Automated License Plate Readers and the Fourth Amendment: A Public‑Safety‑by‑Design Perspective from Flock

Executive summary

• ALPRs create discrete, point‑in‑time observations, still images at fixed locations, rather than an always‑on dossier of anyone’s movements. The output is fundamentally different from technologies that continuously log location data, such as cell‑site records or vehicular GPS devices.
  
  
• ALPRs observe vehicles, not people. They read license plates and vehicle characteristics that are meant to be publicly visible. From these images, one cannot know who drove, whether drivers changed, who rode as passengers, or what anyone did before, during, or after an ALPR’s roadway observation.
  
  
• License plates are public by design. They are government‑issued identifiers intended to be visible for roadway safety and enforcement; a plate number does not, on its own, describe or index anything about a specific individual.
  
  
• Flock builds privacy protections into the product. We impose a standard 30-day retention limit that can be modified further based on state laws and community preferences, ensure end‑to‑end encryption, and create transparent audit trails. Agencies exclusively own their data and set local policy.
  
  
• Courts, including decisions across multiple states and in the Ninth and Eleventh Circuits, widely agree that using fixed‑location ALPRs without a warrant does not violate the Fourth Amendment. One Virginia state court decision (Commonwealth vs Bell) is the only case to have concluded otherwise; since then, multiple courts, including the Virginia Court of Appeals, have rejected Bell and declined to suppress Flock evidence.
  
  
• ALPRs are a traditional investigative aid, not a stand‑alone means of continuously following a person.
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• ALPRs are typically adopted through democratic processes, including city council action.
  
  
• The public‑safety value is real. Agencies use ALPRs to find missing persons, recover stolen vehicles, and identify vehicles linked to violent crime, with documented outcomes in diverse communities. 

1. What ALPR is and is not

What it is: A fixed camera system that reads license plates and vehicle attributes and logs the date, time, and location of each roadway observation. Officers can search by full or partial plate, make/model, color, or unique features (e.g., roof racks) using Flock’s Vehicle Fingerprint® technology. This is objective, vehicle‑level evidence—not identity or personal data.

What it is not: Flock ALPR does not perform facial recognition, does not store biometrics, cannot be queried to find people, and is not used to enforce traffic violations. 

How alerts work. Agencies can enable hot‑list alerts for vehicles associated with urgent concerns (e.g., Amber/Silver Alerts, stolen vehicles). These alerts notify when a listed plate is observed by a camera; they do not provide continuous following. 

2. Fourth Amendment framework: from trespass to reasonable expectations

Modern Fourth Amendment analysis considers whether government action intrudes on a reasonable expectation of privacy (the Katz test). 

Courts have consistently recognized that vehicles traveling on public roads are exposed to public view, which reduces privacy expectations. In United States v. Knotts (1983), the Court emphasized that traveling on public thoroughfares reveals direction of movement and stops to anyone watching—without constituting a Fourth Amendment search. 

In United States v. Jones (2012), the Court required a warrant to use vehicular GPS trackers because officers physically installed the device on the vehicle (constituting a trespass). While also noting in concurring opinions that, whereas certain electronic methods might raise different concerns, visual observation is not a search.

In Carpenter v. United States (2018), the Court required a warrant for accessing historical cell‑site location information (CSLI) because it created an “all‑encompassing” log of a person’s whereabouts, roughly 101 data points per day over 127 days, with records retained by providers for up to five years. The Court specifically noted that its narrow ruling did not call into question conventional camera‑based tools. 

In Leaders of a Beautiful Struggle v. Baltimore Police Department (4th Cir. 2021), persistent aerial imaging–covering ~90% of the city for ~12 hours per day for 45 days–was deemed unconstitutional, reinforcing the line between discrete public‑space observations and prolonged, extensive, population‑wide monitoring that exposes intimate details. 

Takeaway: The constitutional concern arises when technology stitches together a comprehensive portrait of someone’s movements over time that can reveal intimate and sensitive details. Fixed, point‑in‑time roadway observations occupy the other side of that line.

