Landmark Carpenter Decision Changes the Game

In the landmark Carpenter v. United States case, the U.S. Supreme Court held that seizure of 7 days or more of historical cell-site location information (CSLI) is a search requiring a probable cause warrant under the Fourth Amendment and that the Third Party Doctrine does not defeat an individual's reasonable expectation of privacy in their locational history. While the Supreme Court’s ruling involved police tracking a suspect using location data obtained from his cellular provider, much of its reasoning applies to ALPRs as well. For example, Chief Justice Roberts wrote that because nearly everyone uses a cell phone, the government’s tracking ability “runs against everyone,” and “[o]nly the few without cell phones could escape this tireless and absolute surveillance.”

ALPR data collection is similarly indiscriminate; anyone who drives on public streets is likely to be tracked and logged in an ALPR database available to police. Roberts also pointed to law enforcement’s ability to retrieve CSLI from years in the past, creating a virtual surveillance time machine which “gives police access to a category of information otherwise unknowable.” ALPR databases, too, facilitate retrospective searches of cars whose drivers were not under suspicion at the time they were photographed by an ALPR camera. As we wrote in our amicus brief in Yang, “The confluence of these factors—detailed location data collection about a vast swath of the American population allowing retrospective searches—is why technologies like ALPRs violate expectations of privacy under the Fourth Amendment."

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Checklist for Challenging an ALPR Search

  1. File a discovery motion requesting the ALPR results, search parameters, and any audits or updates of the ALPR query system.
  2. Where law enforcement fails to obtain a probable cause search warrant for an ALPR search, file a motion to suppress based on the arguments in our Yang amicus brief.
    • If you are in the 9th Circuit, a single, unverified ALPR match (i.e. software match without visual confirmation) does not justify a traffic stop (See Green).
    • But in the Georgia Court of Appeals, an ALPR hit alone does justify a traffic stop (See Hernandez-Lopez v. State).
  3. Where law enforcement does obtain a warrant for an ALPR search, refer to our strategies for How to Challenge a CSLI Search.
  4. Challenge the foundation and authenticity of the proffered ALPR record(s).
  5. Rely on more privacy protective state laws.
    • In some states, ALPR use must meet specific statutory requirements. Some state laws may provide more privacy protection and/or a suppression remedy for statutory violations.
    • For a list of state ALPR laws, visit:

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Public Policy Arguments

This is an outline for our policy arguments as to why ALPR searches should require a probable cause search warrant under the Fourth Amendment taken from our Yang amicus brief:

ALPR Systems Across the Country Collect and Store Massive Amounts of Data That Can be Used to Identify and Track Drivers

Our vehicles take us to sensitive and private places like our homes, doctors’ offices, and places of worship. And yet, for many years now, with little to no oversight, law enforcement agencies and private companies have been quietly scanning and recording the locations of billions of vehicles’ license plates across the country.

  1. ALPRs Automatically and Indiscriminately Capture License Plate Data

    ALPRs are computer-controlled camera systems that automatically capture images of every license plate that comes into view. ALPRs can detect when a license plate enters the camera’s field, capture a photograph of the license plate, the car and its surroundings, capture an infrared image of the plate at night, and convert the image of the plate into alphanumeric data—in effect “reading” the plate.

    By design, ALPR collection is indiscriminate. ALPR data is collected on every vehicle, regardless of whether individual drivers are suspected of criminal activity. ALPR operators turn on vehicle-mounted ALPRs at the start of their shifts, and the devices scan plates continuously until operators turn off the system at the end of their shift. Fixed ALPRs have a continuous connection to an ALPR server. Vehicle plates are scanned not just while cars are in motion or parked on public roads, but also while they are parked in privately owned parking lots, on private streets, and driveways of homes. 

    • ALPRs Collect a Significant Amount of Data

      ALPR systems record extremely detailed GPS coordinates for each plate scanned. ALPR data includes not just the plate number but also a photograph of the vehicle, which may include the driver and passengers, as well as detailed location, time, and date information that can later place the vehicle and its passengers to within feet of the original scan. This data is stored in massive databases that are accessible to federal, state, and local law enforcement agencies, even where those agencies do not collect their own data or maintain their own databases. In many cases, this data is retained for more than five years—sometimes indefinitely.

    • ALPRs Collect Data on Everyone, Without Regard to Ties to Criminal Activities

      ALPRs scan vehicles regardless of any association with criminal activity. This means that the vast majority of data is collected on drivers who are under no suspicion of criminal activity or risk to public safety. Public records requests in California have revealed, for example, that out of nearly 4 million plates scanned by a Northern California regional agency, only 985 plates—0.025%—were linked to criminal activity. That means 99.975% of the data—3,995,111 plate scans—was collected from vehicles under no suspicion. Similar rates were recorded in New York (0.01%) and North Carolina (0.08%). Of the 173 agencies surveyed by EFF and MuckRock, “on average, only 0.5%—that is, one half of one percent—of license plate scans” were linked to a hotlist.

  2. ALPR Data Can Reveal Private and Personal Details About Individuals

    ALPR data can be used not just to identify and locate a particular vehicle, but also, when combined with other easily accessible data, to identify that vehicle’s owner and driver. And because ALPR data is stored for years, ALPR databases allow for retrospective searches that enable law enforcement to infer driving patterns, associations, and sensitive details about drivers’ lives. At bottom, searches of ALPR databases threaten to undermine the “degree of privacy against government that existed when the Fourth Amendment was adopted,” because they give police a capability unimaginable in the past—the ability to enter a virtual time machine and view suspects’ past movements. Carpenter, 138 S. Ct. at 2214.

