Introduction

Recently, concerns have been raised about the legality, even the constitutionality, of surveillance systems used by law enforcement to collect data that aids in the prevention and the prosecution of criminal activity. 

Automatic License Plate Reader (ALPR) technology allows cameras fixed to light poles, other stationary structures or patrol vehicles to capture images of passing vehicles that can be logged into a searchable database.

Commercial ALPR vendors such as Flock provide their services to a range of customers, including municipalities and their police departments. These customers may share access to the data collected by their ALPR cameras, creating a wide-ranging, nationwide information system that law enforcement uses to prevent or address criminal activity. 

But ALPR data collection systems prompt concerns about surveillance and individual privacy, as expressed in recent criticism of governmental use of Flock cameras. The questions posed by ALPRs are whether they violate the Fourth Amendment’s prohibitions on unreasonable governmental searches and how to gain the benefits of ALPRs without the liability that comes with unconstitutional action.

What Is an ALPR?

ALPR is a camera system that captures images of passing vehicles and logs their license plates as searchable data. High-speed, high-resolution cameras take photographs of vehicles’ plates, and optical character recognition software converts those images into text that is uploaded into a searchable database, along with the time, date and location of the capture.

Many ALPRs operate twenty-four hours a day and, when working properly, photograph every vehicle that passes them. In addition to the plates themselves, ALPRs may record a vehicle’s make, model and color, and catalog other visible features, such as roof racks, bumper stickers and body damage, allowing for searches of vehicles even without a plate number.

ALPR customers, including municipalities, police departments, homeowners’ associations, schools and businesses, may share access to their cameras with one another, so that a query in one jurisdiction may identify data captured by cameras in another.

How long the data is retained depends on a customer’s contract and applicable law. Some jurisdictions limit retention to one week, while others allow it for up to five years. For a municipality, then, many of the features that create potential Fourth Amendment risk, such as how long data is kept, who can access it and with whom it is shared, are matters of policy and contract rather than fixed features of the technology.

ALPRs and the Fourth Amendment

A. The Current State of the Law

The Fourth Amendment protects “[t]he right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures[.]” U.S. Const. amend. IV. The United States Supreme Court, in Katz v. United States, 389 U.S. 347, 361 (1967), explained that a government action constitutes a search where it: (1) infringes on an individual’s actual, subjective expectation of privacy, that (2) society is prepared to recognize as objectively reasonable. The question is whether use of novel technologies like ALPRs and the information they collect constitutes a search under the Fourth Amendment.

The courts that have considered ALPRs and the Fourth Amendment find that motorists do not have an expectation of privacy when they travel from one place to another on public roads. No person can subjectively expect to remain private what they knowingly expose to public view; as the function of a motor vehicle is transportation, it exposes its contents and occupants to plain view on public thoroughfares. New York v. Class, 475 U.S. 106, 114 (1986).

By using the public roads, motorists voluntarily convey to anyone who wishes to look:

• the fact of their travelling on particular roads in a particular direction,
• the fact of any stops they may make and
• the fact of their final destination when they exit public roads onto private property. United States v. Knotts, 460 U.S. 276, 281 (1983).

There is likewise no “reasonable expectation of privacy in the information contained on [a] license plate under the Fourth Amendment.” United States v. Ellison, 462 F.3d 557, 561 (6^th Cir. 2006).

Subjectively, because state law universally mandates the public display of registration tags, drivers must expect that they will be observed by law enforcement and the general public. United States v. Sturdivant, 786 F.Supp.3d 1098, 1108-10 (N.D. Ohio). Objectively, society is not prepared to recognize as reasonable Fourth Amendment protection over a vehicle’s exterior appearance, which is knowingly thrust into plain public view. New York v. Class, U.S. at 114.

Based on this “plain-view” doctrine, courts have held with near uniformity that neither the photographing of license plates nor the searching of a license plate database constitutes a Fourth Amendment search. Thus, use of ALPRs currently does not expose a municipality to liability for engaging in an illegal search.

B. Developments in the Law Regarding the Fourth Amendment, Privacy and ALPRs

As the use of ALPR technology expands from a scattering of cameras in local communities into a vast interstate network of cameras that log, store and share the movements of nearly every passing vehicle, the issue of unconstitutional searches remains unsettled.

Flock Safety, a leading provider of ALPR technology to government agencies, reports more than 120,000 cameras across 49 states, recording roughly 20 billion plate scans in more than 6,000 communities each month. This expansion of ALPRs into nationwide information networks poses important questions related to the Fourth Amendment and unconstitutional searches.

