Automatic license plate readers don’t pose much of a threat to our privacy if there aren’t very many of them. Like surveillance cameras, they really only become a problem when the data they collect are situated in a broader context of pervasive monitoring. One data point showing that your car drove past a stationary license plate reader on one highway doesn’t tell the government very much. But the data points begin to pile up when the surveillance cameras and license plate readers are on every street corner and police cruiser. And absent commonsense limits, that means police and prosecutors (and anyone else who gets at the database) can map your movements with the click of a button.

That’s why the ACLU has invested resources in the effort to protect our motoring privacy from unwarranted retroactive government surveillance. We didn’t want to wait until license plate readers (LPRs) were everywhere before we acted to protect our privacy; the trends seemed clear to us.

Now, a report published by the Police Executive Research Forum (PERF) shows that we were right to act early.