License Plate Readers: How Police Track Vehicle Movements

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Where the law allows, police can save a car’s location even when its plate matches none of the vehicles they are looking for. License plate readers photograph plates and store the images with the date, time and location of each capture.

Police track vehicle movements by searching these stored sightings, turning passing observations into leads they can revisit after a car has gone. That can make a car’s past movements easier to investigate without proving who was driving or whether that person did anything wrong. The rules on how long a sighting may be kept and who may search it differ by state and city.

A sighting records a vehicle, not a driver

Fixed readers can sit on stationary objects such as light poles, while mobile readers can ride on patrol cars. In an automated license plate reader (ALPR), software extracts the plate number from the image and stores the image and its capture information in a searchable database.

A record of a vehicle at one location does not show who was driving or why the vehicle was there. The International Association of Chiefs of Police advises officers to confirm the plate, the issuing jurisdiction, the record status and the connection to the driver before taking enforcement action.

A collection of sightings can be more revealing than any one of them, but its gaps matter too. An archive of camera captures cannot establish every movement between the recorded points.

An alert and a history search do different jobs

Police agencies create lists of plates of interest and also obtain such lists from state and federal agencies, as the California State Auditor explains. These lists, often called hot lists, give the camera system something to compare a sighting against; a match produces an alert. The police chiefs association says an ALPR alert should be treated as an investigative lead, not conclusive evidence.

Archived records can be searched by a full plate number, a partial plate number, or vehicles captured at a particular place and time. An alert flags a listed plate as it passes a camera. A history search comes later and can reach vehicles whose plates never triggered an alert.

A camera’s owner is not necessarily the only organization that can use its records. Palo Alto, California, describes sharing among local law enforcement partners and says private entities such as homeowners associations, shopping centers and retailers can elect to share their camera data with local police. For a resident checking the reach of a system, the sharing arrangement can matter as much as the number of cameras on the street.

The Drug Enforcement Administration operates its own readers and also gets access to equipment run by other law enforcement agencies, which send their plate data to regional hubs under sharing agreements, 404 Media reports, citing a December 2024 DEA privacy assessment. Where such agreements exist, a local camera’s sightings can feed a broader federal network. Read a local retention rule alongside the rules of any agency that receives the data, because a shared copy may be kept under different terms.

Useful leads still need verification

The case for the tool is that a plate can supply a concrete lead when finding a vehicle may help find a person or resolve a crime. New Hampshire law limits reader operation and access to official law enforcement purposes, including identifying stolen vehicles, vehicles associated with wanted, missing or endangered people, and vehicles registered to someone with an outstanding warrant.

A 2012 report on license plate reader systems, summarized by the National Institute of Justice, describes a follow-up survey answered by 40 of the 70 agencies identified as ALPR users. Among those respondents, 69 percent used the systems for auto theft, and 68 percent reported increases in stolen vehicle recoveries. These are agency-reported outcomes from an older survey, not proof that a particular camera installation caused a particular reduction in crime.

The police chiefs association calls for trained, authorized users, a documented lawful public safety purpose, and prohibitions on personal, political, retaliatory, discriminatory or commercial misuse. New Hampshire requires daily additions and deletions to keep plate lists current, as well as manual updates during shifts when officers learn that bulletins have been issued or canceled. The point is that a hot list can go out of date: an officer should confirm that the record behind an alert is still active as well as that the plate was read correctly.

The legal limits depend on place and use

Under the Fourth Amendment, the Constitution’s protection against unreasonable searches, police searches for evidence of a crime generally require a warrant unless a specific exception applies, as the Supreme Court explains. A search warrant is a written order signed by a judge that permits an officer to search a particular person, place or thing. Whether a particular ALPR investigation requires a warrant depends on the jurisdiction and the surveillance involved.

A plate is visible on a public road, but courts have not settled whether that makes every search of accumulated plate records constitutional. In Carpenter v. United States, decided June 22, 2018, the Supreme Court held that the government’s acquisition of historical cell-site records, which record the towers a cellphone connected to, was a Fourth Amendment search. The Court described that decision as narrow and said it did not call conventional surveillance tools such as security cameras into question. Carpenter separates historical cell-site records from conventional cameras, but it does not itself decide every use of license plate readers.