The Supreme Court recognizes that “[a] person does not surrender all Fourth Amendment protection by venturing into the public sphere[,]” but instead maintains a reasonable expectation of privacy in the whole of their movements. Carpenter v. United States, 585 U.S. 296, 310, 313 (2018). Based on this view of privacy, the Supreme Court has determined that surveillance that creates a comprehensive record of a person’s movements and “an intimate window into a person’s life” may constitute a search under the Fourth Amendment. Id, at 296.

The question is whether such comprehensive surveillance violates citizens’ Fourth Amendment right to be free from unreasonable governmental searches.

Recent Supreme Court and lower federal court decisions suggest a search may be conducted when information from ALPRs combines discrete moments of data to track the movements of vehicles and the persons in them over an extended period of time. For instance, a federal district court in Oklahoma described how ALPRs can generate personal surveillance data that may constitute a Fourth Amendment search:

To be sure, ALPR technology does not provide the same precision in location information as cell phones. But it could still reveal a lot about a person’s whereabouts, and it is easy to imagine the kinds of sensitive information it collects. It could track and log a couple struggling to start a family at the fertility clinic. It could show how often someone visits a liquor store or casino. It could show which doctor someone goes to or what lawyer’s office someone visits. It also collects mundane information about people’s commutes, but it collects a tremendous amount of such information all the time.

United States v. Kyle, 2026 U.S. Dist. LEXIS 225052, at *39 (N.D. Okla. Oct. 1, 2026). To date, the Supreme Court has not determined whether the use of ALPR data to track the movements of cars and drivers over an extended period of time constitutes a Fourth Amendment search and, if so, whether such a search violates constitutional protections against unreasonable governmental intrusion into individuals’ lives. 

C. Illinois Law Regarding Municipalities Using ALPRs

For an Illinois municipality, current Seventh Circuit case law generally favors the use of ALPRs. In United States v. Tuggle, the Seventh Circuit held that eighteen months of warrantless surveillance via stationary pole cameras on public property outside a suspect’s home was not a search. 4 F.4^th 505, 511, 526 (7^th Cir. 2021). The court distinguished the use of the pole cameras as “prospective and nonhistorical” as the government “had to decide ex ante” to install them and it “did not tap into an expansive, preexisting database” of the defendant’s movements. Id. at 525. Because an ALPR network is precisely such a preexisting, searchable database, Tuggle offers less protection for ALPR queries than its holding alone suggests.

Illinois statutory law also independently regulates ALPR data. Under 625 ILCS 5/2-130, ALPR users

• may not share ALPR information with any jurisdiction to enforce laws that restrict reproductive health care or that permit detention or investigation based on immigration status
• may not share ALPR information with an out-of-state law enforcement agency without first obtaining a written declaration that it will not be used for those purposes. 625 ILCS 5/2-130(b)-(c).
• must hold ALPR information confidentially to the fullest extent permitted by law, including privately owned devices whose data is shared with law enforcement. 625 ILCS 5/2-130(a)-(d).

D. Recommendations

While the current state of the law allows for the use of ALPR systems without violating the Fourth Amendment, there are steps municipalities can take to avoid liability. These steps include the following:

1. Require a documented basis for every search of data. Each unique search of ALPR data should be tied to an investigation or other law enforcement-related need. Data searches that do not support reasonable law enforcement actions should be avoided. Data searches that reconstruct a vehicle’s total movements over multiple days should only occur after consultation with prosecutors about obtaining a warrant.
2. Minimize retention. Data should be retained for the shortest period consistent with operational needs, unless being preserved as evidence in a specific case.
3. Limit data sharing. Municipalities must comply with 625 ILCS 5/2-130, including its written-declaration requirement for out-of-state data sharing. As broader data sharing also increases the risk of an unconstitutional Fourth Amendment search, municipalities should consider prohibiting out-of-state data access, limiting what partner law enforcement agencies may search for and reviewing any integration with privately owned cameras.
4. Adopt a written policy. A written policy or standard operating procedures setting out these limits will help a municipality show, if challenged, that its ALPR program is not the unrestrained, indiscriminate type that courts have disfavored.
5. Monitor developments. The law is evolving quickly in this area.

For questions about ALPRs, please contact Anthony G. Becknek, J. Allen Wall or another member of Klein, Thorpe & Jenkins’ Municipal Law Practice Group.