Flock Safety and Automated License Plate Readers: Balancing Public Safety, Privacy, and the Fourth Amendment: Part Two – The Constitutional Framework Related to ALPRs
As discussed in Part One of this series, automatic license plate readers (ALPRs) such at Flock have played a significant role in helping law enforcement officers solve crimes. This article will first examine precedent from the Supreme Court of the United States that support the legality of the use of such systems and, second, will examine circumstances that could jeopardize the legal of these systems.
Defining a Reasonable Expectation of Privacy
Although the Supreme Court has never directly addressed the constitutionality of automated license plate reader (ALPR) systems such as Flock, its Fourth Amendment decisions establish the constitutional framework for analyzing them. Those decisions answer several important questions that ultimately determine whether the use of ALPR technology constitutes a Fourth Amendment search. Specifically, the Court has considered whether individuals have a reasonable expectation of privacy in information exposed to public view, in information disclosed to third parties, and in their movements on public roadways. More recently, it has examined when advancing technology transforms ordinary observations into constitutionally protected surveillance.
Katz v. United States: The Reasonable Expectation of Privacy
The starting point for every Fourth Amendment analysis is Katz v. United States.[1] There, FBI agents placed an electronic listening device on the outside of a public telephone booth and recorded the defendant’s telephone conversations without first obtaining a warrant. Although the agents never entered the booth, the Supreme Court concluded that the government’s conduct violated the Fourth Amendment.
Justice Harlan’s concurring opinion, which later became the governing Fourth Amendment test, explained that constitutional protection depends upon whether a person has (1) exhibited a subjective expectation of privacy and (2) whether that expectation is one society is prepared to recognize as reasonable.
Constitutional Principle: The Fourth Amendment protects people—not simply places. Whether government conduct constitutes a search generally depends upon whether it intrudes upon a reasonable expectation of privacy.
That principle remains an issue for every constitutional challenge involving emerging surveillance technology, including ALPR systems. The remaining Supreme Court cases simply apply that framework to different types of information and different investigative technologies.
New York v. Class: There Is No Reasonable Expectation of Privacy in a Vehicle’s VIN
Nearly twenty years later, the Supreme Court considered whether officers violated the Fourth Amendment by moving papers on a vehicle’s dashboard in order to view the vehicle identification number (VIN).[2]
The Court concluded that no reasonable expectation of privacy existed because federal law requires the VIN to be displayed in plain view. As the Court explained:
It is unreasonable to have an expectation of privacy in an object required by law to be located in a place ordinarily in plain view from the exterior of the automobile.[3]
The Court further noted that the exterior of a vehicle is “thrust into the public eye,” and examining it therefore does not constitute a Fourth Amendment search.[4]
Constitutional Principle: A motorist has no reasonable expectation of privacy in a VIN or other identifying information that federal law requires to be displayed in plain view on the exterior of a vehicle.
Although Class involved a VIN rather than a license plate, the reasoning extends naturally to license plates because both are government-issued identifiers displayed on the exterior of a vehicle for public observation.
Extending Class to License Plates
Federal appellate courts have consistently applied the reasoning of Class to license plates. One of the leading decisions is United States v. Miranda-Sotolongo, where the Seventh Circuit held that an officer’s use of a license plate number to access a state’s vehicle registration database is not a Fourth Amendment search.[5]
Rather than viewing the database inquiry as an intrusion upon privacy, the court emphasized that officers simply used information already exposed to public view to retrieve official government records. Summarizing the unanimous view of the federal circuits that had addressed the issue, the court explained:
A police officer’s check of a vehicle registration in a database is not a Fourth Amendment search…. The registration check involves only checking publicly displayed registration information against official public records.[6]
The court therefore concluded that neither observing a license plate nor using that publicly visible information to retrieve registration records implicates the Fourth Amendment.[7]
Constitutional Principle: Running a license plate through a government vehicle registration database does not constitute a Fourth Amendment search because officers are using publicly visible information to obtain official public records.
These decisions provide significant constitutional support for ALPR technology. Every automated license plate reader begins with information already exposed to public view—a vehicle’s license plate. If an officer may lawfully observe a license plate and manually check it through a government database without implicating the Fourth Amendment, the mere automation of that process through ALPR technology does not, standing alone, create a constitutional violation. That principle forms one of the primary foundations upon which courts have evaluated modern ALPR systems.
Public Travel and Technological Surveillance
The next series of Supreme Court decisions examines a different aspect of the Fourth Amendment. Rather than asking whether individuals have a reasonable expectation of privacy in information exposed to public view, these cases address whether people have a reasonable expectation of privacy in their movements while traveling on public roadways. As technology evolved from visual surveillance to GPS tracking and historical location data, the Court was required to determine whether traditional constitutional principles remained adequate in the face of increasingly sophisticated surveillance capabilities.
