License Plate Scanning Cars: How ALPR Technology Works and Impacts Privacy

Automated License Plate Readers (ALPRs), also known as License Plate Scanning Cars, are sophisticated, computer-vision camera systems designed for high-speed capture and interpretation of vehicle license plates. These systems are typically mounted in various locations, from street poles and traffic lights to highway overpasses and even police vehicles, marking a significant advancement in surveillance technology. ALPRs work by automatically capturing license plate numbers of every vehicle that comes into their view, along with crucial contextual data such as location, date, and time. This information, often including photographs of the vehicle and sometimes even occupants, is then instantly uploaded to a central server, creating a vast and readily accessible database of vehicle movements.

Vendors of ALPR technology highlight its utility for law enforcement, suggesting that the collected data can be used to retrospectively trace a vehicle’s locations, determine its presence at crime scenes, analyze travel patterns, and identify vehicles potentially associated with each other. The capacity for law enforcement agencies to share this information across thousands of other agencies further amplifies the reach and potential impact of ALPR systems.

However, the aggregation of ALPR data presents a comprehensive and potentially intrusive picture of a driver’s life. This technology can inadvertently monitor and record movements to and from sensitive locations, such as healthcare facilities, immigration support centers, gun stores, labor union offices, protest sites, or places of worship. This raises significant concerns about the chilling effect on First Amendment rights and the potential for unwarranted surveillance of ordinary citizens.

While license plates are a mandatory requirement for public road vehicles, the indiscriminate use of license plate scanning cars to track and record the movements of millions of individuals, the vast majority of whom are not suspected of any wrongdoing, is a matter of growing concern and public debate.

Understanding How ALPR Systems Operate

Automated license plate reader systems can be categorized into three primary types, each with distinct deployment and operational characteristics.

Fixed or Stationary ALPR Cameras

Stationary automated license plate readers mounted on a traffic signal, illustrating a common fixed deployment method for traffic monitoring and law enforcement.

Stationary, or fixed, ALPR cameras are permanently installed at specific locations. These locations can range from traffic signals and utility poles to the entrances of facilities or highway off-ramps. These cameras are designed to capture license plates of vehicles in motion as they pass within the camera’s field of view.

When multiple stationary ALPR cameras are strategically positioned along a roadway, the collected data becomes more insightful. By correlating data from different cameras, it’s possible to determine not only the direction of travel but also the speed of a vehicle. With a dense network of these cameras, law enforcement can achieve near real-time vehicle tracking. Moreover, the long-term storage of this data allows for historical analysis, revealing patterns such as how frequently a particular license plate passes a specific location. This capability can enable inferences about a driver’s residence or workplace. In some instances, smaller municipalities have deployed ALPR cameras at town entrances and exits, essentially creating a digital perimeter where every vehicle entering or leaving is documented. Adding a layer of complexity, some agencies camouflage ALPRs as everyday objects like traffic cones or even desert flora like cacti to minimize public awareness of surveillance.

Fixed ALPR cameras are frequently integrated with other automated traffic management systems, including red-light and speed enforcement cameras, and are also utilized for electronic toll collection on highways and bridges.

Mobile ALPR Cameras

Mobile automated license plate reader affixed to a police patrol vehicle, showcasing the mobility and expanded surveillance reach offered by mobile ALPR systems.

Mobile ALPR systems are typically mounted on law enforcement patrol vehicles. This mobility allows officers to collect license plate data as they patrol their jurisdictions throughout their shifts. In many cases, these systems are activated at the start of a shift and remain operational until the shift concludes, continuously capturing data.

Mobile ALPRs are not limited to moving vehicles; they are also highly effective at capturing license plates of parked cars. For example, a patrol car equipped with a mobile ALPR can systematically drive through a public parking lot, recording hundreds of license plates in a matter of minutes.

Law enforcement agencies also employ ALPRs for a practice known as “gridding.” This involves systematically driving an ALPR-equipped vehicle along every street in a neighborhood to gather comprehensive data on residents’ vehicle usage.

Furthermore, private companies, such as Vigilant Solutions, utilize mobile ALPRs to collect license plate data on a large scale. This data is then compiled and sold to law enforcement agencies and other interested parties, creating a commercial market for vehicle location data.

ALPR Trailers

A speed enforcement trailer from the Walnut Creek Police Department equipped with an automated license plate reader, demonstrating the use of trailers for temporary or strategic surveillance deployments.

ALPR technology is also available in trailer-mounted configurations. These ALPR trailers can be towed and positioned in specific areas for extended periods. They function similarly to fixed ALPRs, collecting data without the need for permanent installation. The Drug Enforcement Administration (DEA), for instance, has previously acquired these systems, often disguised as speed enforcement trailers, to monitor vehicle movements in regions along the U.S.-Mexico border. Police agencies have also strategically deployed trailers or ALPR-equipped vehicles near locations such as gun shows and political rallies to monitor attendees.

