The Mosaic Theory and Location Surveillance

The idea

The usual Fourth Amendment question asks whether a particular act was a search. The mosaic theory asks something different. In the words of a source quoted by the Massachusetts high court in McCarthy, it “asks whether a series of acts that are not searches in isolation amount to a search when considered as a group.”

Applied to driving: one camera seeing your car isn’t a search, because anyone on the street could see it. But a record of every time your car passed every camera for months might show where you live, work, worship and who you visit. The theory holds that the whole can be protected even when the pieces aren’t.

Where it comes from

• Knotts (1983) held that following a car on public roads wasn’t a search, but reserved the possibility that “dragnet-type law enforcement practices” could call for different principles.
• Jones (2012): five Justices, across two concurrences, took the view that “longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy.” Justice Sotomayor asked whether people reasonably expect their movements to be “recorded and aggregated” in a way that lets the government learn their beliefs and associations. The majority decided the case on a different ground, physical intrusion.
• Carpenter (2018): the Court held that 127 days of cell-site records was a search because they provide “an all-encompassing record of the holder’s whereabouts.” The decision drew on the Jones concurrences, though the Court didn’t use the label “mosaic theory.”

How courts have applied it

In McCarthy, the court described the theory as “an aggregation principle for the technological surveillance of public conduct” and accepted that a person has a protected interest “in the whole of his public movements.” It just found that four bridge cameras didn’t reveal enough. In Leaders, the court found that the aerial program did.

Why it matters for license plate readers

A single ALPR camera looks like ordinary observation. A network of cameras with stored history and search tools looks more like the long-term tracking the Jones concurrences and Carpenter were concerned with. McCarthy identified the features that push ALPRs toward that end: long retention, capture of nearly every vehicle, round-the-clock operation, and location tagging. It also identified what pulls the other way: few cameras, in limited places.

That’s why the facts of a specific search matter so much. See the factors in is ALPR a Fourth Amendment search?

Open questions

• Where is the line? No court has set a number of cameras, days or data points. McCarthy expressly declined to.
• What gets counted? Only the data police looked at, or everything they could have searched?
• Has the Supreme Court adopted it? Not by name. Carpenter rests on similar reasoning but called itself narrow, and the Jones majority relied on a different test.
• Do states go further? State constitutions may be read more broadly. McCarthy analyzed Massachusetts’s Article 14 alongside the Fourth Amendment.