If the Electronic Frontier Foundation and a San Diego-based Republican state senator have their way , it will soon become legal for Californians to cover their license plates while parked as a way to thwart automated license plate readers.

Those devices, now commonly in use by law enforcement nationwide, can capture license plate numbers at a very high rate of speed, as well as record the GPS location, date, and time that a particular plate is seen. Those plates are then run against a "hot list" of stolen or wanted cars, and a cop is then alerted to the presence of any vehicle with a match on that list.

As written, the new senate bill would allow for law enforcement to manually lift a cover, or flap, as a way to manually inspect a plate number. The idea is not only to prevent dragnet license plate data collection by law enforcement, but also by private companies. A California company, Vigilant Solutions, is believed to have the largest private ALPR database in America, with billions of records.

Ars is unaware of a commercially available product that would allow a license plate to be easily blocked in this fashion. A man in Florida was arrested earlier this year for using a miniature black screen that could be activated via remote control as a way to block his plate number when he passed through automated toll booths.

The new bill will come up before the California State Senate Transportation and Housing Committee on Tuesday, May 9—the first stop in the legislative process.

The California Police Chiefs Association has already filed its opposition to the bill. In a letter to Sen. Joel Anderson, the group argued that the bill would only benefit one group: "those who are trying to evade law enforcement and detection." Similarly, the bill has faced resistance from the California Public Parking Association, among other groups.

Blanket coverage

While law enforcement agencies have generally favored the use of this technology—it makes locating wanted or stolen cars easier, as well as issuing parking tickets—the reality is that nearly all of the data collected is for cars that do not generate "hits." For example, in April 2008, the Oakland Police Department reported to the city council that after using just four LPR units for 16 months, the units had read 793,273 plates and had 2,012 hits—a "hit rate" of 0.2 percent. In other words, nearly all of the data collected by an LPR system concerns people not currently under suspicion. Data released by other cities shows a similarly low hit rate.

Privacy activists, though, are concerned that with enough data a pattern of behavior can be kept for lengthy periods of time showing movement near potentially sensitive locations: marijuana dispensaries, abortion clinics, churches, gun stores, and more.

Under current state law, it is legal for parked cars to be entirely blanketed in a car cover as a way to protect them from the elements. However, cars in California cannot legally have their license plates covered while driving.

Dave Maass of the EFF told Ars that he was partially inspired by the webcam-covering stickers the EFF sells that have become practically ubiquitous amongst the privacy-minded.

"Why can’t we make that for a car? Why can’t we make a version of Privacy Badger for a car?" he said, referring to the EFF browser plugin.

Last month, Vigilant began enticing more law enforcement agencies to use its services by waiving "any and all data migration and hosting fees" and giving customers access to the company’s "LEARN Basic Analytics" package for three years.

In March 2015, Ars obtained the Oakland Police Department’s 4.6 million reads of more than 1.1 million unique plates, which were gathered between December 23, 2010 and May 31, 2014, as part of a public records request. The dataset showed precisely how revelatory such information can be—we were able to discern the home of a city council member with little difficulty.