What are license-plate readers?

A license-plate reader photographs passing vehicles and uses software to read their plates. The Congressional Research Service describes the process this way: "ALPR systems work by automatically capturing images or videos of passing vehicles. A computer algorithm then detects, reads, and converts the license plate characters within the image into readable data." The systems can also record a vehicle's type and color, its location, and the date and time.¹ According to the National Conference of State Legislatures, readers can be mounted on police cars, road signs, traffic lights or trailers, some record distinctive features such as bumper stickers, and they differ from red-light and speed cameras.²

Each read can be checked at once against "hot lists" of plates, such as those of stolen vehicles or vehicles tied to a crime or a missing person, and the system alerts officers to a match. The reads are also stored, so officers can later look up a plate or follow a vehicle's movements across cameras.² A September 2026 statement by the International Association of Chiefs of Police and other law-enforcement groups says an alert is a lead, not proof: "A plate detection does not establish guilt and does not, by itself, provide probable cause for an arrest."³ Readers also misread plates. Business Insider reported that Flock Safety's system misread plates in 71 percent of the alerts it sent to police in Roseville, California, in 2023 and 2024.⁴

Large police agencies are the most likely to use readers. Citing the Bureau of Justice Statistics, the Congressional Research Service reports that nearly 90 percent of sheriffs' offices with 500 or more sworn deputies, and all police departments serving more than 1 million residents, used the technology, compared with 20 percent of all sheriffs' offices and nearly 22 percent of all police departments.¹ State transportation departments use readers to study traffic, and homeowners' associations and private businesses use them as well.²

The data may sit in public or private systems that law enforcement can search. Flock Safety runs one of the largest networks: crowdsourced data from an advocacy organization had mapped more than 137,000 readers in the United States by the end of August 2026, with more than 80 percent attributed to Flock, according to the Congressional Research Service.¹ Agencies on Flock's network can search one another's cameras. In August 2026, the company said cities could choose the types of offenses for which other agencies may access their cameras.⁵

No federal statute sets rules for police use of plate readers. "There is no specific federal legislative framework that governs law enforcement use of ALPRs," the Congressional Research Service wrote in September 2026.¹ Limits come instead from state law, local policy and the settings vendors offer.

As of September 2026, at least 28 states had statutes that expressly address readers or the data they collect, according to the National Conference of State Legislatures. At least 19 states limit how long the data may be kept, from three minutes in New Hampshire to 21 days in Maine, 18 months in Montana and five years in Alabama. Several states bar the use of readers for traffic or immigration enforcement, or to investigate people engaged in constitutionally protected activity or seeking certain health care.² Florida took a different route on its own roads. On August 31, 2026, its transportation department revoked local police permits for readers on state highway rights-of-way and gave agencies 30 days to remove them.⁶ Flock changed its own default that month, cutting its recommended retention period from 30 days to seven.⁵

The Fourth Amendment bars unreasonable searches, and a search usually requires a warrant. Courts disagree over whether looking up a vehicle's stored plate-reader history counts as a search.

The older precedent favors the police. In United States v. Knotts (1983), the Supreme Court said that "[a] person travelling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another."⁷ Later cases about phones point the other way. In Carpenter v. United States (2018), the Court held that "accessing seven days of CSLI," or historical cell-site location records, "constitutes a Fourth Amendment search." It called the decision narrow and said it did not call into question conventional tools such as security cameras.⁸ In Chatrie v. United States (2026), the Court held that police conducted a search when they acquired a person's location data from Google.⁹

Most courts that have ruled on plate readers have found no search. The Fifth Circuit held in March 2026 that police use of about ten fixed cameras in Gautier, Mississippi, was not a search.¹⁰ A federal judge in Virginia reached the same conclusion in January 2026 about Norfolk's 176 Flock cameras, and that ruling is on appeal.¹¹ On October 1, 2026, a federal judge in Oklahoma ruled the other way. Judge Sara E. Hill held that a deputy's warrantless query returning a month of a car's locations across several states was a search, and she called the systems "a type of indiscriminate mass surveillance." She acknowledged that nearly every court to examine the question had concluded otherwise.⁷

Police and prosecutors' groups defend the readers as investigative tools. In September 2026, the International Association of Chiefs of Police and 12 other organizations wrote that the systems can alert officers to a plate tied to "a stolen or wanted vehicle, a missing or endangered person, a person with an outstanding arrest warrant, or a vehicle connected to a crime." They proposed principles on written policies, audits, documented queries and data sharing.³ Some in law enforcement draw a line at newer systems. Sheriff Ross Teeple of Pinal County, Arizona, wrote in testimony for a Senate Judiciary subcommittee hearing that plate readers "are a great tool" but that "the latest camera networks, powered by artificial intelligence and machine learning, are capable of violating our citizens' Fourth Amendment rights."¹²

Civil liberties groups go further. The Electronic Frontier Foundation says that "ALPR mass surveillance," which it defines as the continuous collection and retention of location data on every driver regardless of suspicion, "should not exist," and that because it does, courts and state legislatures should impose warrant requirements and deletion deadlines.¹³ Chad Marlow of the American Civil Liberties Union wrote in testimony for the same hearing: "The time has come for Congress to ban any law enforcement entity who receives federal funding from using ALPRs."¹⁴ The Institute for Justice attached to its testimony a model bill that would require a warrant for location data more than two hours old and cap retention at seven days without one.¹⁵