US federal judge rules warrantless Flock license plate searches violate Fourth Amendment

A U.S. District Court in Oklahoma has suppressed evidence obtained through a warrantless Flock ALPR search, calling the system "indiscriminate mass surveillance."

Today brings further evidence that legal sentiment is shifting against Flock, an automated license plate reader (ALPR) system widely used across the United States. A federal court in Oklahoma granted a woman's motion to suppress evidence found in her car after a search conducted based on information obtained through Flock, describing the practice as "indiscriminate mass surveillance."

The Court's Reasoning

Judge Sara E. Hill of the U.S. District Court for the District of Oklahoma delivered a sharp rebuke of warrantless Flock searches:

"Think about it this way. We might reasonably expect our friends and family to tell us something like: 'Hey, I saw you out driving by the store last week.' But we do not expect people we know—let alone strangers, and especially not law enforcement—to randomly approach us to say: 'Hey, I have been following your car for the last 30 days, and here is a detailed log of every place your vehicle has been with supporting photographic evidence. I am also going to share this information with law enforcement throughout the country so they know your whereabouts, too.'"

Hill added: "Freedom from persistent, dragnet-style surveillance while in public…is a reasonable expectation," rejecting previous court holdings that law enforcement use of ALPR systems does not constitute a "search" under the Fourth Amendment.

The Fourth Amendment Quandary

The Flock system—and similar programs—capture images of vehicles and store them along with:

• Time and location of capture
• Vehicle make, model, and features

Authorities can later query the system using license plate numbers or vehicle descriptions, or search for all vehicles that passed through a given area at a specific time.

High-profile instances of police using Flock for stalking have made news, but the broader concern involves warrantless use in legitimate cases—often without reasonable suspicion. The prevailing protocol around ALPR systems does not require officers to present probable cause or obtain a warrant before conducting a search.

This protocol appears to conflict with the Fourth Amendment, though at least one federal court has endorsed it. In September, the U.S. District Court for the Central District of California held that querying the Flock database is not a "search" for Fourth Amendment purposes. Judge Virginia A. Phillips ruled that no probable cause or particularized warrant is required.

The Oklahoma court's contrary conclusion marks a significant departure.

The Traffic Stop

The case involves the Tulsa County Sheriff's Office and Deputy Sheriff Freddie Alaniz. After an ALPR system on his patrol car detected a vehicle with a California license plate on the highway, Alaniz began following the car and searched Flock for its plate number.

In the query's reason field, he wrote "traffic violation" and "investigation"—even though, as Judge Hill noted, "at the time of those queries, all Alaniz knew was that the vehicle had a California license plate." The Flock query revealed the car had traveled from Oklahoma toward the California/Arizona border before heading back east.

Alaniz pulled over the driver, Melisa Kyle, alleging failure to signal lane changes sufficiently early and following too closely. He asked about her recent travel. Kyle explained she had been visiting a cousin and provided details about her trip and the half-dozen suitcases in her backseat.

After issuing a warning and returning her license, Alaniz told Kyle that "people who do very short trips to California and come right back are usually transporting large amounts of drugs" and requested to search her car. She refused.

Alaniz then detained Kyle, called for backup, and had his dog sniff her car. Officers allegedly found methamphetamine, and Kyle was charged with possession with intent to distribute.

Kyle moved to suppress the evidence, arguing:

• The stop was unjustified
• Extending the stop beyond the initial conversation was unjustified
• The license plate surveillance system search was unlawful

Suspicious Travel?

Judge Hill determined that Alaniz had an objectively reasonable basis to stop Kyle for following too closely. However, he lacked reasonable basis to extend the traffic stop after returning her license:

Kyle's short trip to California might not have been something Alaniz would have personally done. But Kyle had a reasonable explanation for the short turnaround—that the drive was as fulfilling as seeing the people she visited. As the Tenth Circuit has recognized, '[t]here is nothing criminal about traveling by car to view scenery.'

Hill also rejected the government's assertions that other elements of Kyle's road trip justified the search, writing:

"In the Court's view, Alaniz did not have enough information to reasonably believe that Kyle was engaged in unlawful conduct. The factors that the government relies upon are the same type of circumstances that everyday Americans encounter on long road trips for many legitimate reasons. Many of us drive longer than we want to get to a desired destination, or to no destination at all other than the road and sights ahead. Many of us lose track of what exact day of the week it is when we are traveling, even if it is for just a moment. Many of us travel for myriad reasons (some better than others), cross state lines, pack too much, buy too much, smoke cigarettes, and rent cars. And many of us even get a bit nervous when talking to law enforcement."

A 'Constitutionally Problematic' Search

The government argued the Flock search was constitutional because the data is collected from public roadways, where individuals lack a reasonable expectation of privacy. Hill acknowledged that "nearly every court examining the issue to date has determined that [ALPR] technology use is not a search under the Fourth Amendment because people have no reasonable expectation of privacy in their movements on public streets."

She disagreed:

"This is a type of indiscriminate mass surveillance. It is not targeted on a single individual….It is a tool that collects information about all vehicles that pass by any network-connected camera at all times, and it serves up the information to law enforcement on demand."

In Kyle's case, "Alaniz had access to an entire month's worth of location data tied to the license plate on Kyle's car, including more than fifty sightings across multiple states," Hill noted. "Alaniz and other officers using these systems have access to a continuously updated location history for all vehicles caught on ALPR cameras within the network."

Hill cited Chatrie v. United States (2026), in which the Supreme Court held that geofence warrants—demanding data on all phones within a given location at a given time—constitute searches triggering due process protections:

"Cell phones also move in public places where people can see you in plain view. But it becomes constitutionally problematic when law enforcement can indiscriminately and passively catalog your whereabouts over an extended period of time and then use that information for any purpose whenever convenient."

Evidence Suppressed

Hill ruled that "the only reason Alaniz conducted that search was because he saw her license plate was from California. That search was not supported by probable cause, and it was done without a warrant in violation of Kyle's Fourth Amendment rights."

Consequently, "all evidence obtained after Alaniz initiated the [license plate reader] search must be suppressed as fruit of the poisonous tree."

Tom Bowman of the Center for Democracy and Technology said in a statement: "This decision is an important next step from the Supreme Court's pivotal recognition this year in Chatrie v. U.S. that Americans have a reasonable expectation of privacy in the whole of their physical movements."

He added: "Previously, courts generally did not require a warrant for police to search the information collected by license readers, despite the fact that the technology allows easy reconstruction of Americans' movements over time. License plate readers enable indiscriminate mass surveillance, and as other courts should, Judge Hill's decision brings constitutional protections in line with that reality."

This is the second federal court decision in just over a week to suggest that driver surveillance systems can go too far. On September 28, a jury in a federal case from San Antonio held that a Bexar County, Texas, policing program involving surveillance cameras that recorded license plates and pretextual traffic stops was unconstitutional.

Institute for Justice Attorney Christie Hebert called the Bexar County verdict "a huge win," adding: "Motorists across this country are being watched by local, state and federal agents, flagged by AI-powered cameras, and then pulled over for a bogus offense that exists only as a cover story for an interrogation and a search. The Fourth Amendment does not permit this."

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