Automated License Plate Readers and the Fourth Amendment: Balancing Policing and Privacy

Automated License Plate Readers (ALPRs) have become a common tool for law enforcement, aiding in everything from recovering stolen cars to tracking suspects. These systems – whether fixed cameras on poles or mobile units on patrol cars – automatically photograph license plates and convert them into data . The plate number, timestamp, and GPS location are instantly checked against hotlists (e.g. stolen vehicles or wanted suspects) and stored in databases for later analysis. This powerful capability raises an important question: How does ALPR use intersect with Fourth Amendment protections against unreasonable searches? Law enforcement officers must understand the constitutional concerns of ALPR use, including what courts have said about privacy expectations, data retention, mass surveillance concerns, and recent media coverage on the topic. This article examines key court decisions and constitutional arguments, and summarizes legislation shaping how ALPRs can be deployed. 

Understanding ALPR Technology and Uses

Beyond real-time alerts, detectives increasingly rely on historical ALPR data during investigations – for example, to retrace a suspect’s movements or place a vehicle near a crime scene at a specific time. ALPR data has been used to solve violent crimes and find missing persons by revealing patterns of travel. Some ALPR models can scan up to 1,800 plates per minute, and one private ALPR provider boasts a database of over 6.5 billion plate scans, growing by 120 million a month.

However, this indiscriminate collection of all vehicles’ movements (not just suspects) raises privacy questions, since the vast majority of scanned plates belong to ordinary people not suspected of any wrongdoing. It’s this tension between investigative value and potential “dragnet” surveillance that brings ALPRs under Fourth Amendment scrutiny.

Fourth Amendment Basics: No Privacy in a License Plate’s Public Display

The Fourth Amendment protects people from unreasonable searches and seizures, generally requiring a warrant when a person has a reasonable expectation of privacy in the thing searched. Courts have long held that license plates – which are mandated by law to be displayed publicly on vehicles – carry no reasonable expectation of privacy. In plain terms, a plate is in public view, so police are free to observe and record it just as any member of the public could. Running a plate number through a database to check registration or warrants is likewise permissible without any suspicion, as it’s considered a minimal intrusion into something exposed to public view. Officers around the country routinely engage in this practice to discover suspended licenses, stolen cars, or wants on a vehicle, and courts have consistently upheld its legality.

This traditional view means that an ALPR snapping a photo of a license plate as a car drives by is not a “search” in the Fourth Amendment sense. The U.S. Supreme Court noted that because of the “pervasive regulation of vehicles” and their operation in public, drivers cannot claim privacy in the outward content of their license plates. In one federal case, for example, the court observed that “a motorist has no privacy interest in her license plate number,” which is displayed for all to see (United States v. Ellison, 462 F.3d 557 (6th Cir. 2006)). Thus, using ALPR technology to automatically check plates against law enforcement databases (for warrants, stolen status, etc.) is generally considered equivalent to an officer’s plain-view observation and radio lookup – no warrant or particular suspicion is required.

Traffic stops initiated from ALPR “hits” (such as a stolen vehicle alert) have likewise been upheld as reasonable seizures, so long as the underlying database information is reliable. In practice, agencies instruct officers to verify an ALPR hit (confirm the plate number and match with the correct state, vehicle description, etc.) before acting, to avoid false positives. But the act of scanning plates en masse – at a toll plaza, along a highway, or by a roving cruiser – has been seen as simply capturing what any passerby could observe. In sum, for one-off, real-time uses of ALPR to detect specific vehicles of interest, Fourth Amendment concerns are minimal under current case law.