United States v. Knotts: There Is No Reasonable Expectation of Privacy in Public Travel
The Supreme Court first addressed this issue in United States v. Knotts.[8] Investigators suspected several individuals of manufacturing illegal drugs and secretly placed a radio “beeper” inside a container of chemicals purchased by one of the suspects. Officers then used the beeper, together with visual surveillance, to follow the vehicle carrying the container over public roadways to a secluded cabin where the drugs were ultimately discovered.
The defendants argued that the electronic monitoring constituted a Fourth Amendment search because it allowed officers to track the vehicle’s movements without a warrant. The Supreme Court disagreed.
The Court reasoned that a person traveling on public streets has no reasonable expectation of privacy in movements that are “voluntarily conveyed to anyone who wanted to look.”[9] Because the vehicle traveled only on public roadways where it could have been observed by any member of the public, the use of the beeper merely enhanced the officers’ ability to observe what was already exposed to public view.
The Court summarized the principle succinctly:
A person traveling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another.[10]
The Court emphasized that the beeper did not reveal information that was otherwise hidden from public observation. Instead, it merely assisted officers in following a vehicle that could have been observed visually throughout its journey.
Constitutional Principle: Motorists generally have no reasonable expectation of privacy in their movements on public roadways because those movements are knowingly exposed to public observation.
For decades, Knotts served as the primary constitutional authority supporting the use of electronic technologies to assist officers in monitoring vehicles traveling on public roads. It remains one of the strongest Supreme Court decisions supporting the constitutionality of ALPR technology because every ALPR camera records vehicles traveling in places already open to public observation.
United States v. Jones: When Technology Becomes Something More
Nearly thirty years after Knotts, the Supreme Court revisited electronic vehicle surveillance in United States v. Jones.[11] In this case, officers installed a GPS tracking device on the defendant’s vehicle based on an expired warrant and monitored its movements continuously for twenty-eight days after the warrant authorizing the installation had expired.
Unlike Knotts, the Court did not resolve the case by focusing solely on reasonable expectations of privacy. Instead, Justice Scalia’s majority opinion concluded that officers had physically trespassed upon the defendant’s private property by attaching the GPS device to the vehicle for the purpose of gathering information. That physical intrusion constituted a Fourth Amendment search.[12]
Although the Court unanimously agreed that the search violated the Fourth Amendment, several concurring Justices focused on a different concern that would later prove significant in Carpenter. Justice Alito observed that prolonged GPS monitoring allows the government to assemble an extensive record of a person’s movements, revealing far more than isolated observations ever could. Justice Sotomayor similarly questioned whether individuals reasonably expect the government to secretly monitor every movement they make over an extended period of time.[13]
Those concurring opinions introduced what has become known as the mosaic theory, which is the idea that while a single observation on a public street may not implicate the Fourth Amendment, the aggregation of thousands of observations over time may reveal a detailed picture of a person’s private life deserving constitutional protection.
Constitutional Principle: While officers may observe vehicles traveling on public roads, prolonged and comprehensive electronic surveillance may present constitutional concerns beyond those addressed in Knotts.
For agencies using ALPR technology, Jones marked an important turning point. The Court did not retreat from the principle that motorists generally lack a reasonable expectation of privacy in their public movements. Instead, it recognized that advancing technology may eventually permit surveillance so pervasive that traditional Fourth Amendment analysis becomes more complicated.
That concern reached its fullest expression six years later in Carpenter v. United States, where the Supreme Court directly confronted the constitutional implications of long-term historical location tracking.
When Technology Crosses the Constitutional Line
The decisions discussed thus far establish two important constitutional principles. First, individuals generally have no reasonable expectation of privacy in information knowingly exposed to public view, such as a vehicle’s license plate or VIN. Second, motorists ordinarily have no reasonable expectation of privacy in their movements on public roadways because those movements are visible to anyone who chooses to observe them.
Carpenter v. United States required the Supreme Court to determine whether those traditional principles remained adequate in an era when advancing technology could record, store, and reconstruct virtually every movement a person makes over an extended period of time.
Carpenter v. United States: Comprehensive Historical Location Tracking
Carpenter arose from a series of armed robberies in which federal investigators obtained 127 days of historical cell-site location information (CSLI) from the defendant’s cellular service providers without first obtaining a search warrant. By examining which cell towers the defendant’s phone connected to over time, investigators reconstructed his movements and placed him near the locations of several robberies.[14]
The government argued that no warrant was required because the records belonged to the cellular providers rather than to the defendant. Relying upon the Court’s earlier decisions in United States v. Miller and Smith v. Maryland, the government maintained that individuals generally have no reasonable expectation of privacy in information voluntarily conveyed to third parties.[15]
The Supreme Court disagreed.
