The Case for Warrant-Based License Plate Reader Searches

By Andrew Wheeler

As someone who frequently consults for law enforcement agencies, I believe it is paramount that we maintain a standard of reasonableness in how technology is deployed. While this stance might seem contradictory to some of my clients, I served as an expert witness for the Institute for Justice (IJ) in the case of Schmidt v City of Norfolk.

(Disclaimer: The views expressed here are mine alone and do not represent the Institute for Justice.)

The central conflict of that trial was whether the act of searching through archived Automated License Plate Reader (ALPR) data constitutes an illegal search under the law.

"Consistent with Plaintiffs’ claims in this case and controlling precedent involving mass surveillance in public spaces, ALPR surveillance could become too intrusive and run afoul of [constitutional privacy standards] at some point." — Court Judgment

The court ultimately decided that, for the time being in Norfolk, Virginia, this threshold had not yet been crossed. However, I believe the proliferation of cameras is inevitable. Therefore, the real question isn't if this data will eventually require a warrant, but rather when.

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⚖️ Legal Framework and Precedents

To understand why a warrant should be required, we have to look at how the definition of a "search" has evolved. Historically, the "third-party doctrine" suggested that you have no expectation of privacy if a private company holds your data. However, recent case law has shifted this:

1. Carpenter v US: The court ruled that police need a warrant for Cell-Site Location Information (CSLI). Because cell data is so comprehensive, it allows for the total tracking of a person's movements, violating a reasonable expectation of privacy.
2. Chatrie: This decision reaffirmed the Carpenter logic regarding "geofence warrants" (requesting all devices in a specific area X during time Y).
3. Beautiful Struggle v Baltimore: This case established that searching historical drone imagery also constitutes a search.

The Logic Flow of Privacy Law

If a city eventually places a camera at every single intersection, ALPR data becomes functionally identical to the CSLI data discussed in Carpenter.

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🛠️ The Technology: Costs and Utility

I am a former crime analyst, and I believe ALPRs are generally a smart investment. Unlike some police tech—specifically acoustic gunshot detection, which I find overpriced—ALPRs are highly cost-effective.

Tech Comparison Table

The hardware requirements are minimal; the machine learning and computation needed to identify a plate can run on a modern smartphone. The primary challenges are simply power and weatherproofing.

The ROI Calculation

While the data on whether ALPRs actually reduce crime is "meh," the low cost means they only need to facilitate a few arrests to be profitable. If we assume the value of solving a case is $V$, the return on investment can be expressed as:

$\text{ROI} = \frac{(\text{Cases Solved} \times V) - \text{Cost of Hardware}}{\text{Cost of Hardware}}$

Even if $V$ is as low as \500$, a single camera solving a few cases a year yields a positive return.

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🔍 Active Flags vs. Historical Searches

It is vital to distinguish between how these cameras are used. I am not suggesting all ALPR use be restricted, but rather specific types of queries.

• Active Flags (The "Hot List"): A stolen car drives past a camera $\rightarrow$ the system pings the PD in real-time. This is a reasonable use of tech.
• Historical Searches: An officer searches for plate XYZ1000 to see every location it has been for the last 30 days. This should require a warrant.

Example of a Historical Query

SELECT location, timestamp 
FROM alpr_archive 
WHERE license_plate = 'XYZ1000' 
AND date > CURRENT_DATE - INTERVAL '30 days';

Currently, some departments avoid this legal headache by simply not retaining data. This is a VERY BAD strategy because:

1. It doesn't actually stop illegal searches.
2. It destroys the utility of the data for legitimate, legal investigations.

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🚩 Conclusion and Path Forward

The current safeguards against the abuse of ALPR data are, quite frankly, laughable. We are heading toward a future where private systems (like an advanced version of Ring) will capture this data regardless of city policy.

My proposed solution:

• Move away from vague "policies."
• Mandate warrant procedures for historical ALPR searches via state statutes.

Ultimately, everyone—citizens and police alike—would be better served by clear, statutory legal requirements.

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