From Mosaic Theory to Carpenter: Long-Term Tracking and Privacy Expectations

While a single license plate scan is not intrusive, the cumulative effect of ALPR data is a different story. As police agencies retain months or years of plate scans, they can effectively chronicle a car’s (and by extension, a person’s) movements over time – which may encroach on privacy in a way traditional methods never could. The Supreme Court’s recent decisions hint at this distinction. In United States v. Jones (2012), five Justices acknowledged that prolonged GPS monitoring of a vehicle might impinge on privacy, even if each individual trip is public. And in Carpenter v. United States (2018), the Court held that accessing historical cell phone location records (which reveal a detailed, long-term log of a person’s whereabouts) requires a warrant. The Carpenter Court emphasized the “deeply revealing nature” of location data that “provides an intimate window into a person’s life” when collected over extended periods. Although Carpenter dealt with cell-site location info held by a phone company, its reasoning has prompted courts and advocates to re-examine other forms of pervasive location tracking – including ALPR databases.

The “mosaic theory” of the Fourth Amendment suggests that while a short glimpse of public travel is insignificant, a mosaic of someone’s movements over weeks or months is something society might recognize as private. ALPR systems make creating that mosaic trivially easy. Every scan is time-stamped and mapped, and when thousands of data points are aggregated, police can retrospectively trace a vehicle’s route, determine patterns (such as daily routines or visits to sensitive locations like medical clinics, churches, or political meetings), and even predict future movements. Importantly, ALPR deployments tend to collect data indiscriminately on everyone, not just suspects – one audit in Los Angeles found 99.9% of ALPR scans were of vehicles not on any hotlist (i.e. not linked to any wanted suspect or case). This raises the question: At what point does querying or data-mining a massive license plate database become a Fourth Amendment “search”?

Courts are beginning to grapple with this question. In a 2020 Massachusetts case, Commonwealth v. McCarthy (484 Mass. 493, 142 N.E.3d 1090 (2020)), the state’s highest court acknowledged that ALPRs “raise clear privacy issues” under both the Fourth Amendment and the state constitution. The court noted that a sufficiently extensive network of ALPR cameras “would invade a reasonable expectation of privacy and would constitute a search for constitutional purposes”. However, because the defendant had not shown that the police had used a truly pervasive network in his particular case and the case involved ALPRs on two specific bridges, the court stopped short of ruling the ALPR data search unconstitutional. Similarly, the Ninth Circuit in U.S. v. Yang (2020) avoided the core Fourth Amendment question by finding the defendant lacked “standing” to object – since he was driving a rental car past its return date, the court said he had no reasonable expectation of privacy in that vehicle’s movements. Notably, the Ninth Circuit did recognize that ALPRs collect “massive amounts of data” on Americans, but it left the constitutional issue unresolved due to the defendant’s vehicle status.

Recent Court Decisions: Diverging Approaches to ALPR Data

While higher courts have been cautious, at least one trial-level court has taken a bold stance to protect privacy. In 2024, a Virginia state court confronted a situation where 172 ALPR cameras blanketed the city of Norfolk, logging every vehicle’s movements continuously . In Commonwealth v. Jayvon Bell, the defendant faced armed robbery charges, and police evidence included several ALPR photographs placing his car at certain locations . The Norfolk Circuit Court suppressed the ALPR evidence, ruling that warrantless use of the citywide ALPR network violated the Fourth Amendment . The court was persuaded that constant, citywide monitoring creates a “dragnet over the entire city” – precisely the kind of pervasive surveillance Carpenter warned against. The judge distinguished this from the limited ALPR use in the Massachusetts McCarthy case, finding Norfolk’s extensive 24/7 ALPR grid far more intrusive . This landmark suppression order (currently a single trial court’s decision) indicates that some judges see retained ALPR data as fundamentally different from a fleeting plate scan, at least once the technology’s reach becomes “unavoidable” for anyone driving in the city.  While this case is from the trial court level and received pushback (Virginia legislature bill HB2724, effective July 1, 2026, included provisions for state approval of ALPR systems and data collection reporting), it may be a preview of coming appeals-level rulings.