3. Why ALPR fits within constitutional boundaries

A. Discrete roadway observations, not a record of “the whole” of one’s movements

ALPR cameras sit at fixed and obvious locations and register still images of vehicles at that location and time. They do not follow a vehicle from origin to destination, cover only a small fraction of a city's roads, and provide samples, not a seamless diary of one's movements. This is categorically different from CSLI’s ~12,898 location points over 127 days in Carpenter. 

B. Vehicles, not people

ALPR logs vehicles. People frequently borrow, rent, or share them. From an image of a rear plate, one cannot know who drove, who else was inside, or whether drivers changed. Courts have long recognized diminished privacy expectations in vehicles on public roads. 

C. Plates are public by design

Courts have held that a license plate number is not personal information and is meant “to be seen by all who care to look at it.” 

D. Plain view in public places

ALPR cameras are placed on public thoroughfares, fully visible to any passerby. They do not intrude into homes, curtilage, or private spaces and are nothing like a hidden GPS tag that follows movements beyond public spaces. 

E. Limited retention and scope

Flock’s standard 30‑day retention ensures data cannot accumulate into long‑term dossiers, and our systems are limited to still, fixed‑location imagery. Retaining the data to only a short, specific period of time ensures data cannot accumulate into long time periods. This time-and-space boundedness addresses the mosaic‑theory concern that otherwise could arise with many months of continuous, city‑wide recording.

4. What courts are saying about ALPR

Although the U.S. Supreme Court has not yet decided an ALPR case, state and federal courts across the country, at both the trial and appellate levels, have concluded that using fixed‑location ALPRs without a warrant does not violate the Fourth Amendment. Jurisdictions include Alabama, California, Florida, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Montana, Nebraska, Nevada, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Texas, Utah, and Virginia. 

Illustrative cases:

• Commonwealth v. McCarthy (Mass. 2019): Using ALPR data from two fixed bridges during a drug investigation did not invade the defendant’s protected interest in the “whole” of his movements; the limited extent and use mattered.
  
  
• United States v. Yang (9th Cir. 2019): No reasonable expectation of privacy in historical location data of a rental vehicle derived from ALPR; Carpenter does not apply to license‑plate observations.
  

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Recent challenges and the outlier Virginia decision:

In Commonwealth v. Bell (Norfolk Cir. Ct., 2024), one judge concluded that accessing Flock ALPR data without a warrant violated the Fourth Amendment. However, since Bell, the Virginia Court of Appeals in Commonwealth v. Church reversed a lower court ruling, issued by the same judge who decided Bell, and held that ALPR technology does not require a warrant. 

In its ruling the Court held that: “Because the Flock system simply took pictures of the license plate and Church’s vehicle as he drove it down public thoroughfares in the City of Norfolk, the circuit court erred in ruling that a search warrant was required for police to access the Flock system.” Numerous other judges in Virginia and across the country have denied suppression of Flock evidence since Bell, finding no constitutional violation. 

Other active matters: Courts in Florida, Illinois, Oklahoma, and South Carolina have addressed or are considering ALPR claims; outcomes to date have generally distinguished ALPR from cellphone/GPS systems that log movements beyond public thoroughfares. ‍

Bottom line: The weight of authority supports warrantless use of fixed‑location, short‑retention ALPR as consistent with the Fourth Amendment.

5. Flock’s privacy‑by‑design and accountability

Flock’s product and program design embed technical and policy guardrails that align with constitutional principles and community expectations:

• Data minimization: 30‑day default retention with hard deletion.
  
  
• Security: End‑to‑end encryption; agencies own their data.
  
  
• Human‑image restraint: No ability to search specifically for images of people.
  
  
• Immutable accountability: Every user action and search reason is recorded in an indefinitely available audit trail.
  
  
• Transparent governance: We encourage agencies to make adoption a city‑council decision and to publish a Transparency Portal with usage details. 

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These controls, combined with the product’s limited, fixed‑location imagery, are precisely the kinds of checks and balances courts consider when distinguishing conventional observation from systems that risk compiling the “whole of a person’s movements.”

