Court warns against all encompassing surveillance networks, but says for now, it's legal for the state to keep records of every vehicle that crosses over the Cape Cod Canal for up to a year

The Supreme Judicial Court ruled today that prosecutors can use license-plate data from cameras mounted on both Cape bridges to make their case against an alleged heroin dealer who was nabbed in part because the data showed he was making frequent trips on and off the Cape - and alerted Barnstable police to the specific trip he made that led to his arrest.

The state's highest court spent much of its decision today considering the privacy implications of automated license-plate readers and said that at some point, it may have to consider limits on the type and use of data collected from devices, which are now found along the Massachusetts Turnpike and on other roads in the state.

But for now, the court said, Barnstable police did nothing wrong in going through data collected from four cameras mounted on the Bourne and Sagamore bridges to monitor, without a warrant, the comings and goings of a man they were already investigating as a heroin supplier in 2017. On Feb. 22 of that year, one of the cameras pinged Barnstable police the car was coming on-Cape and officers set out to follow it - and ultimately arrested its driver after they watched him meet with a suspected buyer.

The court said it would continue to use a particular legal theory, known as the "mosaic theory" for figuring out how much information is too much in a world where modern technology makes data collection incredibly easy even as we continue to have constitutional privacy rights under the Fourth Amendment and the similar section of the state constitution. Simply put, the theory compares data to colored stones: One stone by itself, seen close up, would not tell much of a story, but an observer who stands back could view a design, or mosaic, based on its positioning with multiple other stones.

The court said that a large enough network of license-plate readers could provide such a "mosaic" view of somebody's comings and goings that, when used by police, would constitute an unwarranted privacy invasion, an unconstitutional "search:"

A network of ALPRs that surveils every residential side street paints a much more nuanced and invasive picture of a driver's life and public movements than one limited to major highways that open into innumerable possible destinations. ...

Similarly, with cameras in enough locations, the hot list feature [which lets police get notified when a particular plate passes a particular location] could implicate constitutional search protections by invading a reasonable expectation of privacy in one's real-time location. If deployed widely enough, ALPRs could tell police someone's precise, real-time location virtually any time the person decided to drive, thus making ALPRs the vehicular equivalent of a cellular telephone "ping." See Almonor, 482 Mass. at 55 (Lenk, J., concurring) ("When police act on realtime information by arriving at a person's location, they signal to both the individual and his or her associates that the person is being watched. . . . To know that the government can find you, anywhere, at any time is -- in a word -- 'creepy'"). Of course, no matter how widely ALPRs are deployed, the exigency exception to the warrant requirement would apply to this hot list feature.

Finally, like carrying a cellular telephone, driving is an indispensable part of modern life, one we cannot and do not expect residents to forgo in order to avoid government surveillance.

But the case at issue is not such a case, and so the court said now is not the time to try to codify just how pervasive a network of plate readers has to be to be unconstitutional.

The cameras in question here gave police only the ability to determine whether the defendant was passing onto or off of the Cape at a particular moment, and when he had done so previously. This limited surveillance does not allow the Commonwealth to monitor the whole of the defendant's public movements, or even his progress on a single journey. These particular cameras make this case perhaps more analogous to [cell-phone location data], if there were only two cellular telephone towers collecting data. Such a limited picture does not divulge "the whole of [the defendant's] physical movements," Carpenter, 138 S. Ct. at 2217, or track enough of his comings and goings so as to reveal "the privacies of life." Id., quoting Riley, 573 U.S. at 403. See Boyd v. United States, 116 U.S. 616, 630 (1886).

While we cannot say precisely how detailed a picture of the defendant's movements must be revealed to invoke constitutional protections, it is not that produced by four cameras at fixed locations on the ends of two bridges. Therefore, we conclude that the limited use of ALPRs in this case does not constitute a search within the meaning of either art. 14 or the Fourth Amendment.

In a footnote, the court did allow as how its decision today not to establish a "brightline" rule on plate-reading pervasiveness could lead to some confusion - if four readers aren't enough to trigger constitutional issues, what is the limit?

We trust, however, that as our cases develop, this constitutional line gradually and appropriately will come into focus.