Commonwealth v. McCarthy
The Massachusetts Supreme Judicial Court decision holding that a limited ALPR deployment at two Cape Cod bridges did not constitute a constitutional search, while recognizing that a sufficiently extensive ALPR network can reveal a protected mosaic of a person's movements.
Executive Summary
Barnstable police suspected Jason McCarthy of supplying heroin to a Cape Cod distributor. Investigators placed McCarthy's vehicle on an ALPR hot list, received real-time alerts when the vehicle crossed the Bourne or Sagamore bridges, and reviewed historical records showing when it had crossed those bridges during an approximately two-and-one-half-month period. The Massachusetts Supreme Judicial Court recognized that people have a constitutionally protected privacy interest in the whole of their public movements and that widespread ALPR deployment can implicate that interest. On the record before it, however, four cameras at two fixed bridge locations revealed only whether and when the vehicle entered or left Cape Cod. They did not expose an entire journey or a detailed account of McCarthy's movements. The court therefore held that this limited use was not a search under either the Fourth Amendment or Article 14 of the Massachusetts Declaration of Rights and affirmed the denial of the motions to suppress.
Commonwealth v. McCarthy is a leading state decision addressing automated license plate reader data as location information. It rejects the simplistic proposition that ALPR use is always constitutionally insignificant merely because license plates and road travel are exposed to public view.
The decision is equally careful not to declare every ALPR scan a search. Its analysis is fact-dependent. Constitutional significance turns on what the queried system can reveal, including the number and placement of cameras, duration of the query, density of detections, use of historical records, and ability to reconstruct sensitive activities and associations.
Key Holdings at a Glance
How ALPR Technology Works
An ALPR combines a camera with software that detects and interprets license-plate characters. A typical record may include an image of the plate or vehicle, the interpreted plate number, date, time, camera location, direction of travel, and travel lane. Systems may compare a new scan against a hot list, store detections for later searching, or both.
| Function | What It Does | Legal Significance |
|---|---|---|
| Single scan | Records a vehicle at one place and time. | Often resembles an officer observing a publicly displayed plate. |
| Hot-list alert | Notifies users when a listed plate is detected. | Can become real-time tracking when network coverage is sufficiently broad. |
| Historical query | Retrieves prior detections associated with a plate. | May reconstruct travel without advance physical surveillance. |
| Network analytics | Connects detections, vehicles, locations, and patterns. | Aggregation can reveal associations and sensitive destinations. |
| Data sharing | Expands searches across agencies or private providers. | Effective coverage may be far broader than agency-owned cameras alone. |
Facts of the Investigation
Barnstable police developed evidence that a codefendant was distributing heroin from his residence. Through surveillance, controlled purchases, confidential informants, and observation of a black Hyundai, investigators came to suspect that McCarthy supplied the codefendant.
On February 1, 2017, police added the Hyundai's plate to the ALPR hot list. On February 8, officers received an alert that the vehicle had crossed the Sagamore Bridge onto Cape Cod. Police followed McCarthy and the codefendant to a brief meeting in a residential area. Officers did not observe a physical exchange.
Investigators also generated a spreadsheet of every recorded crossing of the Bourne and Sagamore bridges between December 1, 2016, and February 12, 2017. It contained dates, times, directions, and lanes. The vehicle was recorded traveling onto Cape Cod on nineteen days in December, twenty-one days in January, and eight days in February, sometimes more than once on the same day.
On February 22, police received another alert and observed another short meeting. Officers stopped both vehicles. Heroin was recovered from the codefendant, and McCarthy later made incriminating statements after receiving and waiving Miranda rights.
Procedural History and Result
McCarthy moved to suppress the ALPR data and evidence flowing from his arrest. A Superior Court judge held an evidentiary hearing and denied the motions. A single justice permitted an interlocutory appeal to proceed to the Supreme Judicial Court.
Privacy in Public Movements
McCarthy displayed a license plate on public roads, and an officer ordinarily may observe and record that plate. The court nevertheless distinguished an isolated observation from the government's ability to collect, retain, search, and analyze many observations over time.
Automated systems remove the practical limits that traditionally constrained surveillance. They operate continuously, record nearly every passing vehicle, preserve data for retrospective searching, and link a vehicle to specific locations and times. Those capabilities can turn otherwise ordinary observations into a revealing account of a person's life.
The Mosaic Theory
The mosaic theory asks whether a series of observations that might not be searches individually becomes a search when collected and analyzed together. Aggregation may reveal patterns, routines, relationships, religious attendance, medical visits, political activity, and other details that no isolated observation would expose.
| Factor | Lower Privacy Impact | Higher Privacy Impact |
|---|---|---|
| Coverage | Few cameras at isolated locations | Dense regional or nationwide network |
| Duration | Brief, event-specific query | Weeks, months, or years of history |
| Location | Major highway or bridge | Homes, worship, medical, political, or residential locations |
| Detail | Entry or exit at one boundary | Full routes, routines, stops, and associations |
| Data sources | Agency's limited camera set | Interagency, commercial, and private-camera aggregation |
| Analytics | Plate match only | Pattern analysis, association mapping, and predictive alerts |
The court declined to announce a numeric threshold for when an ALPR mosaic becomes a search. That omission is important: camera count or number of days cannot be treated as a universal safe harbor. The central question is how much of the person's movements the system actually or potentially revealed through the challenged query.