Writing for the majority, Chief Justice Roberts emphasized that historical CSLI is fundamentally different from the business records at issue in Miller and Smith. Unlike bank records or telephone numbers dialed, historical cell-site data creates an extraordinarily detailed record of a person’s movements over time. The Court explained that this information provides law enforcement with “an all-encompassing record of the holder’s whereabouts” and enables the government to travel back in time to reconstruct a person’s movements with remarkable precision.[16]
Unlike the limited surveillance at issue in Knotts, historical CSLI does not merely reveal where a person happened to be at one moment in time. Instead, it allows investigators to reconstruct where a person has been over days, weeks, or even months. As the Court observed:
Cell phones “faithfully follow[] their owners beyond public thoroughfares and into private residences, doctor’s offices, political headquarters, and other potentially revealing locales.[17]
The Court suggested that the government’s acquisition of seven days or more of historical CSLI constitutes a Fourth Amendment search and generally requires a warrant supported by probable cause.[18]
Constitutional Principle: Long-term historical location information that enables the government to reconstruct the whole of a person’s physical movements implicates a reasonable expectation of privacy and ordinarily requires a search warrant.
An Exception—Not a Rejection—of Earlier Cases
One of the most misunderstood aspects of Carpenter is the scope of its holding. The decision did not overrule Knotts, Class, Miller, or Smith. Nor did it establish that every use of technology to assist law enforcement constitutes a Fourth Amendment search.
To the contrary, the Court repeatedly emphasized that its decision was intentionally narrow. Chief Justice Roberts explained:
Our decision today is a narrow one. We do not express a view on matters not before us.[19]
The Court specifically stated that it was not questioning conventional surveillance techniques, security cameras, or other traditional investigative methods. It likewise declined to disturb the third-party doctrine announced in Miller and Smith except as applied to the unique characteristics of historical cell-site location information.[20]
Those limitations are particularly significant when analyzing ALPR technology. Carpenter does not hold that every historical database search constitutes a Fourth Amendment search. Rather, it recognizes that certain technologies become constitutionally significant when they permit the government to compile a comprehensive, retrospective chronicle of a person’s movements that could not realistically have been assembled through ordinary police observation.
That distinction explains why Carpenter has become the central case in modern ALPR litigation. Defendants routinely argue that historical ALPR databases create the same type of comprehensive location history condemned in Carpenter. As the next article demonstrates, however, most courts have rejected that comparison, concluding that current ALPR systems differ in several constitutionally significant respects.
The issue, therefore, is not whether Carpenter governs ALPR technology—it undoubtably provides the constitutional framework for analyzing advanced surveillance systems. The more difficult question is whether today’s ALPR networks have reached the level of pervasive, comprehensive, and retrospective surveillance that concerned the Supreme Court in Carpenter. That question has largely been answered by the lower courts, which is the focus of the next article in this series.
Applying the Constitutional Framework to Emerging Technology
The Supreme Court’s decision in Carpenter did not end the constitutional analysis of emerging surveillance technologies. Instead, it established the framework that lower courts continue to apply as new technologies develop. Since Carpenter, courts have been asked to evaluate a variety of investigative techniques capable of collecting historical location information, including geofence warrants, cell phone location records, and automated license plate reader databases. Each presents the same fundamental question: Has technology advanced to the point that it invades a reasonable expectation of privacy by enabling the government to reconstruct the whole of a person’s physical movements?
One of the most recent examples is United States v. Chatrie.[21] In this case, investigators obtained a geofence warrant directing Google to identify cellular devices that were present within a defined geographic area surrounding the scene of a bank robbery during a limited period of time. The case required the courts to determine whether obtaining historical location information from Google constituted a Fourth Amendment search under the principles announced in Carpenter.
Although Chatrie involved a different technology than ALPR systems, it illustrates the continuing effort by courts to identify where the constitutional line should be drawn as digital surveillance capabilities expand. The central question was not whether technology assisted the investigation, but whether the government’s acquisition of historical location information intruded upon a reasonable expectation of privacy protected by the Fourth Amendment.[22] The Court held that officers did conduct a search under the Fourth Amendment by acquiring Chatrie’s cell-phone location information because a person has a reasonable expectation of privacy in that information. The Court reasoned that
[E]ven short-term monitoring” of a person’s physical movements can provide “a wealth of detail about [his] familial, political, professional, religious, and sexual associations.[23]
The same question now confronts courts evaluating ALPR technology.
Defendants challenging ALPR and Flock systems routinely argue that modern license plate reader networks permit law enforcement to reconstruct historical travel patterns in much the same manner that historical cell-site location information allowed investigators to reconstruct the defendant’s movements in Carpenter. According to this argument, sufficiently large ALPR networks create a digital record of a person’s movements that should receive the same constitutional protection as historical CSLI.
