk about data retention, and what it would mean for them to be able to go back into a database to see where and when anyone has driven over the past year (or five). But that's the central, largely unattended problem with license plate readers, and we need to shout it from the rooftops. Unrestricted license plate data retention is a serious threat to our motoring privacy.

Our fight therefore isn't license plate readers or no license plate readers. The nuance is important, because so often privacy and civil liberties advocates are painted as being opposed to effective approaches to combat crime. That's never the case, but with license plate readers the straw man is even more absurd because we can effectively limit the privacy harms while still allowing police to use the technology to great effect. 

The threat is growing everyday, as law enforcement agencies pool our license plate information into regional databases, a trend that is spreading nationwide.

Many law enforcement agencies say they need to retain all captured license plate data for long periods of time (in Boston it's three months; New Jersey's state limit is five years) so they can use it to go back and solve serious crimes. But if police need to identify which cars were in an area during a murder or armed robbery, they can do it with a retention period of a few days or a week, tops. Storing the data for months or years is excessive and violates the privacy of everyone who drives.

Police don't need to know when my colleague went to the laundromat, what time you get home from work, which gun store your uncle frequents, or when your cousin had an abortion. And they shouldn't know where I was every day last month, either, unless they got a warrant to track me.

If you want to get involved here in Massachusetts to stop police from collecting detailed records of our movements, take action. Want to do more, and get involved in grassroots organizing to push privacy legislation in Massachusetts? Please get in touch. And stay tuned to learn more about how police and federal agencies are using license plate readers.