Why the Challenged Use Was Not a Search
The evidentiary record established only four cameras at two fixed bridge locations. Those cameras revealed whether McCarthy's vehicle passed onto or off Cape Cod at particular times. They did not track his progress between the bridges and his destination, monitor an entire journey, or expose the whole of his public movements.
Real-Time Hot Lists
The system allowed authorized users to add a plate to a hot list and receive an email or text when a reader detected it. The alert included the plate image, time, location, and direction of travel. In McCarthy, the limited network did not provide continuous real-time tracking.
The court warned, however, that sufficiently widespread hot-list coverage could allow police to locate a person almost whenever that person drives. At that scale, ALPR can begin to resemble a real-time cellphone ping or continuous tracking system. Exigent circumstances remain available when immediate location is reasonably necessary to respond to an emergency.
Data Retention, Queries, and Discovery
At the time of the case, the state system had a stated one-year retention policy, although alert emails could persist longer on recipients' servers. Retention expands the system's retrospective reach: a query can reconstruct travel that officers did not decide to monitor when it occurred.
Chief Justice Gants emphasized that courts need to know what data a query could retrieve, not merely what prosecutors later choose to use. His concurrence urged preservation of the search request and the full responsive dataset so the defense and reviewing court can evaluate the duration, scope, and density of the resulting mosaic.
McCarthy and Carpenter v. United States
McCarthy drew heavily from Carpenter v. United States, which treated prolonged historical cell-site location information as a revealing record of physical movements. Both decisions focus on aggregation, retrospective reach, low-cost surveillance, and the extent to which technology eliminates the practical limits of traditional observation.
The analogy is not exact. CSLI can generate frequent location points wherever a phone connects to the network, while ALPR records depend on where cameras are located and whether the vehicle passes them. For that reason, ALPR coverage and camera placement are essential to the analysis.
McCarthy and United States v. Knotts
United States v. Knotts permitted officers to use a beeper to follow a vehicle on public roads during a particular journey. McCarthy recognized the continuing relevance of conventional visual surveillance but also explained why networked ALPR can be different: it is continuous, retrospective, inexpensive, searchable, and capable of recording nearly every vehicle passing each collection point.
The key distinction is between augmenting officers during limited public surveillance and building a durable database that can reconstruct movements on demand. As scale and analytic capacity grow, Knotts becomes a less complete analogy and Carpenter-style aggregation concerns become stronger.
Chief Justice Gants's Concurrence
Chief Justice Gants agreed with the result but wrote separately to propose a framework for future queries of government-held location databases. He reasoned that a sufficiently comprehensive ALPR or facial-recognition location mosaic would require a probable-cause warrant.
For intermediate datasets, the concurrence identified two possible approaches: a single threshold at which probable cause becomes necessary, or a two-tier framework allowing a court order based on reasonable suspicion for a lesser mosaic and a warrant based on probable cause for a more revealing one. The majority did not adopt either framework.
Other Issues Resolved by the Court
Written ALPR policy
The court rejected the claim that suppression was required because police failed to establish an adequate written ALPR policy. Written rules may be wise and may be required by other law, but they did not determine the threshold question whether the limited conduct was a constitutional search.
Federal communications statutes
The Stored Communications Act did not apply because the government was not compelling an electronic communications provider to disclose communications. The Electronic Communications Privacy Act did not apply to officers receiving their own system's alerts in the manner argued by McCarthy.
Target standing
McCarthy could not challenge the seizure of evidence from his codefendant merely because he was a target of the investigation. The court found no distinctly egregious police conduct warranting a different state-law result.
Miranda and voluntariness
The court upheld the finding that McCarthy knowingly and voluntarily waived his rights. Police statements suggesting he was not under arrest were part of the totality analysis but did not, standing alone, make the waiver or statements involuntary.
ALPR Technology in 2026
Modern ALPR systems may combine fixed cameras, patrol-car readers, mobile trailers, commercial databases, neighborhood cameras, vehicle descriptions, route analysis, plate associations, and automated notifications. A single query may reach data collected far outside the requesting agency's jurisdiction.
