No User, Only a Subject Chatrie Tests Meet Plate Readers

Orin Kerr posted a first draft of The Fourth Amendment After Chatrie on SSRN yesterday. It is the first serious map of what June’s Chatrie decision means, and it concludes that the case can be read three ways: a decision about location records, about cell phone records, or about all records. The article doesn’t commit to any of them. It also names ALPR litigation as the first wave of cases that will have to choose, citing the Eleventh Circuit’s argument in US v. Slaybaugh last week.

I wrote about Chatrie the day it dropped; I have since realized that my first thoughts have some gaps and flaws, but I believe its underlying ideas are correct: Chatrie is a “blockbuster,” as Kerr puts it; it will affect ALPR; and it will land on the side of privacy.

This post looks at Kerr’s analysis and the tests he distills from the court’s opinion, and tries to apply them to ALPR. I say “try,” not because I think I failed, but because, as an incoming 1L in early August, I am only nominally a law student, and definitely not a legal scholar. Some additional audacity will have to carry the work, or maybe the other way around.

With Schmidt v. Norfolk still pending at the Fourth Circuit, the only circuit holding on plate readers I was able to find was United States v. Porter, 170 F.4th 381 (5th Cir. Mar. 17, 2026). It predates Chatrie by three months, so we’re venturing out into some uncharted territory here.

Here Be Dragons.

#Kerr’s three tests

Kerr flags three tests. He doesn’t name them, so I did. The first quote is the Court’s, the other two are Kerr’s summaries. (Section numbers refer to the draft; they may shift.)

1. The relationship test—§II(D): protection follows what “a user reasonably understands as his own.”
2. The nature test—§III(A): “the private nature of the information in the abstract—rather than where it is, who holds it, or how much was obtained” controls.
3. The capacity test—§II(C): “the capacity to easily obtain the whole of a person’s physical movements” is what makes accessing records a search.

The draft also flags the Court’s “novice user” choice when faced with Kerr’s “magic box” problem^[1] in §III(A), as well as the apparent scope problem should a court try to decide what is “ordinary use” of a cell phone.

#The ownership question

Chatrie appears to rest in part on a largely presupposed notion that Chatrie’s location history data was his in some sense. If you were to assume that Google owns the location history data, Gorsuch’s concurrence^[2] would collapse in its entirety and the majority’s strongest analogies (the personal journal, records a user “reasonably understands as his own”) would weaken.^[3]

While an argument that “Google owns the software, therefore Google’s observation of a location is Google’s property” might appear strained here, it is available for data gathered on unwitting subjects by cameras and other sensors in the public sphere.

Taking Flock as an example, the government’s vendor observes a subject (not a user, like Google’s) and then creates a record using hardware and software owned and operated by that vendor. The vendor then stores the resulting record(s) on its servers.

The government has various arguments available to say it owns that record.^[4] If a court were to accept the argument the government owns the record, it could shift the posture of any ALPR case to the government querying its own records, making it less likely that any “search,” for purposes of the Fourth Amendment, occurred.

Even if a court accepted government ownership of the file, that has never been the dispositive factor. The carrier owned the CSLI in Carpenter, and the DEA owned the beeper in Karo.

If the record and the information about a person contained in the record were to be considered one and the same, government ownership could kill a Gorsuch-style property theory. But his theory is not the majority’s.

#A secret database of public information

The tension shines through in the question Judge Abudu raised at the Slaybaugh oral argument: even if the information contained in Flock’s database was gathered in public, if Flock’s database is not public, why should the court treat it as public information?

The government had no real answer. The one it gave was grounded in Knotts and collection: each individual observation or collection was of something non-private (a car on a public road). It never addressed Carpenter and Chatrie’s post-collection frameworks dealing with the aggregate and the resulting search of aggregate data.

#No user, only a subject

When applied to a vendor-operated, government-accessed ALPR model like Flock’s, Chatrie may already reveal some answers Kerr’s draft article tells lower courts to look for.

Most importantly: with ALPR, there is no user, only a subject. There is nobody who can consent, disclose, own, or control information.