In Commonwealth v. McCarthy, decided April 16, 2020, Massachusetts’ highest court said the defendant had a protected privacy interest in the whole of his public movements but concluded that the limited extent and use of ALPR data in that case did not invade it. The court also said that enough cameras in enough locations could make historical ALPR data in Massachusetts a search for constitutional purposes. What mattered to the court was how far the collection reached and how it was used, not the name of the technology.

Schmidt v. City of Norfolk, a federal case from Virginia, asks whether Norfolk’s 176-camera network is a Fourth Amendment search. Lee Schmidt and Crystal Arrington brought the suit, arguing that Norfolk’s camera program violates their Fourth Amendment privacy rights. The district court ruled for the city on January 27, 2026, granting it summary judgment.

The plaintiffs appealed to the U.S. Court of Appeals for the Fourth Circuit; its docket records a tentative December 8 through 11, 2026 argument session and notices that lawyers have scheduling conflicts. Until the appeals court rules, the district court’s decision stands.

According to his petition, a camera alert led police in Gautier, Mississippi, to stop Elijah Porter while he was driving his brother’s car. The petition says the Fifth Circuit upheld the officers’ warrantless access to ALPR data, ruling that it was not a Fourth Amendment search.

A petition in Porter v. United States asks the Supreme Court whether an officer’s access to an ALPR database that stores location history for later lookup is a Fourth Amendment search. The petition was sent to the justices on August 20, 2026, for their September 28, 2026 conference. That is the last action the docket lists; it shows no decision on whether the Court will hear the case. Asking the Supreme Court to take the case does not establish that it will hear it or change the governing law.

California requires ALPR operators to maintain reasonable security safeguards and a written usage and privacy policy available to the public, including conspicuous posting on an operator’s website if it has one. Constitutional protections, state statutes and an agency’s own policy can each limit a search, which is why another city’s policy cannot settle the rules where someone lives.

Retention and access decide the privacy stakes

New Hampshire’s statute uses “alarm” for the listed-plate match: before a stop based on an alarm, an officer must try to verify that the plate on the list matches the displayed image.

Under New Hampshire’s statute, plate records ordinarily may not be recorded or transmitted anywhere and must be destroyed irrecoverably within three minutes. The exceptions apply when an alarm leads to an arrest, citation or protective custody or identifies a vehicle subject to a missing-person or wanted broadcast. In those specified cases, the particular plate’s data may be kept until final court disposition of the case. Palo Alto’s published policy instead describes automatic deletion after 30 days unless a specific record has been identified as relevant to a specific criminal investigation. One is a state statute and the other a city policy; neither is a national standard.

California requires an operator’s policy to state how long information is retained and how destruction decisions are made, and requires access records identifying the time, search terms, user and purpose. A log makes a search traceable only if the stated purpose is specific enough to tell an authorized search from misuse. A San Mateo County Civil Grand Jury report released August 3, 2026, found that some users entered generic query descriptions such as “Investigation.” Such entries were not necessarily improper but limited the ability to check whether searches had authorized purposes.

The American Civil Liberties Union (ACLU) argues that location tracking can reveal visits to friends, doctors, protests, political events and churches. It also objects to collecting information about innocent activities just in case someone later does something wrong. The objection is to the accumulated record of ordinary life, even though each sighting occurs in public.

In February 2026, the ACLU released model legislation with what it called strict ALPR rules. In September 2026, its senior policy counsel Chad Marlow argued at a Senate Judiciary subcommittee hearing that the best privacy protection would be to ban ALPRs altogether. The police chiefs association’s safeguards address controlled investigative use, while the ACLU’s objection reaches the creation of searchable records of innocent movements itself.

How to check the rules in your community

Start with the agency’s written policy and the state law that governs its collection and use of plate data. In California, the state attorney general says that policy must be posted conspicuously on the agency’s website. Look for the default deletion period, exceptions, authorized search purposes, which other agencies receive the data, and who reviews access logs.

The ACLU recommends using a city or county’s public-records request process to ask whether it has a camera contract, then requesting details such as the number and location of cameras. To check oversight, ask for the policy and how compliance is tested; the existence of an audit log does not prove anyone regularly examines it. New Hampshire authorizes the attorney general or designated officials to examine readers, servers and records to check compliance or investigate misuse complaints. California also permits a person harmed by a violation to bring a civil action against someone who knowingly caused the harm.