To date, however, most courts have rejected that comparison.
Rather than viewing ALPR systems as the functional equivalent of the continuous location tracking at issue in Carpenter, courts have generally concluded that current ALPR technology remains fundamentally different in several important respects. Unlike historical CSLI, which continuously tracks a person’s cell phone wherever it travels, ALPR systems capture only isolated photographs of vehicles as they pass fixed camera locations. They do not continuously monitor a vehicle’s movements, reveal where it travels between camera locations, or provide the type of comprehensive location history that concerned the Supreme Court in Carpenter.[24]
Likewise, unlike the GPS surveillance discussed in Jones, ALPR systems do not follow a vehicle continuously for days or weeks. Instead, they record only those occasions when a vehicle happens to pass a particular camera. The resulting information consists of a series of discrete observations made from locations where both the vehicle and its license plate are already exposed to public view.
These distinctions have proven significant. Courts evaluating ALPR systems have consistently emphasized that Carpenter was intentionally limited to the unique characteristics of historical cell-site location information and expressly declined to question conventional surveillance techniques or other forms of technology-assisted observation.[25] As a result, most courts have concluded that existing ALPR systems remain closer to the public-roadway surveillance approved in Knotts than to the pervasive, retrospective location tracking condemned in Carpenter.
That does not mean the constitutional analysis is complete. To the contrary, several courts have recognized that future advances in technology could eventually present a different question. As camera networks become denser, retention periods grow longer, and artificial intelligence enhances the government’s ability to analyze historical travel patterns, the constitutional balance described in Carpenter may continue to evolve. The Fourth Amendment has always adapted to technological change, and there is little reason to believe that ALPR technology will prove to be an exception.
For now, however, the constitutional framework established by the Supreme Court provides a clear starting point. Individuals generally have no reasonable expectation of privacy in information knowingly exposed to public view, including vehicle identifiers and travel on public roadways. At the same time, technology that enables the government to compile an exhaustive history of a person’s movements may implicate the Fourth Amendment. Where current ALPR systems fall between those two principles has become one of the most significant issues confronting the lower courts.
The next article examines how federal and state appellate courts have answered that question. As will be seen, although the opinions differ in their reasoning, a clear judicial consensus has emerged regarding the constitutionality of accessing historical ALPR and Flock Safety databases.
Note: Court holdings can vary significantly between jurisdictions. As such, it is advisable to seek the advice of a local prosecutor or legal adviser regarding questions on specific cases. This article is not intended to constitute legal advice on a specific case.
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[1] Katz v. United States, 389 U.S. 347 (1967)
[2] New York v. Class, 475 U.S. 106 (1986)
[3] Id. at 114 (emphasis added)
[4] Id.
[5] United States v. Miranda-Sotolongo, 837 F.3d 663 (7th Cir. 2016)
[6] United States v. Miranda-Sotolongo, 837 F.3d at 667-668 (emphasis added)
[7] Id. at 667–68
[8] United States v. Knotts, 460 U.S. 276 (1983)
[9] United States v. Knotts, 460 U.S. at 281-282
[10] Id. at 281 (emphasis added)
[11] United States v. Jones, 565 U.S. 400 (2012)
[12] United States v. Jones, 565 U.S. at 404-405
[13] Id. at 413–426 (concurring opinions)
[14] Carpenter v. United States, 585 U.S. 296 (2018)
[15] United States v. Miller, 425 U.S. 435 (1976); Smith v. Maryland, 442 U.S. 735 (1979)
[16] Carpenter, 585 U.S. at 311–13
[17] Id. at 311
[18] Carpenter, U.S. at fn. 3 (“It is sufficient for our purposes today to hold that accessing seven days of CSLI constitutes a Fourth Amendment search.”)
[19] Carpenter, 585 U.S. at 316
[20] Id. at 315–18
[21] United States v. Chatrie, 136 F.4th 100 (4th Cir. 2025), vacated and remanded, Chatrie v. United States, 225 L. Ed. 2d 793 (2026)
[22] Carpenter v. United States, 585 U.S. 296, 138 S. Ct. 2206 (2018)
[23] Chatrie, 225 L. Ed. 2d at 813 (quoting Jones, .” 565 U. S., at 415, 429-431, 132 S. Ct. 945, 181 L. Ed. 2d 911).
[24] See, e.g., United States v. Martin, 753 F. Supp. 3d 454 (E.D. Va. 2024); Robinson v. Commonwealth, 87 Va. App. 262, 927 S.E.2d 822 (2026); United States v. Porter, 170 F.4th 381 (5th Cir. 2026). These decisions are discussed in detail in Article III of this series.
[25] Carpenter, 585 U.S. at 316–18, 138 S. Ct. at 2220–23