That idea culls some of Chatrie’s open questions and transforms how the rest can be applied.

#The tests that drop out

The third-party doctrine as a whole is not in play, because the subject never disclosed information to a third party — at most, you might argue the license plate requirement forces the subject to make disclosures to the government, which would weigh toward a “search,” not away from it. The Slaybaugh argument tracks this: in thirty-five minutes, neither side mentioned the third-party doctrine.

To the extent voluntary disclosure turns on whether records arise from “ordinary use” of a device (a cell phone in Chatrie, a car in an ALPR case): driving on a public road is the epitome of ordinary use of a car. Waiver-by-conduct is not plausible.

The relationship test drops out too: a “novice subject” may not even be aware that (ALPR) location data is being collected. Even for a subject who is fully aware of ALPR conceptually, whether the information is collected on any particular trip is unknown: the cameras are often inconspicuous by design, and the government typically does not disclose their locations.^[5] There can be no perceived ownership of records the subject doesn’t know exist.

While that answers the relationship test in the negative, the information is not left unprotected; Carpenter protected records that, as Chatrie put it, “most cell-phone users have no awareness of” and “would never try to retrieve.” Perhaps that means the test is generally unnecessary, but we don’t need to look at that here.

#The deciding tests

The remaining two questions are about the nature of the information, and the government’s capacity to assemble the information into something revealing more than its constituent parts. The answers work in tandem, and, like a lithium-battery fire, cause the uncontrolled reaction Alito fears in his dissent. It’s wonderfully simple and destructive.

First, ALPR records are similar in substance to Google’s location history, but less avoidable (and so less voluntary) than either CSLI or location history: as Kerr’s draft notes, the defendants in Carpenter and Chatrie “could have just turned their phones off.” A subject can’t (legally) remove their license plate or turn off third-party cameras whenever they go out criming, having abortions, or visiting their therapists.

Control over the recording therefore also shapes the nature of the record. A Google user can pause or disable location history ahead of the trips he most wants kept private, like closing the blinds on the front window. An ALPR subject has no control over the recording and is compelled to display identifying information at all times.^[6]

Chatrie warned that even two hours of location data “allows officers to target one-off events of potential interest: a gun show, say, or a political rally.” A plate query against the same parking lot does the same work with less effort, and the subject, unlike the user, could not have kept that trip out of the database. If a sanitizable location history satisfies the nature test, an unsanitizable one must.

Second, the government has the capacity to easily assemble the information into something of a far more private nature. In Chatrie, a search occurred when the government accessed Chatrie’s location history; combining a location history with more information can hardly result in less private information being exposed.

Yet that’s what happens for Flock/ALPR data. The data is often accessed through Flock’s portal, or through RTCCs or fusion centers. These use the type of combining software that creates a more invasive record. Entering a license plate in software like FlockOS, Flock Nova,^[7] Fusus (now Axon’s), or Peregrine may bring up ALPR location history, the location of the vehicle on non-ALPR cameras, DMV records, criminal history information, previous 911 calls, OSINT, related drone and body cam footage, and so on.

The system queried spans databases and jurisdictions to deliver an ever-expanding aggregate of information of a highly private nature; its compiled result is not something anyone would ever expect to expose to the government through the act of driving on a public roadway. It is also exactly the “effortless” assembly of information required to satisfy the capacity test. And it answers Judge Abudu’s question from the other end: whatever the components were, the compiled result is not public.

Chatrie also distinguished Knotts because the beeper surveillance “was confined to public roads,” while “the movements that Location History reveals are not limited to public streets” (slip op. at 22). The government has already planted its flag there: its post-Chatrie supplemental authority letter in Slaybaugh leads with this passage, improperly reads footnote 10 as a safe harbor for “public-movements-only technology,”^[8] and adds that ALPR cameras don’t collect information a driver “reasonably views as his own.” But the flag doesn’t reach the queried system: the fusion aggregate of private-property cameras, 911 records, drone and body cam footage is no more confined to public streets than Location History was.

