Late last week, the state's Supreme Judicial Court issued a decision concerning whether law enforcement can use the data collected by automatic license plate readers, or ALPRs, in criminal cases. WGBH Morning Edition host Joe Mathieu spoke with Northeastern University law professor and WGBH News legal analyst Daniel Medwed to learn more about the decision. The transcript below has been edited for clarity.

Joe Mathieu: For starters, before we turn to the case itself, there are general concerns about whether automatic license plate readers are even constitutional.

Daniel Medwed: That's right. Some civil libertarians across the country have raised concerns about whether these devices, sometimes called ALPR's, invade the privacy rights of motorists. However, for one thing, motorists have a reduced expectation of privacy when they're on the road. The exteriors of their cars are "knowingly exposed" — that's the language from U.S. Supreme Court precedent — and that knowing exposure is a deal that you take in exchange for the privilege of driving [and] for being regulated. For another thing, these devices are typically used for fairly innocuous and legitimate law enforcement purposes. You mentioned at the top as a stand-in for tollbooths. They're also used to monitor potential speeders or if you've run through a red light. The real constitutional question arises when these devices stray beyond the motor vehicle infraction context and are used for criminal investigations into bigger matters.

Mathieu: So tell us what happened in the SJC case.

Medwed: Well, this case involved four readers that were put in place in 2015 on the Bourne and Sagamore Bridges on Cape Cod. What these readers do is they convert your license plate images into readable data that's stored by the Executive Office of Public Safety, and that data is accessible by the state and local police. The police specifically can rely on it for what's called historical location data to determine when and how often you criss-cross these bridges. They can also set up real time alerts — they can tag your license plate and be tipped off when you're actually crossing the bridge. So in this case, a New Bedford man named Jason McCarthy was suspected of supplying heroin to a Barnstable County drug dealer. The police monitored his activities on the bridge and they used that data to build a case against him for a drug distribution charge. After his conviction, McCarthy challenged the use of this evidence as a Fourth Amendment violation. He said that it was an unlawful search and seizure [and that] the police should have had a warrant.

Mathieu: What did the courts hold here, Daniel?

Medwed: The court ultimately sided with the government, emphasizing that these readers only captured very limited data about McCarthy's movements — his comings and goings on these bridges over a three-month period. In the eyes of the court, that wasn't a Fourth Amendment violation. But the court did hint that in other contexts and in other scenarios, there could be a problem with the use of these automatic license plate readers.

Mathieu: What were those scenarios, if you will?

Medwed: The court in particular mentioned that if there were enough cameras in enough different locations in the commonwealth to capture the "whole of a person's movements," then that's a problem. In particular, the court relied on a famous 2018 U.S. Supreme Court case called Carpenter that held that in order for the police to access cell phone geolocation data of a suspect, they needed a warrant. Why? Because this historical geolocation data created a whole picture of the person's habits, lifestyle and movements. It created what's called a mosaic. And under the mosaic theory of the Fourth Amendment, that whole tableau is an unwelcome intrusion into your privacy interests. So the court distinguished this case by saying it just captured a narrow slice of McCarthy's life, not the whole picture.