Through a public records request, Ars Technica's Cyrus Farivar has obtained the Oakland police department's entire license plate reader database, just under 4.7 million scans taken over more than three years. The database is available to police without a warrant, and thus was accessible through public records law, but the mere existence of the dataset raises profound privacy issues.

"Do you think that anyone with a badge should be able to search through that data?"

Only 0.16 percent of the database relates to suspected criminal activity, but there are no minimization policies in place, allowing anyone with access to the database to trace back a subject's movements over the course of years. In one case, Ars traced the data back to a subject's home and work place, although the article stresses that "in cases where we searched a known individual's plates, we did so only with their explicit consent." Oakland operates 33 license plate readers, each capable of scanning up to 60 plates per second, but that relatively small number of devices adds up to a remarkably comprehensive portrait of the city's inhabitants, particularly when combined with other publicly available data.

In 1983, the Supreme Court ruled that cars on public roads do not have a reasonable expectation of privacy, but many think the ruling should be revisited in light of modern database technology. "Project forward to a world where LPR technology is cheap and they can be mounted on every police car and posted at every traffic light," UC Berkeley professor Catherine Crump told Ars. "Do you think that anyone with a badge should be able to search through that data at their discretion?"