So while I work with police departments regularly, I think it is critically important that technology be used reasonably.

While this may be off-putting to some of my clients, I worked with the Institute for Justice as an expert witness in their trial Schmidt v City of Norfolk. (Any opinions herein are my own and not those of IJ, to be clear.) The gist of that case was whether searches of historically cached ALPR data (automated-license-plate-reader) constituted an illegal search.¹

The judge ruled against plaintiffs in that case. Here is a quote from the judgment:

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. But when? While a definitive answer to that question is elusive, what is readily apparent to this Court is that, at least in Norfolk, Virginia, the answer is: not today.

The important point to note about this quote is “not today”. This will be a long winded post, but to try to keep it simple:

• I think cameras will become ubiquitous in the foreseeable future. So the question is not if this data will require a warrant, it is when. It is going to happen eventually under current case law.
• I think cameras are good, and can be used to reduce crime in a cost effective manner.
• There is a difference between active flags (e.g. this car is stolen and it pings the PD when it drives past a camera) vs historical searches (e.g. look to see where license plate XYZ1000 was the last 30 days).
• Requiring a warrant for historical searches will not seriously impede police investigations.
• The current status quo of not retaining data is VERY BAD; it does not prevent illegal searches, and currently limits the utility of actually using that data for legitimate investigations.
• Current standards to prevent abuse of the searching ALPR data systems are laughable.

Long story short in my opinion everyone would be better off if states just mandated warrant procedures through state statutes.

Current Legal Standards

To try to not get too much into the weeds of what historically constitutes a search, I think the easiest place to start is via Carpenter vs US. So current US case law requires police departments to obtain a warrant to request cellular providers provide law enforcement with cell phone tower pings (cell-site location information, CSLI).

This deviated from historical precedent in requiring a warrant mainly because it was private companies that had the information. Before Carpenter, mostly it was argued you did not have a reasonable expectation of privacy if a private company could access the same data. The court in Carpenter basically made a determination that cell phone data was so comprehensive it justified a different standard – that you could track the whole of a person’s movements with the detailed CSLI data. And that this level of invasiveness violated a reasonable person’s expectation of privacy. Even if Google had all that info, you did not expect them to give it away.

This opinion was reaffirmed with the recent Chatrie decision (for geofence warrants, e.g. give me a ping for all cell phones in area X and datetime-range Y). Another relevant decision to be aware of is also Beautiful Struggle v Baltimore, in which searching historical aerial imagery via drones also constituted a search.

So this is why I am saying the question is when, not if, ALPR data will require a warrant. If a city happened to have a camera on literally every intersection (which I think will happen in the future), under current case law it would clearly be the same situation as you have for your cell phone data.

Cameras are Good

To be brief, again I mostly work with police departments in my career and was a former crime analyst. I do think ALPR cameras are good investments, mainly because they are cheap enough to have a reasonable return on investment. (Note I do not think this about all police tech, I am particularly critical of the price tag for acoustic-gun-shot-detection.)

So ALPRs are well under $3,000 per camera. The machine learning models, camera, and computation necessary to flag a plate when it passes can easily fit on current cell phones. (The harder part is powering the phone and protecting it from the elements.) ALPRs for the most part just take static images and then extract out the license plate (and for some vendors extract out additional information, like car make and color).

The overall evidence that ALPRs reduce crime is pretty meh at the moment (see my slides at a Wake Libertarian talk I did in 2024), but because they are so cheap they really only need to increase a few arrests per camera to likely have a positive return on investment.

It is pretty hand-wavy, as we do not have estimates for the value of increased clearances I find persuasive. But I think saying “I would pay $500 to help solve one case” is on the low side if anything. So a single camera if it helps catch just a handful of crimes a year is likely in my opinion to be a positive ROI.

I think cameras in all public spaces are going to happen. Imagine Ring comes out with a nicer camera system for homeowners that has more comprehensive views around your house and is just as cheap. And we will ultimately be safer for it. So even for folks advocating that cities do not pay for Flock, this is coming anyway in the near future.

Historical Searches vs Active Flags

ALPRs have been around a long time. The first ones I worked with at Troy, NY when I was an analyst were in-car cameras. Basically a go pro attached to the window that alerted when an officer drove by a stolen plate.

While ALPRs initial use was always pitched as this active flagging of stolen vehicles, they were used right away to retroactively search the historical locations of plates. They had a log of every plate, lat/lon, and timestamp of when that car passed a camera.

So imagine you are conducting an investigation of Joe Schmo, you know his license plate, and then you can type in his plate and see where his car passed a camera. Based on this information, same as CSLI data, you can basically trace where Joe went, where he repeatedly visited, where he likely slept, etc. (The first time I used this at Troy, we figured out a particular individual we were actively investigating was living with his girlfriend for example. I was honestly amazed how densely filled in the map was of hits for a single plate based on the in-car cameras.)

You technically do not need to cache any data at all to accomplish this “flag a stolen vehicle” (or any other scenario where you are actively looking for a specific license plate). There are legitimate scenarios though where ALPR searches for recent data in a real time context can be very helpful.

One of the more common examples – someone robs a gas station, and they drove a vehicle. You don’t know the plate, but can look at the images that passed by the fixed location ALPRs in the time range, and then especially if you have a car description from the gas station attendant can figure out the plate associated with the vehicle.

To be clear I am not a lawyer, but in my opinion I think exigent circumstances make searching a few minutes of cached ALPR location data totally reasonable. In practice, New Hampshire’s 3 minute data retention is far too short. I could see arguments for several hours (imagine “I found a dead body on the side of the road”, that requires more time for it to be reported.) But we are meandering into the territory where it is not an active emergency “need to find someone who may have a gun and hurt people” that would justify those exigent circumstances. Those are the scenarios where getting a warrant is reasonable (no different than a geofence warrant if you do not have a plate and want to just search what cars passed by a camera within a certain date-time window, or no different than a CSLI warrant if you have an active suspect and want to search for a specific license plate).

