Police Car License Plate Scanners: Understanding ALPR Technology and Privacy Concerns

Automated License Plate Readers (ALPRs) are sophisticated, computer-driven camera systems designed for rapid data collection. You’ll typically find these systems mounted in various locations: on street poles, traffic lights, highway overpasses, mobile trailers, or directly on police squad cars. These Police Car License Plate Scanners and their counterparts automatically capture license plate numbers from all vehicles in their field of view, recording the precise location, date, and time of each capture. This data, often including photographs of the vehicle and sometimes even its occupants, is then transmitted to a central server for storage and analysis.

Vendors, like Vigilant Solutions, promote these systems by highlighting their utility for law enforcement. They argue that the collected information allows police to trace a vehicle’s past locations, determine its presence at a crime scene, analyze travel patterns, and identify potentially associated vehicles. Crucially, law enforcement agencies have the option to share this sensitive information with numerous other agencies, expanding the reach and scope of this surveillance technology.

However, the aggregation of ALPR data raises significant privacy concerns. This data can create a detailed and intimate picture of a driver’s life, potentially chilling activities protected by the First Amendment. Police car license plate scanners and other ALPR technologies can inadvertently target individuals visiting sensitive locations such as healthcare facilities, immigration services, gun stores, union offices, protest sites, or places of worship.

While drivers are legally obligated to display license plates, making vehicle identification possible, the widespread and automated tracking enabled by license plate readers is deeply concerning. These systems record the movements of millions of ordinary citizens, the vast majority of whom have no connection to any criminal activity. The indiscriminate nature of this surveillance raises critical questions about privacy and government overreach.

Decoding ALPR Systems: How They Operate

Automated license plate readers can be broadly categorized into three main types, each with its own deployment strategy and capabilities.

Stationary or Fixed ALPR Cameras

Stationary ALPR cameras mounted on a traffic signal, used for fixed location surveillance.

Stationary, or fixed, automated license plate readers are permanently installed in specific locations. These locations can range from traffic lights and utility poles to facility entrances and freeway off-ramps. These cameras are designed to capture license plates of vehicles moving within their field of view.

The strategic placement of multiple stationary ALPR cameras along a road allows for the tracking of a vehicle’s direction and speed. With a sufficient network of these cameras, law enforcement can achieve real-time vehicle tracking. Furthermore, the long-term storage of this data enables the government to analyze historical travel patterns, potentially inferring a driver’s home or work location based on frequently recorded plates in certain areas. Smaller municipalities sometimes deploy ALPR cameras at town entrances and exits, effectively creating a digital perimeter where every vehicle entering or leaving is documented. In some instances, police may camouflage ALPRs to blend into the environment, disguising them as ordinary objects like traffic cones or even cacti.

It’s also common to find ALPR cameras integrated with automated red-light and speed enforcement systems, and they are increasingly used for electronic toll collection on roads and bridges.

Mobile ALPR Cameras: The Role of Police Car License Plate Scanners

Mobile ALPR system mounted on a police car, enabling license plate scanning while on patrol.

Mobile ALPRs are frequently mounted on police patrol cars, transforming these vehicles into police car license plate scanners. This mobile configuration allows law enforcement officers to gather license plate data as they patrol city streets throughout their shifts. Typically, these systems are activated at the beginning of a shift and remain operational until the shift concludes, continuously collecting data.

Beyond capturing plates of moving vehicles, mobile ALPR cameras are particularly effective at scanning license plates of parked cars. For instance, a patrol car equipped with a police car license plate scanner can systematically drive through a public parking lot, recording hundreds of license plates in a matter of minutes.

Law enforcement also utilizes police car license plate scanners for a practice known as “gridding.” This involves systematically driving an ALPR-equipped vehicle up and down every block of a neighborhood to gather intelligence on residents and their vehicles.

Adding another layer to this data collection landscape, private vendors such as Vigilant Solutions operate mobile ALPRs to gather plate data independently. This privately collected data is then sold to law enforcement agencies and other interested parties, further expanding the reach of vehicle surveillance.

ALPR Trailers

ALPR trailer deployed by police, offering temporary surveillance capabilities in specific areas.

ALPRs are also available in trailer form, providing a mobile and temporary surveillance solution. Police can tow these trailers to specific locations and deploy them for extended periods. ALPR trailers function similarly to fixed ALPRs, collecting data without requiring permanent camera installations. The Drug Enforcement Administration has utilized these trailer systems, sometimes disguised as speed enforcement trailers, to monitor vehicles in areas along the U.S.-Mexico border. Additionally, police have parked trailers or vehicles equipped with ALPRs near gun shows and political rallies rallies to monitor attendees.

ALPR Databases: Centralizing Surveillance Data

The vast amount of data collected by ALPR systems is typically stored in databases for extended periods, often for as long as five years. These databases may be managed by police departments themselves, or increasingly, by private companies like Vigilant Solutions or Flock Safety. Law enforcement agencies that do not operate their own ALPR systems can gain access to data collected by other agencies through regional sharing systems and networks managed by these private companies. Furthermore, several companies operate independent, non-law enforcement ALPR databases. They contract with individuals to install cameras on private vehicles to collect data, which is then sold to entities like insurance companies. However, law enforcement can also subscribe to these commercial databases, gaining access to an even broader pool of surveillance information.

