Short answer
Generally, yes. We’re not aware of any federal law that bans license plate readers, and observing a plate on a public road generally isn’t a Fourth Amendment search. But legality depends on how they’re used. Some states regulate collection, retention and sharing, and courts have said enough cameras and stored history could amount to a search.
The cameras themselves
A license plate is displayed on the outside of a car on public roads. Under Katz v. United States, “what a person knowingly exposes to the public” generally isn’t protected by the Fourth Amendment. Courts have applied that reasoning to plates. See does the Fourth Amendment protect your license plate?
How the data is used
The harder question is what happens when a network of cameras stores reads over time and police search that history. In Commonwealth v. McCarthy (2020), Massachusetts’s highest court held that four cameras on two bridges weren’t a search, but said: “With enough cameras in enough locations, the historic location data from an ALPR system in Massachusetts would invade a reasonable expectation of privacy and would constitute a search for constitutional purposes.” The U.S. Supreme Court hasn’t decided the question for ALPRs. See is ALPR a Fourth Amendment search?
State rules
Some states regulate ALPRs by statute. California requires usage and privacy policies, a public comment opportunity before a public agency starts an ALPR program, and limits on sharing. Tennessee caps retention at 30 days outside ongoing investigations and makes the data confidential. Other states have different rules or none. See license plate reader laws by state.
Local decisions
Whether a city or county uses Flock is often a local decision. Police1 reported in 2026 that several cities had ended or reconsidered their contracts over privacy, security and oversight concerns. A camera being legal doesn’t mean every use of its data is lawful. If camera data was used against you, the questions are about the specific stop or search. See license plate readers and the Fourth Amendment.
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.
Sources
- OpinionKatz v. United States389 U.S. 347 (1967)
- OpinionCommonwealth v. McCarthy (slip opinion, SJC-12750)484 Mass. 493 (2020)
- Government reportCalifornia Department of Justice, Information Bulletin 2023-DLE-06, “California Automated License Plate Reader Data Guidance”October 27, 2023
- PolicyKnoxville Police Department, General Order No. 4.26, “Automatic License Plate Readers”Revised July 28, 2026 · ¶¶ 15, 27, 29–30
- SecondaryPolice1, “Flock Safety sets 7-day default for ALPR data, adds mandatory misuse safeguards”August 14, 2026
Related
- Is 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.
- Does 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.
- Commonwealth v. McCarthy (Mass. 2020)Massachusetts's high court held four bridge ALPR cameras weren't a search, but warned widespread ALPR use could implicate constitutional protections.