6. Public‑safety value

ALPR, used responsibly, saves time for investigators, helps find vulnerable people, and deters opportunistic crime by increasing the perceived likelihood of accountability. Two examples include:

• Rapid progress in a shooting investigation: After installing a dozen LPRs in June 2021, Rantoul (IL) PD promptly arrested suspects in a July 4 shooting that injured five, and later identified a second suspect in 14 minutes using Flock data.
• Interjurisdictional Silver Alert: Taylorville (IL) PD entered a missing senior’s vehicle information; successive observations in neighboring jurisdictions enabled a safe reunion with family. 

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These are representative of thousands of time‑bounded, case‑specific uses—not continuous, person‑focused surveillance.

7. From principle to practice: Policies that protect rights and outcomes

Flock supports agencies in adopting clear, public rules that reflect community norms and legal guidance. Based on the questions courts regularly ask and our experience with successful programs:

1. Adopt and publish an ALPR policy covering authorized uses, search reasons, and supervisory review. Provide mandatory training before access.
   
2. Evaluate the retention policy that best fits your community needs.
3. Define alert governance (e.g., criteria for hot lists and who can add/remove plates).
   
4. Ensure camera placement is appropriate and visible, focusing on major ingress/egress points and public roads—not private property.
   
5. Publish a transparency portal with usage statistics, audit summaries, and policy links.
   
6. Use the audit trail for routine supervisory audits and case‑specific reviews; retain access logs.
   
7. Court‑ready evidence handling:
   
   • For observations within 30 days, export the self‑certifying image verification affidavit from the search card.
     
   • For older evidence that was previously downloaded, request an affidavit confirming camera health and user activity at the time.
     
   • When needed, subpoena a Custodian of Records.
     
   • Flock’s Evidence Team (Legal) supports affidavits, certifications, limited subpoenas, and testimony.

8. Addressing common misconceptions

• “ALPRs follow people.” False. They log vehicles at fixed points. There is no identity inference about who was driving or riding, and the imagery does not continue beyond the camera location.
  
• “ALPRs build a detailed diary like cell‑phone data.” False. CSLI in Carpenter involved ~101 pings per day over months with multi‑year retention, enabling a revealing chronicle of a person’s life and intimate details. ALPR observations are sparse, roadway‑bound, and retained 30 days by default.
  
• “Plates are private.” False. Courts consistently hold that plate numbers are meant to be seen and are not personal information.
  
• “Courts are turning against ALPR.” The broad trend remains the opposite. The Bell decision is an outlier and the Virginia appellate court has rejected its holding; multiple judges since have denied suppression of Flock evidence, and many jurisdictions have upheld ALPR’s constitutionality. 

9. How Flock’s implementation aligns with the law

• Consistent with Katz and Knotts: Roadway observations of vehicle exteriors are in plain view and do not invade a reasonable expectation of privacy.
  
• Distinguishable from Jones and Carpenter: No physical trespass on a vehicle, and no comprehensive, long‑term, always‑on logging akin to CSLI or persistent aerial programs.
  
• Narrow, conventional tool: The Supreme Court has expressly noted that its Carpenter holding does not call into question conventional camera tools; courts repeatedly uphold fixed‑location ALPRs with limited retention.

10. Democratic governance and community trust

Flock encourages democratic adoption: elected bodies decide whether, where, and how to deploy; agencies must implement explicit policy, training, and transparent auditing. We also support public portals that provide community visibility into how the system is used. 

Conclusion

From Flock’s perspective, ALPR is a limited, vehicle‑focused, point‑in‑time tool that helps communities improve safety without creating the kind of pervasive, identity‑based surveillance the Fourth Amendment forbids. Our design choices: short retention, fixed‑location imagery, no biometrics, immutable audits, and visible camera placement align with decades of law that recognize driving on public roads as public conduct while reserving heightened protection for intimate, private spheres.

Courts across the country largely agree: fixed‑location ALPR, used with strong governance, is constitutional. We will continue to work with communities, elected leaders, and through the courts to ensure that what we build protects people and their rights and that outcomes like finding missing seniors or solving violent crimes remain possible when minutes matter.