The government knows it’s on shaky ground; the same hastily-written letter tries a chiasmus: because a cell-phone user “is not to be viewed as sharing private information … which then can be freely passed on to the government,” a driver on public roads “is viewed as sharing public information which can be freely accessed by the government.” Sharing with whom? The third-party doctrine’s sharing applies to a counterparty who might turn the information over, but a driver never hands over his plate to a third party.

The last word of the government’s formulation was accessed. Not collected. (The defense’s reply letter engaged none of this.)

#Downward decomposition

Kerr’s draft raises the classification problem with a public-camera hypothetical: a massive camera system can be decomposed into arbitrarily small pieces—one jurisdiction, one database, one camera, one photograph—and a single photograph does not seem sweeping. So at what point does the capacity exist? Are judges prepared to act, in Kerr’s words, as “the program designers who say exactly how much assembly and which querying capabilities” trigger the Fourth Amendment?

It’s the question Flock and the government love to ask, and, to an extent, it is the right question for Carpenter and the mosaic theory, where the quantum obtained decides the case. But asking what “make[s] the result sufficiently sweeping to trigger the Fourth Amendment” is patently the wrong question for Chatrie.

The question treats the unit of analysis as something a court must construct after the fact. But the government has already constructed the unit: it bought or contracted for cameras, pooled them into a shared database, possibly enriched it with other data, and then queried the pooled system. Chatrie’s reasoning attaches to the system as queried, not as it might hypothetically be sharded or decomposed. A general warrant is not saved by the observation that it could have been drafted narrower.

The Court said as much: the “sweep of the official invasion is not made less because the government, with the benefit of hindsight, can pinpoint exactly which few hours of movements it wants to review” (slip op. at 21).

Porter shows decomposition at work: the Fifth Circuit conceded that ALPR’s “historical record” lets police “travel back in time to retrace a person’s whereabouts,” then held there was no search because the system at issue was ten cameras in one Mississippi town providing “only periodic information.” The court ignored the system, measured its output, and disposed of the question based on the quality and quantity of the information obtained. Chatrie rejected that:

Still more fundamentally, we have never understood Fourth Amendment protections as kicking in only once an intrusion “goes too far.” … Where the Fourth Amendment applies, it applies—regardless of “the quality or quantity of information” the government obtains. … That approach makes all the more sense when, as with Location History, officials can select the time-limited set of materials they want from an all-encompassing database.

— Chatrie, slip op. at 20–21 (citations omitted)

The government’s Slaybaugh letter shows where the quantum argument lives now: it runs Porter’s “far less detailed log” point through footnote 10’s “less ‘comprehensive’ log” phrase — the government’s team of legal necromancers attempt to revive the mosaic method using the same opinion that killed it.

Slaybaugh shows that new problem in practice. The government asked the Eleventh Circuit to measure capacity jurisdiction by jurisdiction. It discussed two counties and a handful of cameras. But at argument, the government conceded “the query was actually much broader” than those counties.^[9]

Chatrie’s message is that whenever the government queries its RTCC systems and fusion centers, it doesn’t matter whether that query seeks a detailed, most intimate, history of a person’s every move from their birth through today, or whether it seeks one snapshot image from a particular camera at a particular time. What matters under Chatrie is the government’s effortless capacity to assemble information of a private nature “from an all-encompassing database.”

That means a warrant becomes the default for querying integrated surveillance systems, such as those in RTCCs and fusion centers today. It is not a defect in the argument; it is the “seismic waves” Alito’s dissent predicted, and exactly what we need to bring equilibrium^[10] to a world where the government has already built a secret “all-encompassing database” it can query at any time, in secret, for any reason, or no reason at all.

If the government wants effortless warrantless information, it can still have it, but it must decompose its systems ahead of time — not in court, after the fact.^[11] If querying ten cameras in one Mississippi town is not a search, the government can query a system that makes only information from ten cameras in one Mississippi town accessible. Or it can get a warrant.

At its core, Chatrie requires the Fourth Amendment to be evaluated at the point where the search happens, not at the point where the collection happens. It’s the “private database,” not the “public information” that matters now.