ALPR Databases

The vast amounts of data collected by ALPR systems are typically stored in databases for considerable durations, often up to five years or even indefinitely. These databases can be maintained by the police departments themselves, or, increasingly, by private companies specializing in data management, such as Vigilant Solutions and Flock Safety. Law enforcement agencies that do not operate their own ALPR systems can gain access to data collected by other agencies through regional data-sharing networks and platforms managed by these private companies. Beyond law enforcement, several companies operate independent ALPR databases, contracting with individuals to install cameras on private vehicles for data collection. This commercially gathered data is then sold to various businesses, including insurance companies, but law enforcement agencies can also subscribe to access this commercial data.

Hotlists

A critical feature of ALPR systems is the use of “hotlists.” Law enforcement agencies often pre-program these systems with lists of license plates of interest, such as those associated with stolen vehicles or outstanding warrants. Officers can also create and add to these hotlists dynamically. When an ALPR camera scans a license plate that matches an entry on a hotlist, the system immediately sends an alert. For mobile ALPRs, this alert is directed to the officer in the patrol car, while for fixed ALPRs, the alert goes to the relevant agency. It’s important to note that some hotlists include plates associated with relatively minor offenses, such as low-level misdemeanors and traffic violations. In some cases, agencies utilize these hotlists to generate revenue by targeting individuals with unpaid citations.

The Scope of Data Collected by ALPRs

License plate scanning cars capture a range of data points beyond just the license plate number itself. Crucially, they record location data, along with the precise date and time each license plate is encountered. Advanced systems can also identify the make and model of the vehicle. The speed and volume of data collection are remarkable; some systems can process thousands of license plates every minute. One major vendor boasts a dataset exceeding 6.5 billion scans, growing by an estimated 120 million data points monthly.

By combining data points, ALPR systems can triangulate vehicle movements to determine direction and speed. Over time, aggregated data reveals a detailed history of a vehicle’s travels. Sophisticated algorithms can be applied to this data to identify regular travel patterns and even predict future locations. The data also reveals all vehicles that have visited a specific location.

While ALPR data typically does not directly include the driver’s name, law enforcement agencies have the capability to cross-reference license plate numbers with other databases to link plates to registered owners.

Beyond license plate data, the photographs captured by ALPR systems can contain images of the vehicle itself, its occupants (drivers and passengers), and the immediate surroundings. In some instances, they can even capture images of people entering or exiting vehicles. Certain ALPR products go further, creating “vehicle fingerprints” that include details like vehicle color, make, model, physical damage, and even bumper stickers, painting a comprehensive picture of the vehicle and potentially its owner.

How Law Enforcement Utilizes ALPR Technology

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A time-lapse visualization of ALPR data collection in Oakland, demonstrating the scale and scope of vehicle surveillance.

ALPR technology facilitates indiscriminate data collection, gathering information on the movements of millions of ordinary individuals. By mapping vehicle locations and times and tracing historical movements, law enforcement can construct detailed profiles of drivers’ lives, identifying past behavior patterns and potentially even forecasting future actions. This level of surveillance is achieved despite the fact that the vast majority of individuals whose data is collected are not suspected of any criminal activity. Prior to ALPR technology, law enforcement relied on manual license plate recording, a process that was inherently limited in scale and required officers to make selective decisions about which vehicles to track. ALPR technology eliminates these limitations, enabling comprehensive and continuous tracking of virtually all vehicles, significantly increasing data collection while reducing manpower requirements.

A 2021 report by the Electronic Frontier Foundation (EFF) analyzing data from 63 California law enforcement agencies found that only 0.05% of the data collected by ALPRs was relevant to an immediate public safety interest at the time of capture.

Law enforcement agencies primarily utilize license plate scanning cars for two main purposes: real-time investigations and historical investigations.

Real-Time Investigations

By adding a specific license plate to a “hot list,” law enforcement can use ALPR systems to automatically identify and track vehicles of interest in real-time. License plates are typically added to hotlists for reasons such as the vehicle being stolen or associated with an outstanding warrant. Plates may also be added if a vehicle has been observed at a crime scene, if the registered owner is a suspect in an investigation, or if the vehicle is believed to be linked to gang activity. Notably, hotlists often include vehicles associated with minor offenses as well.

Historical Investigations

Because ALPR systems collect data on all vehicles, not just those on hotlists, officers can search and analyze historical data using a license plate, partial plate, or physical address. For instance, in the case of a convenience store robbery, an officer could input the store’s location to identify all vehicles recorded in the vicinity at the time of the incident. The officer could then further investigate the movements of those identified vehicles by querying the database for other locations where those license plates have been scanned.

It is important to note that some jurisdictions have training materials, policies, and laws that caution officers against initiating stops solely based on a hotlist alert. Officers are typically instructed to visually confirm the license plate match. Failures to manually verify plate numbers, compounded by occasional machine errors, have resulted in wrongful stops.

While law enforcement agencies cite successes such as recovering stolen vehicles and locating abducted children as benefits of ALPR technology, ALPR data has also been used for broad enforcement of less serious offenses, including identifying uninsured vehicles or tracking individuals with outstanding court fees.

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

Key Vendors of ALPR Technology

Vigilant Solutions (a subsidiary of Motorola Solutions) and Flock Safety are prominent vendors in the ALPR technology market within the United States. Other companies in this sector include Rekor, Elsag, Axon, Perceptics, and Jenoptik.

