Guide · Surveillance law
License Plate Readers and the Fourth Amendment
The Fourth Amendment protects against unreasonable searches and seizures. Whether, and when, license plate reader surveillance counts as a search is a question courts are still working through. This guide explains the decisions that frame it and the questions that remain open.
Short answer
The U.S. Supreme Court hasn’t decided whether license plate readers are a Fourth Amendment search. Courts generally treat a single observation of a car on a public road as not a search, while long-term location records can be. Where a given ALPR use falls depends on how many cameras, how much history, and how the data was used, and the answer can differ by jurisdiction.
The four questions in an ALPR challenge
A Fourth Amendment challenge involving license plate reader evidence usually turns on four separate questions. Each has its own guide.
| Question | What it turns on | Guide |
|---|---|---|
| 1. Is it your privacy interest? | Ownership, lawful possession, rentals and borrowed cars | Standing |
| 2. Was it a search? | Single read vs. stored history vs. real-time tracking; network size | Is ALPR a search? · Mosaic theory |
| 3. Was a warrant required, and was there one? | Carpenter, exceptions, agency policy | Warrants and ALPR data |
| 4. Was the stop lawful? | Reasonable suspicion, verification of the alert | Alert-based traffic stops |
A related question is who collected the data. Cameras owned by HOAs, businesses or data companies raise their own issues. See private, business and HOA cameras. If a violation is found, the remedy question is covered in motions to suppress.
The decisions that frame the law
- United States v. Knotts (1983): following a car on public roads isn’t a search, but “dragnet-type” practices were reserved.
- United States v. Jones (2012): attaching a GPS tracker is a search; five Justices said longer-term monitoring impinges on privacy.
- Carpenter v. United States (2018): historical cell-site records require a warrant; the Court called the decision narrow.
- Commonwealth v. McCarthy (Mass. 2020): four bridge ALPR cameras weren’t a search; widespread use could be.
- United States v. Yang (9th Cir. 2020): a driver of an overdue rental lacked standing to challenge an ALPR database search.
- Leaders of a Beautiful Struggle (4th Cir. 2021): accessing citywide aerial surveillance data was a search.
What’s unsettled
- Whether, and at what scale, a network like Flock’s turns ALPR use into a search. No court has drawn a precise line.
- Whether searching stored ALPR history requires a warrant, and for how much data.
- How courts will treat police access to privately owned ALPR networks.
We add decisions to the case law library only after reading the opinion and verifying its citation.
State law can protect more
The Fourth Amendment is a minimum. State courts can read their own constitutions more broadly; McCarthy, for example, analyzed Article 14 of the Massachusetts Declaration of Rights alongside the Fourth Amendment. Some states also regulate ALPRs by statute. We’re researching state law jurisdiction by jurisdiction and publish a state only when we can verify what’s different there. See license plate reader laws by state.
Law varies by state. State constitutions, statutes and courts may treat this issue differently from federal law. A licensed attorney in your jurisdiction can tell you how it applies.
In this guide
- GuideIs a License Plate Reader a Fourth Amendment Search?Whether ALPR and Flock camera use is a Fourth Amendment search: what Knotts, Jones, Carpenter and Commonwealth v. McCarthy say, the factors courts weigh, and what's unsettled.
- GuideWarrants and License Plate Reader DataWhether police need a warrant to search stored license plate reader data: Carpenter, McCarthy, Leaders of a Beautiful Struggle, exceptions, and why case codes aren't warrants.
- GuideThe Mosaic Theory and Location SurveillanceThe mosaic theory of the Fourth Amendment explained: where it comes from (Knotts, Jones, Carpenter), how courts have applied it, and why it matters for ALPR networks.
- GuideTraffic Stops Based on a License Plate Reader AlertWhen a Flock or ALPR alert can justify a traffic stop: reasonable suspicion, unconfirmed hits, Kansas v. Glover, verification, and what a defense may investigate.
- GuidePrivate, Business and HOA Flock Cameras and Police AccessWhen HOAs, businesses and private companies run license plate readers: the private-search rule, how police get access, and when it becomes a Fourth Amendment question.
- GuideState Constitutional Privacy Protections and ALPRWhere state constitutions protect privacy more than the Fourth Amendment (Hawaii, New Mexico, New Jersey, Oregon and others) and how that can matter in ALPR cases.
- GuideStanding to Challenge ALPR Evidence in Borrowed and Rental CarsWho can challenge an ALPR search: standing under Rakas and Byrd, the Ninth Circuit's Yang decision on an overdue rental, and what it means for borrowed and shared cars.
- Quick answerAre Flock Cameras Legal?Are Flock cameras legal? Generally yes, but how the data is used matters: the Fourth Amendment, McCarthy's warning about camera networks, and state ALPR laws.
- Quick answerDoes the Fourth Amendment Protect Your License Plate?Your license plate itself isn't private under the Fourth Amendment. Why, and where protection may begin when many plate reads are stored and searched.
Coming to this guide
- Do Police Need a Warrant for Flock Camera Data?
- Can Police Use HOA Flock Cameras?
Sources
- OpinionKatz v. United States389 U.S. 347 (1967)
- OpinionUnited States v. Knotts460 U.S. 276 (1983)
- OpinionUnited States v. Jones565 U.S. 400 (2012)
- OpinionCarpenter v. United States585 U.S. 296 (2018)
- OpinionCommonwealth v. McCarthy (slip opinion, SJC-12750)484 Mass. 493 (2020)
- OpinionUnited States v. YangNo. 18-10341 (9th Cir. May 4, 2020)
- OpinionLeaders of a Beautiful Struggle v. Baltimore Police Department (en banc)No. 20-1495 (4th Cir. June 24, 2021)