Is a License Plate Reader a Fourth Amendment Search?

Why “search” is the threshold question

The Fourth Amendment protects against unreasonable searches and seizures. If police activity isn’t a “search” at all, the Fourth Amendment generally doesn’t require a warrant for it. If it is a search, police generally need a warrant or an exception to the warrant requirement. That’s why the first question about ALPR evidence is often whether using it was a search, and the answer can decide whether a motion to suppress is possible.

How courts decide what counts as a search

The Supreme Court uses two main approaches:

• Reasonable expectation of privacy. From Justice Harlan’s concurrence in Katz v. United States (1967): a search occurs when the government intrudes on an expectation of privacy that a person actually holds and that “society is prepared to recognize as ‘reasonable.’” The Katz majority also said that “what a person knowingly exposes to the public” generally isn’t protected.
• Physical intrusion. In United States v. Jones (2012), the Court held that attaching a GPS device to a vehicle to gather information was a search because of the physical intrusion onto the vehicle.

Fixed ALPR cameras don’t touch the vehicle, so ALPR cases turn on the first test: did the collection or search of plate data intrude on a reasonable expectation of privacy?

A single plate read

In United States v. Knotts (1983), the Court held that “a person travelling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another.” A license plate is displayed in public, and an officer may read and write down a plate number. The Massachusetts high court made the same point in McCarthy: a single license plate reader “is similar to traditional surveillance techniques.”

So a single camera recording a single passing vehicle is unlikely, by itself, to be treated as a search under current case law.

Stored location history: the aggregation question

The harder question is what happens when many reads are stored and searched together. Knotts itself reserved this, saying that if “dragnet-type law enforcement practices” occurred, “there will be time enough then to determine whether different constitutional principles may be applicable.”

Later decisions took up that idea:

• Jones concurrences (2012): five Justices took the view that “longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy.”
• Carpenter (2018): the Court held that obtaining 127 days of historical cell-site location records was a search, because it “provides an all-encompassing record of the holder’s whereabouts,” and that police generally need a warrant. But the Court called the decision “a narrow one” and said it didn’t “call into question conventional surveillance techniques and tools, such as security cameras.”
• McCarthy (Mass. 2020): applying this reasoning to ALPRs, the court recognized a protected privacy interest “in the whole of [a person’s] public movements,” but held that four cameras on two bridges didn’t reveal enough to invade it.

The aggregation idea is sometimes called the mosaic theory. The core question for ALPRs is whether a given network and a given search reveal enough of someone’s movements to cross that line. No court has drawn a precise line. McCarthy expressly declined to set a bright-line rule.

Real-time alerts

ALPR hotlist alerts tell police where a vehicle is right now. McCarthy warned that “with cameras in enough locations,” this feature could invade a reasonable expectation of privacy in one’s real-time location, making ALPRs “the vehicular equivalent of a cellular telephone ‘ping.’” The same court noted limits: the exigency exception to the warrant requirement may apply to alerts such as missing-child alerts, and using ALPRs to find a vehicle reported stolen would not be impermissible.

Whether a stop based on an alert was lawful is a separate question from whether ALPR use was a search. See ALPR hotlists and alerts for how alerts are supposed to be verified.

Factors courts have pointed to

Flock networks can include many more cameras than the four at issue in McCarthy, and Flock’s data-sharing features can widen what an agency can search. Whether that changes the answer in a particular case depends on the record: which cameras, how many reads, over what period, and what police actually searched.

State constitutions can go further

The Fourth Amendment sets a minimum. State courts can read their own constitutions to give more protection. McCarthy analyzed both the Fourth Amendment and Article 14 of the Massachusetts Declaration of Rights. State statutes can also limit how ALPR data is collected, kept and shared. See license plate reader laws by state.

What this means in a case

Questions a defense attorney may look into:

• Was the evidence a single read, a search of stored history, or a real-time alert?
• How many cameras captured the vehicle, where, and over what period?
• Did police obtain a warrant before searching stored ALPR data? If not, what exception do they rely on?
• Did the search reach data from other agencies or a shared network?
• Does the state constitution or a state statute give more protection than federal law?

The answers depend on the facts and on the law where the case is brought. For the cases discussed here, see the case law library.