Vigilant Solutions, through its affiliated company Digital Recognition Network, gains access to privately collected ALPR data through partnerships with vehicle repossession companies. These companies passively collect ALPR data using their own vehicles, which is then made available to Vigilant Solutions. Similarly, Flock Safety has established partnerships with numerous homeowners associations, gaining access to ALPR data collected within these communities, which is then offered to law enforcement. Both Vigilant Solutions and Flock Safety facilitate data sharing among law enforcement agencies nationwide, expanding the reach and interconnectedness of ALPR surveillance networks.

Privacy and Civil Liberty Threats Posed by ALPRs

License plate scanning cars represent a potent surveillance technology with the potential to infringe upon individual privacy and violate the rights of communities.

Instances of ALPR technology misuse by law enforcement agencies have been documented. For example, in New York City, police officers were found to have systematically recorded the license plate numbers of all vehicles parked near a mosque. In Birmingham, UK, police targeted a Muslim community with surveillance while misrepresenting the nature of the project to the public. Data obtained by the EFF from the Oakland Police Department revealed a pattern of disproportionate deployment of ALPR-equipped vehicles in low-income communities and communities of color, raising concerns about biased surveillance practices.

Furthermore, individual officers have been found to abuse law enforcement databases, including license plate information and motor vehicle records. A Washington, D.C. police officer was convicted of extortion in 1998 after using license plate lookups to identify patrons of a gay bar and then blackmailing them. More recently, a police officer in Kechi, Kansas, was arrested for allegedly accessing a Flock Safety ALPR database to stalk his estranged wife.

Beyond intentional misuse, ALPR systems are prone to errors in license plate recognition, which can lead to serious consequences. In 2009, Denise Green, an African-American city worker in San Francisco, was subjected to a high-risk traffic stop, handcuffed at gunpoint, and detained because an ALPR system incorrectly identified her car as stolen. This incident led to a Ninth Circuit Court of Appeals ruling that technological errors alone cannot justify such stops, though this ruling is not universally applied, leaving individuals vulnerable to similar errors. In another recent case in Aurora, Colorado, a group of Black youths were traumatized by police after an ALPR system mistakenly flagged their vehicle as stolen.

The long-term storage of aggregated ALPR data significantly increases its invasiveness and vulnerability to misuse and data breaches. Even U.S. Customs and Border Protection, despite its extensive resources, experienced a data breach when its ALPR vendor, Perceptics, was hacked, and sensitive data was released online. Implementing sensible data retention limits, clear policies governing data access within agencies, and robust audit and control mechanisms are crucial steps to mitigate these risks. Ideally, a strong privacy protection measure would involve the automatic deletion of data when a vehicle scan does not match a hotlist entry, minimizing the collection of data on non-suspect individuals.

License plate scanning cars also pose a risk of being used to target specific communities, such as immigrant populations and individuals seeking or providing reproductive healthcare services, raising concerns about the technology’s potential for discriminatory application.

EFF’s Advocacy and Action on ALPR Technology

The Electronic Frontier Foundation (EFF) has been actively engaged in investigating and addressing the privacy threats associated with ALPR technology since 2012 through public records requests, litigation, and legislative advocacy.

ALPR Litigation Efforts

EFF, in partnership with the ACLU of Southern California, initiated legal action against the Los Angeles County Sheriff’s Department and the Los Angeles Police Department after these agencies refused to disclose ALPR data in response to public records requests. The agencies argued that the data was exempt from the California Public Records Act as investigative records, a claim that EFF and the ACLU challenged as an overreach, suggesting it implied all residents were under investigation. In 2017, the California Supreme Court ruled in favor of EFF and the ACLU, directing the case back to the Superior Court.

In 2021, EFF and the ACLU also sued the Marin County Sheriff’s Office on behalf of local activists in the case Lagleva v. Marin County Sheriff. This 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. A settlement was reached where the sheriff agreed to cease data sharing outside of California.

Beyond California, EFF has filed amicus briefs in cases concerning ALPR data collection and usage in states like Virginia, Massachusetts, and Nevada, advocating for stronger privacy protections for drivers against license plate surveillance.

Promoting ALPR Accountability and Transparency

In 2015, the California legislature enacted S.B. 34, a bill aimed at enhancing ALPR data protection and transparency. This law mandates that ALPR users implement data protection measures, maintain access logs, hold public meetings before establishing ALPR programs, adopt usage and privacy policies, and keep access logs. It also restricts public agencies from selling, sharing, or transferring ALPR data, except to other public agencies.

EFF has coordinated volunteer efforts to collect and analyze ALPR policies across California and to highlight agencies that are not complying with the law. Through projects like Data Driven and Data Driven 2, EFF has independently submitted public records requests to numerous agencies to shed light on their ALPR data practices.

In 2019, EFF successfully advocated for a California State Legislature audit of law enforcement agencies’ compliance with S.B. 34. The resulting California State Auditor’s report corroborated many of EFF’s concerns, finding that agencies were not adequately adhering to the law, lacked sufficient policies, and were sharing data too broadly.