  3. The Threats to Privacy and Civil Liberties from ALPRs are Well-Recognized

    It is widely understood that police tracking of the public’s movements can have a chilling effect on civil liberties and speech. The International Association of Chiefs of Police (IACP) has cautioned that ALPR technology creates the risk “that individuals will become more cautious in the exercise of their protected rights of expression, protest, association, and political participation because they consider themselves under constant surveillance.” And, indeed, communities that have faced excessive police surveillance—including ALPRs—have feared engaging in political activism, expressing religious observance, and exercising other constitutional rights.

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Reviewing Collected ALPR Data Constitutes a Fourth Amendment Search

  1. Individuals Maintain a Reasonable Expectation of Privacy in Their Movements

    “A person does not surrender all Fourth Amendment protection by venturing into the public sphere.” Carpenter, 138 S. Ct. 22 at 2217; United States v. Jones, 565 U.S. 400 (2012). As recognized by five concurring Justices in Jones and reaffirmed by the majority in Carpenter, “individuals have a reasonable expectation of privacy in the whole of their physical movements” because of the “privacies of life” those movements can reveal. Carpenter, 138 S. Ct. at 2217 (citing Jones, 565 U.S. at 430 (Alito, J., concurring in judgment)).

  2. ALPR Systems Provide the Government with Unprecedented Powers of Surveillance that Upset Traditional Expectations of Privacy

    The use of modern technology to seamlessly capture, aggregate, and search massive amounts of ALPR data is separate and distinct from the observation of a license plate of a single vehicle by an individual law enforcement officer. Innovations like ALPR systems remove many of the practical limitations associated with constant location surveillance. As Immigration and Customs Enforcement (ICE) explains, use of ALPR data “reduc[es] the work-hours required for physical surveillance.” Recognizing the potential for technologies like these to enable invasive surveillance on a mass scale, the Supreme Court has admonished lower courts to remain vigilant “to ensure that the ‘progress of science’ does not erode Fourth Amendment protections.” Carpenter, 138 S. Ct. at 2223. Automated license plate readers infringe on individuals’ expectations of privacy for much the same reason that the GPS monitoring of vehicles at issue in Jones and the tracking of cell phones in Carpenter do: they facilitate detailed, pervasive, cheap, and efficient tracking of millions of Americans in previously unthinkable ways.

    • Detailed nature of the data.

      ALPR systems capture much of the same information that we are concerned about with GPS tracking and cell-site location information. GPS coordinates associated with ALPR records can place vehicles at highly specific locations at specific times, locating an individual’s car with more precision than the cell phone data at issue in Carpenter or even the GPS tracker in Jones. ALPR location data may be accurate to within 2-4 inches of the camera and within feet of the vehicle. Moreover, ALPR data allows the government to track people to locations that reveal private information about their lives. That is because the geographical precision of ALPR data facilitates inferences about individuals’ locations in homes, offices, hotel rooms, and other spaces that receive the highest protection under the Fourth Amendment, and for which warrantless searches using both traditional and technological means are forbidden. Kyllo, 533 U.S. at 40. 

    • Indiscriminate Collection of Data

      For the vast majority of Americans, the choice to drive on public streets is not a luxury; it is “indispensable to participation in modern society.” In many parts of the country, people have no choice but to drive themselves to work, a grocery store, doctor’s office, place of worship, even in some cases to see a neighbor. In one survey, Gallup found that 84% of Americans drive frequently, and 64% drive every day. And once people drive on the public roads or even park in a privately owned lot or in their own driveway, there is little they can do to avoid having their precise location tagged by an ALPR system and made accessible to law enforcement without any suspicion of wrongdoing.

    •  Retrospective searches

      Like CSLI, the lengthy and frequently unlimited retention periods for ALPR data allow these retrospective searches. See Carpenter, 138 S. Ct. at 2218 (retention periods of up to 5 years). "Unlike the nosy neighbor who keeps an eye on comings and goings, they are ever alert, and their memory is nearly infallible.” Carpenter, 138 S. Ct. at 2219. And access to technologies like these is “remarkably easy, cheap, and efficient compared to traditional investigative tools,” id. at 2218, thereby upending traditional protections against pervasive government monitoring on which Americans have long relied.

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Searches of ALPR Databases Require a Warrant

Because ALPR data can reveal private and sensitive details about a person’s life—details that individuals reasonably expect to remain private—warrantless searches of ALPR databases by law enforcement to find evidence of criminal activity are per se unreasonable. ALPR data can be just as revealing as CSLI, and therefore individuals maintain a similar reasonable expectation of privacy in it. To prevent ALPR searches from feeding “too permeating police surveillance,” the Fourth Amendment’s warrant requirement should apply when law enforcement seeks ALPR records that, much like the GPS tracking in Jones, 565 US 400 (2012), create a detailed, encyclopedic, and effortlessly compiled history of an individual's physical movements. See Carpenter, 138 S.Ct. at 2215, 2217 ("we hold that an individual maintains a legitimate expectation of privacy in the record of his physical movements ...regardless whether those movements were disclosed to the public at large"). See also our Cell Site Location Information cases and our CSLI one-pager. For this reason, ALPR data should be subject to the same warrant requirement as CSLI. See Carpenter at 2223.

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