Warrants and License Plate Reader Data

When the Fourth Amendment requires a warrant

The warrant question only arises if the police activity is a search. If it isn’t, the Fourth Amendment generally doesn’t require a warrant. If it is, police generally need a warrant supported by probable cause, unless an exception applies. So this page builds on is ALPR a Fourth Amendment search?

The Carpenter rule and its limits

In Carpenter v. United States (2018), the government obtained 127 days of a suspect’s cell-site location records using a court order that required only “reasonable grounds,” which the Court said “falls well short of the probable cause required for a warrant.” The Court held that obtaining those records was a search and that the government “must generally obtain a warrant supported by probable cause before acquiring such records.”

The Court also called the decision “a narrow one.” It didn’t address license plate readers, said it didn’t “call into question conventional surveillance techniques and tools, such as security cameras,” and declined to say whether a short enough period of location data could be obtained without Fourth Amendment scrutiny.

Applying it to ALPR data

Courts applying Carpenter’s reasoning to other location surveillance have focused on how much the data reveals:

• Not a search on limited data: in Commonwealth v. McCarthy (Mass. 2020), four ALPR cameras on two bridges didn’t reveal enough of the defendant’s movements to be a search, so no warrant was required. The court said widespread ALPR use could be different.
• A search on comprehensive data: in Leaders of a Beautiful Struggle v. Baltimore (4th Cir. 2021, en banc), a federal appeals court held that accessing data from Baltimore’s aerial surveillance program was a search, because the program enabled police “to deduce from the whole of individuals’ movements,” and that its “warrantless operation violates the Fourth Amendment.” That case involved aircraft photographing the city, not ALPRs, though the program planned to integrate license plate readers.

The practical question for a Flock search is where it falls: a check of whether one plate passed one camera, or a query reconstructing a vehicle’s movements across many cameras and days, possibly including other agencies’ data. The answer can depend on the network, the query and the jurisdiction.

Data held by others: the third-party question

Under the “third-party doctrine” from Smith v. Maryland and United States v. Miller, people generally have a reduced expectation of privacy in information they share with others. In Carpenter, the government argued that doctrine covered cell-site records held by phone companies. The Court rejected “mechanically applying” it to that kind of location data, while saying it didn’t disturb Smith and Miller themselves.

ALPR data raises its own version of this question. A plate is displayed publicly, and the data may be held by a vendor or shared among agencies. How courts treat that is still developing.

Exceptions to the warrant requirement

Even where a warrant would normally be required, exceptions can apply. Carpenter noted that “case-specific exceptions” such as exigent circumstances may support a warrantless search, for example to pursue a fleeing suspect or protect people threatened with imminent harm. McCarthy said the exigency exception could apply to some real-time ALPR alerts, such as missing-child alerts, and that using ALPRs to find a vehicle reported stolen would not be impermissible.

Case codes and policies are not warrants

Agency rules for searching ALPR data are often internal policies, not court orders. Knoxville’s policy, for example, requires an associated report number and a specific reason for each query. Flock announced that officers will need to enter a case code for every search by the end of 2026. These controls create records of why a search was run, which can matter in a case. But they aren’t judicial warrants, and a judge doesn’t review whether there was probable cause.

State law may impose more. Some states regulate ALPR collection, retention and sharing by statute, and state constitutions can protect more than the Fourth Amendment. See license plate reader laws by state.

Questions a defense attorney may investigate

• Did police search stored ALPR data, and how far back and across how many cameras?
• Did they obtain a warrant? If not, what do they rely on?
• Did the search include other agencies’ data through a sharing network?
• What reason or case code was recorded for the search, and what do the audit logs show?
• Does a state statute or the state constitution require more than federal law?

Even if a warrantless search is found unlawful, suppression isn’t automatic. Under Davis v. United States, evidence from a search made in reasonable reliance on binding precedent isn’t excluded. See exceptions to the exclusionary rule and grounds to suppress ALPR evidence.