Federal courts, for their part, are cautiously parsing Carpenter’s application to ALPR. In late 2024, a federal judge in Virginia ruled that using a limited number of ALPR hits (images) to track a suspect did not amount to a Fourth Amendment search in that case . Over a 30-day span, only three sightings of the vehicle were obtained from Flock Safety ALPR cameras – which the court felt was too sparse to implicate the kind of comprehensive “long-term tracking” Carpenter addressed . The court explicitly limited its holding to the facts (few cameras and few data points) , implying that a denser web of ALPR data might trigger a different result. These cases suggest a developing consensus: a short-term or infrequent use of ALPR data is permissible without a warrant, but highly prolonged or bulk location tracking could cross a constitutional line. Until the U.S. Supreme Court provides a definitive ruling, law enforcement should assume that the more ALPR data you aggregate on a person, the more likely a court could deem accessing that data a “search” requiring a warrant .

Data Collection, Retention, and Privacy: Constitutional Arguments

A core constitutional debate centers on ALPR data retention. Simply capturing a plate at one moment may be innocuous, but storing years of records effectively creates a searchable time-machine of someone’s movements . Defense attorneys and civil liberties groups argue that without strict limits on retention and use, ALPR databases enable “retroactive surveillance” of individuals who were not suspected at the time of data collection . This raises an important point: when does data storage become a Fourth Amendment seizure or search? The Supreme Court hasn’t squarely addressed that in the ALPR context, but Carpenter’s logic suggests that government collection of detailed location histories interferes with expectations of privacy in the “whole of one’s physical movements” . Moreover, unlike purely private surveillance, ALPR data is typically used by government agents, so the Fourth Amendment is directly implicated (unlike private store cameras tracking customers).

On the other hand, proponents argue that police can theoretically track a car on public streets without a warrant; therefore, using technology to do the same should not be considered different. The counterargument is that ALPRs remove practical constraints (like manpower and limited memory) that naturally protect privacy. As one court observed, traditional surveillance required police to pick and choose targets, whereas ALPR “allows officers to track everyone” with far less effort . This “tracking everyone” capability is what makes indefinite ALPR data retention suspect. Police cannot feasibly follow every driver in person. Still, if they retain all plate data for months or years, they can later fish through it to find a particular car’s trail – essentially conducting a search of a person’s movements well after the fact and without prior suspicion. Critics liken this to having a GPS on every car by proxy. In response, some courts (and likely future legislation) are considering time limits or use restrictions on ALPR data as a way to balance investigative value with privacy. For example, the Massachusetts Supreme Judicial Court in McCarthy hinted that shorter retention or a narrower camera network might avoid constitutional problems, whereas “enough cameras in enough locations” with long-term data would require a warrant .

ALPR Use Under State and Federal Law: Legislation and Policies

In the absence of definitive Supreme Court guidance, legislatures have stepped in to regulate ALPR use and data practices. At least 16 states have statutes expressly addressing ALPR operation or data retention . These laws vary widely, reflecting different approaches to privacy and law enforcement needs:

• Data Retention Limits: Many states cap how long agencies can keep license plate data. For instance, Arkansas limits retention to 150 days and mandates that data not be preserved beyond that except for legitimate law enforcement needs . Maine goes further – ALPR data not related to an active investigation must be purged after just 21 days . In Tennessee, a 2021 update set a 90-day maximum retention period for ALPR data, unless it’s part of an ongoing investigation (in which case it must be deleted at the investigation’s conclusion) . These limits aim to prevent the build-up of large location databases on innocent drivers. By contrast, states without specific ALPR laws sometimes allow retention for years; for example, police in Virginia historically kept data for up to two years in some jurisdictions , which became a point of controversy.
  