Hotlists: Targeted Vehicle Identification

Law enforcement agencies frequently pre-load “hotlists” of license plates into ALPR systems. These hotlists contain plates of interest, such as stolen vehicles or vehicles associated with outstanding warrants. Police officers can also create their own hotlists for specific investigations. If an ALPR camera, including a police car license plate scanner, detects a plate on a hotlist, the system immediately sends an alert to the officer in the patrol car (for mobile readers) or the relevant agency (for fixed readers). Some hotlists even include plates associated with low-level misdemeanors and traffic offenses. In some cases, agencies use these hotlists to generate revenue by targeting and stopping drivers with unpaid citations.

The Breadth of Data Collected by ALPRs

ALPRs collect a range of data, including license plate numbers, precise location data, and the exact date and time of each plate capture. Advanced systems can also identify the make and model of vehicles. These systems are incredibly efficient, capable of capturing thousands of plates per minute. One vendor boasts a dataset exceeding 6.5 billion scans, growing at a rate of 120 million data points every month.

By combining ALPR data points, it’s possible to determine a person’s direction and speed of travel through triangulation. Aggregated over time, this data reveals a vehicle’s complete travel history. Algorithms applied to this data can uncover regular travel patterns and even predict future locations. The data also exposes all vehicles that have visited a specific location, providing a comprehensive record of vehicular presence.

While ALPR data typically does not include the driver’s name directly, law enforcement can use other databases to link license plate numbers to individual identities.

Furthermore, the photographs captured by ALPRs can reveal images of the vehicle, its drivers and passengers, and the immediate surroundings—even capturing people entering and exiting the vehicle. Some advanced systems create “vehicle fingerprints,” incorporating details such as vehicle color, make, model, physical damage, and bumper stickers for more comprehensive identification.

Law Enforcement Applications of ALPR Technology

Time-lapse visualization of data collected by Oakland Police Department’s police car license plate scanners.

ALPR data collection is indiscriminate, gathering information on millions of individuals. By mapping vehicle times and locations and tracing past movements, police can utilize stored data to construct detailed profiles of drivers’ lives. This includes identifying past behavior patterns and potentially predicting future actions. This is achieved despite the fact that the vast majority of individuals whose license plate data is collected are not suspected of any crime. Prior to ALPR technology, law enforcement relied on manual license plate recording, which was limited by time and resources, forcing officers to selectively track vehicles. ALPR technology removes these limitations, enabling the tracking of everyone and facilitating broader and faster data collection with reduced manpower.

A 2021 report by the EFF analyzed data from 63 California law enforcement agencies and found that only 0.05% of the ALPR data collected was relevant to a public safety interest at the time of capture.

Law enforcement primarily uses license plate readers for two general purposes:

Real-time Investigations

By adding a license plate to a “hotlist,” officers can leverage ALPRs, including police car license plate scanners, to automatically identify and track specific vehicles in real time. License plates are frequently added to hotlists when a vehicle is stolen or linked to an outstanding warrant. Plates may also be added if a vehicle has been observed at a crime scene, if the owner is a suspect in a crime, or if the vehicle is believed to be associated with gang activity. Hotlists often include even low-level offenses.

Historical Investigations

Because ALPRs, including police car license plate scanners, collect data on virtually all vehicles, not just those on hotlists, officers can search and analyze historical data using a plate number, partial plate, or physical address. For example, an officer might input the location of a convenience store robbery and identify vehicles present at that location during the time of the incident. They can then trace the historical ALPR data for those plate numbers to discover other locations the vehicles have been recorded, potentially uncovering patterns or connections.

Training materials, policies, and laws in some jurisdictions caution officers that a hotlist alert alone may not justify a vehicle stop. Officers are typically instructed to visually confirm a plate number match. Failures to manually verify plates, combined with machine errors, have led to wrongful stops and detentions.

While law enforcement emphasizes the use of ALPR data in recovering stolen vehicles or finding abducted children, police have also utilized ALPR data for broad enforcement of less serious offenses. This includes searching for uninsured vehicles or tracking individuals with overdue court fees.

Data retention policies for ALPR data vary significantly across agencies, ranging from a few days to several years. However, some entities, including private companies, may retain this data indefinitely, raising further privacy concerns.

Key Vendors in the ALPR Technology Market

Vigilant Solutions (a subsidiary of Motorola Solutions) and Flock Safety are leading vendors of ALPR technology in the United States. Other significant vendors include Rekor, Elsag, Axon, Perceptics, and Jenoptik.

Vigilant Solutions, through its sister company Digital Recognition Network, provides access to privately collected ALPR data obtained through partnerships with repossession companies, who passively collect ALPR data using their own vehicles. Flock Safety has established similar partnerships with numerous homeowners associations, gaining access to data from residential areas and feeding it back to law enforcement. Both Vigilant Solutions and Flock Safety offer data-sharing capabilities, enabling law enforcement agencies to exchange ALPR data across the country, creating a vast network of surveillance information.