Most states are retaining ALPR data for longer periods. While the Norfolk case was ongoing, Virginia set a standard across the state at 21 days. Before that it was up to the individual agency. It varies state by state, but states often mandate data retention around 30 days, or leave it up to the discretion of the police department.

Deleting Data does not prevent abuses

These data retention statutes are argued as a mechanism to prevent abuse. They do not accomplish this.

If you look through the cases in which officers abused the system to search for individuals, all of them searched for specific plates over-and-over again, sometimes hundreds of times.

If you retain data for 20 days, you can just go and do a search every 20 days, keep notes on the data as you so wish, and then do another search 20 days later. Getting rid of old data, in-and-of-itself, does nothing to prevent that abuse. In fact if someone is actively stalking a person, you would expect them to regularly do searches, seeing where their victim is going on a regular basis while they have access to the system.

Simultaneously, deleting data does prevent its legitimate use in long term law enforcement investigations. It is totally normal for a murder investigation to take more than 30 days to identify a suspect. Gosh, sure would be nice to be able to then query the ALPR data to show whether a person was in the vicinity of the murder. Simultaneously it could be used by the defense for exculpatory purposes (which assuredly would take longer than 30 days).

So folks advocating for deleting data as a mechanism to prevent abuse are making things worse. It does not prevent abuse, and limits the utility of ALPR for historical investigations. The only way data retention by itself prevents abuse is if you do not cache data at all (like in New Hampshire), and only use ALPRs for the active alert situation.

What Smart Regulation Looks Like

One of the reasons I say that the current standards to prevent abuse are laughable is that data retention policies and internal PD policies on when the data should be searched have been in place in most departments for years (if not a decade) at this point. The examples where searching ALPR data to stalk an intimate partner were obviously not prevented via data retention policies.

Alas, my suggestion that some data is cached for real time investigations (longer than 3 minutes), and that a warrant should be required outside of this window, does not prevent that type of abuse either. Most departments have in place reasons why a search can be conducted, and some states have specific statutes identifying impermissible reasons for conducting searches. In the Norfolk IJ case, officers, when entering a reason for a search (which was often omitted), sometimes supplied reasons that appeared prima facie illegal, such as “protest”.

Departments, even if they have a standard to do internal audits, often do not follow them. It took Tyler Dukes asking Raleigh PD for their audit results for them to even conduct their first audit.

This is a long standing problem for PDs, not just with ALPRs, but also with searching criminal history illegally. IJ collating a dozen cases of arrests of ALPR misuse across the country is not evidence these systems are working, as it is likely the case that only the most egregious abuses are ever caught.

In addition to creating state statutes to mandate that a warrant be used for historical ALPR searches, states should, at a minimum, have clear punishments for illegal searches. These should include at a minimum losing your job, and being banned from accessing the system forever. When I was a crime analyst in New York (and ditto for when I worked at DCJS), this was the standard for misusing the criminal history search database.

If there is a standard for just retaining active search data for less than 24 hours, it does present a potential simple check that should be flagged – if a specific plate or specific camera is searched twice within 2 days, it should be flagged to review more closely. Flock does have their own system to identify suspicious search history.

The bigger issue to me though is who is doing the reviewing. It does not make sense to put this on vendors, and PDs just have not seriously devoted resources to this, even in response to public criticism. This audit mechanism should be delegated to a third party, either a specific group in the state attorney general’s office, or a state criminal justice agency (like DCJS in New York).

So that of course needs to be explicitly set by state statute as well. Who is doing the auditing?

My focus so far has been on abuses via police departments themselves, but smart regulation should also specify auditing of the vendors themselves, as well as punishments if they fail to meet data standards. (I am not thinking so much TEMPEST attacks here, but more so “I left an unauthenticated endpoint willy nilly on the internet”.)

Indeed, many of the requirements I am suggesting are likely already on the books; the problem is that the entity responsible for auditing is often unspecified or lacks the resources to do the work. (Also it is often unclear what the punishments are for failing to abide by statutes. That also needs to be specifically stated.)

The Future

So while I hope (although I have no expectation) that my blog post can somehow influence current standards across the country, I think it is important to keep in mind surveillance not just as the world exists now, but how it may look in the foreseeable future.

I think states should just pull the band aid off and create statutes that require a warrant to search the historical ALPR data. (And this makes data sharing between agencies mostly moot, the real time searches only need to be done within your own jurisdiction.) Like I said at the beginning, the current case law on being able to reconstruct the whole of a person’s movements (which I think is quite reasonable) will eventually be met if the ALPR cameras become dense enough. So states can either create the statutes to dictate that a warrant is necessary themselves, or eventually have the court system thrust it upon them.

In a world filled with privately owned cameras in public spaces, I think these suggestions are still relevant. So similar to Carpenter for CSLI data, and Chatrie for geofence warrants, there should just be warrant standards for historically searching any surveillance footage. There need be no special distinction between ALPR data (public or private) or video cameras.

Even if the groups calling for the banning of Flock cameras get their way, this does not stop private owners from collecting the data. So banning Flock, by itself, does not prevent abuse of searching private cameras. Again I think it is better to just let the government retain the data (same as private vendors will retain the data), and have consistent warrant standards for police to obtain that historical data.

This, of course, is a burden to detectives. I believe that trade-off in protecting our personal liberties while still allowing police effective means to investigate cases is a reasonable one.