  

• Usage Restrictions and Purpose Limitations: States like New Hampshire tightly restrict ALPR use. New Hampshire law allows only certain law enforcement uses and famously requires that all non-hit data be purged within three minutes of capture – effectively preventing any long-term database except for plates tied to immediate incidents. Arkansas outright prohibits private individuals or non-law enforcement entities from using ALPRs, with narrow exceptions (e.g. parking enforcement), and it disallows any state agency from ALPR use outside of law enforcement or security purposes . Utah permits ALPRs for public safety and investigations but forbids police from using privately collected plate data (such as data from commercial ALPR companies) if it’s older than 30 days unless a court order or warrant is obtained . This innovative provision closes a potential loophole where police might otherwise buy historical location data from third-party aggregators to sidestep warrant requirements.
  
  

• Privacy and Security Requirements: California took a lead in 2015 by classifying ALPR data as “personal information” under its data privacy laws and requiring any operator of ALPR systems – public or private – to implement a privacy policy with robust safeguards . California law (Cal. Civil Code § 1798.90.5) mandates training for users, security measures to protect the data, audits of use, and it bars sharing ALPR information with unauthorized third parties . Minnesota and North Carolina similarly require agencies to adopt written ALPR policies governing approved uses, retention schedules, access controls, and audit logging of queries . The emphasis is on accountability – ensuring ALPR is used only for legitimate law enforcement purposes and that misuse (like an officer looking up a plate for personal reasons) can be detected and addressed.
  
  

• Public Disclosure and Oversight: Because ALPR data often contains information about individuals’ movements, many states exempt it from public records disclosure to protect privacy. For example, Florida declares ALPR records confidential and not subject to Sunshine Law requests, except to the registered owner of a plate seeking their own data . Nebraska requires annual reports on ALPR usage and makes agencies post their ALPR policies publicly, increasing transparency of how the technology is used . And in Virginia, after a high-profile legal battle, the state’s Supreme Court ruled in 2020 that simply storing plate images and times was not “personal information” under state privacy law (because a license number alone doesn’t identify an individual without a separate DMV lookup) . This meant police in Virginia could continue to keep ALPR data indefinitely under state law , absent new legislation. However, that ruling sparked legislative interest in imposing new rules. In 2025, Virginia’s General Assembly considered a bill to limit ALPR data retention and require state-issued permits for installing ALPR cameras on highways . Privacy advocates there worry that without careful drafting, such laws might unintentionally expand ALPR deployments while merely formalizing minimal restrictions . Detectives in every state should be aware of their own jurisdiction’s ALPR statutes or policies, as they dictate what time limits, sharing restrictions, or documentation requirements apply to ALPR data in investigations.
  
  

At the federal level, there is currently no single comprehensive law governing law enforcement use of ALPRs . Federal agencies like the DEA and DHS have built extensive ALPR programs, operating under general agency policies and the Privacy Act. 

As for pending federal legislation, one notable proposal is the Fourth Amendment Is Not For Sale Act, which seeks to bar law enforcement from buying data on individuals from commercial data brokers without a warrant. Early versions of this bill prompted clarification that publicly obtained data like ALPR records would be exempted (so as not to hamper legitimate ALPR use) . The mere fact that ALPR was explicitly discussed in Congress shows the growing awareness in D.C. of surveillance technologies. Moving forward, we may see federal guidelines or laws that standardize best practices – for example, imposing minimum privacy standards on any agency tapping into private ALPR databases, or providing funding incentives for agencies that adopt stricter data hygiene (short retention and robust audit trails).

Best Practices and Takeaways for Law Enforcement

For detectives and officers, ALPR is a force-multiplier – it can pinpoint a suspect’s car in seconds or generate leads that would take days of manual surveillance. However, with great capability comes the responsibility to use ALPRs in a manner that upholds constitutional rights and public trust. Here are some practical guidelines:

• Know Your Agency’s Policy and State Law: Given the patchwork of ALPR laws, always follow the strictest applicable rules on data use and retention. Ensure you adhere to any mandated retention purge deadlines (e.g. 30 days, 90 days, etc.) and document any exceptions if data must be kept longer for an investigation. If your state requires a usage log or audits, keep meticulous records of queries (who searched, for what purpose) – this not only maintains compliance but also strengthens the admissibility of evidence by showing a chain of custody and proper use.
  
