automated license plate reader (ALPR)
Automated license plate readers (ALPRs) are digital cameras that are equipped with corresponding sensors and software. ALPRs are utilized to capture and store the license plate data of drivers. They can be either fixed or mobile: Fixed ALPRs are typically mounted on existing infrastructure, such as traffic signals and street lights, while mobile systems may be installed in vehicles or on trailers. Organizations may own these systems or license them from private third party entities. Data is collected from the ALPRs and stored in databases maintained by the organization or private entity. ALPR data may be used in criminal investigations, to help locate missing persons, and in an attempt to prevent crime.
ALPRs have often been the subject of Fourth Amendment challenges as an unreasonable search. Under the Fourth Amendment, government conduct generally constitutes a "search" when it infringes upon an individual's reasonable expectation of privacy. However, federal courts have consistently upheld the constitutionality of ALPRs. Justice Harlan’s concurrence in Katz v. United States, 389 U.S. 347 (1967) laid out the framework for Fourth Amendment “search” analysis. Before the proliferation of ALPRs, courts consistently held that individuals do not have a reasonable expectation of privacy in their license plates. See: United States v. Matthews, 615 F.2d 1279 (10th Cir. 1980), and United States v. Ellison, 462 F.3d 557 (6th Cir. 2006).
In Carpenter v. United States, 585 U.S. 296 (2018), the U.S. Supreme Court held that law enforcement needed a search warrant to access historical cell site location information (CSLI). Similarly, in Chatrie v. United States (2026), the U.S. Supreme Court ruled that geofencing is a search under the Fourth Amendment. Many defendants have argued to extend the requirement for CSLI to ALPRs, but they have not succeeded. In the case United States v. Porter (2026), the 5th Circuit Court of Appeals distinguished cell site location information from automated license plate readers in that CSLI provides 24-hour surveillance of someone, whereas an ALPR “provides a snapshot of the suspect’s location at a discrete time while traveling in an automobile on a public road.” However, recent cases have opened the door to constitutional challenges for misuse and prolonged surveillance on individuals using ALPRs without reasonable articulable suspicion of criminal activity.
For more information, see the CRS Report on Automated License Plate Readers: Background and Legal Issues.
[Last reviewed in July of 2026 by the Wex Definitions Team]
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