These developments make McCarthy's functional inquiry more important. Agencies should assess the effective network searched—not merely the cameras they own. They should also document whether a query used plate number, vehicle characteristics, location, time, pattern, association, or an algorithmically generated lead.
| Modern Capability | Question Raised by McCarthy |
|---|---|
| Nationwide commercial search | How comprehensive is the returned movement history? |
| Cross-agency sharing | What is the total geographic and temporal coverage? |
| Vehicle fingerprint search | Is the system identifying a vehicle without a known plate? |
| Pattern and convoy analysis | Does the system reveal relationships or repeated associations? |
| Real-time alerts | Does network density permit persistent location monitoring? |
| AI-assisted lead generation | Can investigators explain and validate the source data and inference? |
Practical Guidance for Law Enforcement Agencies
ALPR Investigator Checklist
| Question | Why It Matters |
|---|---|
| What specific offense or public-safety purpose supports the query? | Establishes legitimate use and investigative relevance. |
| Which databases and partner networks will be searched? | Determines the true scale of collection. |
| What time period and geographic area are necessary? | Limits the location mosaic and supports particularity. |
| Are sensitive locations likely to appear? | May increase constitutional and First Amendment concerns. |
| Could the results reveal prolonged or near-continuous movement? | May move the query toward a constitutional search. |
| Is a warrant, court order, exigency, or consent available? | Identifies the appropriate legal authority. |
| Has the plate and underlying alert been independently confirmed? | Reduces misreads and unlawful enforcement action. |
| Are the query and full results preserved? | Supports audit, discovery, and suppression review. |
| Does sharing comply with policy, contract, and applicable law? | Controls downstream access and secondary use. |
Litigation Checklist for Agency Counsel and Prosecutors
- Identify every ALPR system and data source searched.
- Establish who performed the query, when, and for what documented purpose.
- Preserve the original query, filters, audit logs, and complete responsive dataset.
- Quantify the duration, number of detections, camera locations, and geographic coverage.
- Determine whether the query exposed sensitive destinations, routines, or associations.
- Separate historical searching from real-time hot-list monitoring.
- Analyze the Fourth Amendment and any more protective state constitution or statute.
- Identify the warrant, court order, consent, exigency, or other authority if one was used.
- Establish alert validation and the independent basis for any stop, arrest, or search.
- Document vendor, private-camera, and partner-agency involvement.
- Address retention, deletion, exports, and downstream copies.
- Review subsequent authority in the controlling jurisdiction before relying on McCarthy.
Frequently Asked Questions
What did Commonwealth v. McCarthy hold?
It held that police use of data from four ALPR cameras at two Cape Cod bridge locations did not reveal enough of the defendant's movements to constitute a search under the Fourth Amendment or Article 14.
Did the court hold that ALPR use never requires a warrant?
No. The court expressly recognized that widespread ALPR deployment can implicate a protected expectation of privacy in the whole of a person's public movements.
Why did the limited system not constitute a search?
It showed when the vehicle entered or left Cape Cod but did not reveal a complete journey, continuous location, or a sufficiently detailed mosaic of public movements.
Does the number of cameras determine the constitutional result?
Not by itself. Courts should consider camera placement, duration, detection density, geographic coverage, data sharing, and the detail revealed by the query.
Are real-time hot-list alerts constitutionally different?
They can be. A limited alert may resemble ordinary observation, but a dense network capable of locating a person whenever the person drives may implicate constitutional protections.
Does an agency need a written ALPR policy?
McCarthy did not make a written policy a prerequisite to the limited use at issue. Agencies should nevertheless maintain detailed policies to govern access, validation, retention, sharing, auditing, legal process, and accountability, and other laws may require them.
What is the most important discovery record?
The agency should preserve the exact query and the complete data returned, together with audit logs and any exports or analytic products. The constitutional analysis may depend on what the search exposed, not merely what was introduced at trial.
Is McCarthy binding outside Massachusetts?
No. It is binding Massachusetts state authority on the issues it decided and persuasive elsewhere. Federal circuits and other states may apply different constitutional rules, statutes, or thresholds.
Primary Authorities
Massachusetts Supreme Judicial Court opinion addressing historical ALPR data, hot-list alerts, aggregate location privacy, Article 14, and the Fourth Amendment.
Read Commonwealth v. McCarthy
Official court summary issued when the decision was released.
Read the official court summary
United States Supreme Court decision addressing comprehensive historical location records.
Read Carpenter v. United States
United States Supreme Court decision addressing electronic monitoring of public-road travel.
Read United States v. Knotts
Final Assessment
Commonwealth v. McCarthy is neither a broad approval nor a categorical rejection of ALPR. It is a scale-sensitive decision. Limited observations at two travel chokepoints did not reveal the whole of McCarthy's movements, but the court recognized that a broader, denser, and more revealing system can cross the constitutional line.
The decision's lasting importance lies in its focus on function rather than labels. Agencies must evaluate the complete surveillance capability produced by camera placement, historical retention, hot lists, shared databases, and analytics. As ALPR networks grow, the distinction between a single roadside observation and persistent location tracking becomes increasingly important.