  

• Use ALPR Data Purposefully, Not Pervasively: Avoid the temptation to go on “fishing expeditions” through ALPR databases without a focused investigative purpose. Courts are wary of broad, suspicionless searches of historical location data . If you need to perform an extensive query – for example, tracking a vehicle’s pattern over months – consider whether you have sufficient cause to obtain a warrant. Even if not strictly required by settled law, getting a warrant (or at least documenting probable cause for the query) can provide a safety net if the data is later challenged in court under evolving Fourth Amendment doctrines.
  
  

• Mind Sensitive Locations: Be aware that recurring hits at locations like homes, hospitals, places of worship, or political meetings could elevate privacy concerns. The Supreme Court in Carpenter noted location data’s potential to reveal “the privacies of life” . If ALPR data places a suspect’s car at a constitutionally sensitive location (e.g. in a home driveway, which might raise curtilage issues, or at a religious service implicating First Amendment associative privacy), consult with legal advisors about whether additional steps (like a warrant) are prudent before using that information.
  
  

• Verify and Corroborate ALPR Leads: Technological errors and misreads can occur – an ALPR might misinterpret a character or a hotlist might be outdated. There have been incidents of innocent people detained at gunpoint due to mistaken ALPR alerts . Always confirm critical ALPR hits through other evidence. For example, if an ALPR places a suspect vehicle near a crime scene, try to obtain CCTV footage, witness observations, or make note of unique vehicle descriptors (many ALPR systems capture color, make, and even bumper stickers now) to ensure it’s the same car. This not only improves your case but also mitigates the chance of a false lead that could harm innocent drivers or derail your investigation.
  
  

• Secure the Data and Respect Privacy: Treat ALPR data with the same care as other sensitive investigative data. Ensure that only authorized personnel access the databases and that there’s a legitimate law enforcement reason for each query. Never use ALPR systems to look up personal curiosities or non-official matters – aside from being unlawful in many states, such misuse could result in discipline and undermines the credibility of the technology. Remember that ALPR data often contains information about people not charged with any crime; safeguards and restraint in handling this data demonstrate professionalism and respect for the public’s privacy.
  
  

• Stay Informed on Legal Developments: The legal landscape for ALPR use is continuously evolving. Keep abreast of new case law (such as any appellate decision affirming or overturning the Norfolk suppression in Bell, or a future Supreme Court ruling on ALPRs) and changes in legislation (e.g. if Congress or your state enacts new ALPR limits or reporting requirements). Updated training and legal bulletins can help ensure that your use of ALPR evidence will hold up in court. It’s wise to liaise with your prosecutor’s office when using novel ALPR tactics – they can advise on how to best document the process to withstand judicial scrutiny.
  
  

Conclusion

ALPR technology has proven to be a valuable asset in modern policing – it has enabled faster recovery of stolen cars, apprehension of fugitives through their vehicles, and the ability to solve crimes by placing suspects at crucial locations. Yet, it also illustrates the classic trade-off between security and privacy. The Fourth Amendment does not forbid police from using new tools, but it does demand that those tools not be used in a way that tramples reasonable expectations of privacy. Detectives leveraging ALPR should do so with an understanding that while a license plate in public is fair game, the comprehensive tracking of a person’s movements is approaching uncharted constitutional territory . By following the spirit of emerging guidelines – using ALPR data in targeted ways, minimizing indiscriminate retention, and securing proper authorization for broad searches – law enforcement can harness ALPRs effectively without inviting legal challenges. In the coming years, court decisions and legislation will continue to refine the boundaries. For now, the best practice is to stay on the leading edge of compliance, ensuring that ALPR remains a powerful but rights-respecting tool in the detective’s toolkit.