Privacy Threats Posed by ALPR Technology

ALPR technology is a powerful surveillance tool with the potential to significantly infringe upon individual privacy and violate the rights of communities.

Numerous instances of abuse of this technology by law enforcement agencies have been documented. For example, New York City police officers drove down a street and electronically recorded the license plate numbers of every vehicle parked near a mosque. Police in Birmingham, UK, targeted a Muslim community while misrepresenting the nature of the project to the public. ALPR data obtained by the EFF from the Oakland Police Department revealed that police disproportionately deploy ALPR-equipped vehicles, including police car license plate scanners, in low-income communities and communities of color, raising concerns about discriminatory surveillance practices.

Moreover, individual officers have abused law enforcement databases, including license plate information and motor vehicle department records. In 1998, a Washington, D.C. police officer “pleaded guilty to extortion after looking up the plates of vehicles near a gay bar and blackmailing the vehicle owners.” More recently, an officer in Kechi, Kansas was arrested for allegedly accessing a Flock Safety ALPR database to stalk his estranged wife, highlighting the potential for misuse in personal contexts.

Beyond intentional misuse, ALPR systems are not infallible and can misread plates, leading to serious and unjust consequences. In 2009, Denise Green, an African-American city worker in San Francisco, was pulled over, handcuffed at gunpoint, forced to her knees, and searched, along with her vehicle, because an ALPR system incorrectly identified her car as stolen due to a plate reader error. Her experience led the U.S. Ninth Circuit Court of Appeals to rule that technology alone cannot be the sole basis for such a stop, although this ruling is not universally applied, leaving individuals vulnerable to similar errors. More recently, in Aurora, Colorado, a group of Black youths were traumatized by police after an ALPR system wrongly flagged their vehicle as stolen.

The accumulation of ALPR data over extended periods, or indefinitely, increases its invasiveness and vulnerability to misuse and data breaches. Even Customs & Border Protection, a well-resourced law enforcement agency, experienced a data breach when its ALPR vendor, Perceptics, was hacked and sensitive data published online. Implementing sensible data retention limits, clear policies regarding data access within agencies, and robust audit and control processes could mitigate some of these risks. One of the most effective privacy safeguards would be for police to automatically delete data when a vehicle does not match a hotlist, minimizing the collection of data on innocent individuals.

Automated license plate readers can also be used to target immigrant communities and individuals seeking or providing reproductive health services. This raises concerns about the potential for ALPR technology to be weaponized against vulnerable populations.

EFF’s Advocacy and Action on ALPR Technology

The Electronic Frontier Foundation (EFF) has been actively investigating and challenging the privacy threats posed by ALPR technology through public records requests, litigation, and legislative advocacy since 2012.

ALPR Litigation: Fighting for Transparency and Accountability

EFF and the ACLU of Southern California jointly sued the Los Angeles County Sheriff’s Department and the Los Angeles Police Department after these agencies refused to release ALPR data to public records requests. The agencies argued that the records were exempt from the California Public Records Act because they were classified as investigative records. This argument was challenged as implying that all residents of Los Angeles were under investigation, a point that was described as “Orwellian” by both an LAPD lawyer and a California Supreme Court Justice during oral arguments. In a significant victory in 2017, the California Supreme Court ruled in favor of EFF and the ACLU, ordering the case back to the Superior court and setting a precedent for public access to ALPR data.

EFF and the ACLU also filed a lawsuit against the Marin County Sheriff’s Office in 2021, Lagleva v. Marin County Sheriff, on behalf of local activists. The lawsuit challenged the sheriff’s practice of sharing ALPR data with out-of-state agencies, including Immigration & Customs Enforcement (ICE), in violation of California laws regulating ALPR use and prohibiting the sharing of criminal justice data for immigration enforcement. As part of a settlement, the sheriff agreed to stop sharing data outside of California, limiting the reach of this surveillance data.

Beyond California, EFF has submitted amicus briefs in a Virginia lawsuit concerning the excessive collection and storage of ALPR data, and in cases in Massachusetts and Nevada challenging law enforcement searches based on license plate data.

ALPR Accountability and Transparency Initiatives

In 2015, the California legislature passed S.B. 34, legislation designed to enhance ALPR data protection, requiring ALPR users to safeguard data, maintain access logs, hold public meetings before implementing ALPR programs, establish usage and privacy policies, and maintain access logs. The law also restricts public agencies from selling, sharing, or transferring ALPR data except to other public agencies.

EFF has coordinated volunteer efforts to gather and analyze ALPR policies across California and to expose agencies failing to comply with the law. EFF has also independently submitted public records requests to numerous agencies to shed light on their ALPR data usage through its Data Driven and Data Driven 2 projects, promoting transparency and public awareness.

In 2019, EFF successfully advocated for a California Legislature-ordered audit of law enforcement agencies’ compliance with S.B. 34. The California State Auditor subsequently released a critical report confirming many of EFF’s concerns regarding agency non-compliance, inadequate policies, and overly broad data sharing practices.