License Plate Scanner Cars: Understanding Automated License Plate Recognition (ALPR) Technology

Automated License Plate Readers (ALPRs), also known as License Plate Scanner Cars, are advanced, computer-vision camera systems designed for rapid and comprehensive vehicle surveillance. These systems are typically deployed in various locations, from fixed positions on street poles and traffic lights to mobile setups on police vehicles and trailers. License plate scanner cars operate by automatically capturing license plate numbers of all vehicles within their field of 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 transmitted to a central server for storage and analysis.

Vendors in the ALPR industry emphasize that the collected data provides law enforcement with powerful tools. This data can be used to retrospectively track vehicle movements, establishing if a vehicle was present at a crime scene, identifying travel patterns, and even linking vehicles that may be associated with each other. The capability for law enforcement agencies to share this vast network of information with numerous other agencies significantly amplifies the scope and impact of license plate scanner car technology.

However, the aggregation of ALPR data paints a detailed and potentially intrusive picture of a driver’s life, raising concerns about chilling effects on First Amendment rights. License plate scanner cars can inadvertently or intentionally monitor individuals visiting sensitive locations such as healthcare facilities, immigration support centers, gun stores, labor union offices, protest sites, or places of worship.

Given that vehicle owners are legally obligated to display license plates, there is no way for drivers to avoid this type of mass surveillance. The widespread use of automated license plate readers to track the movements of ordinary citizens, the vast majority of whom are not suspected of any wrongdoing, is a deeply concerning aspect of modern policing.

How License Plate Scanner Car Systems Work

License plate scanner car systems can be broadly categorized into three main types based on their deployment and functionality.

Stationary or Fixed ALPR Cameras

Stationary license plate scanner cameras mounted on a traffic signal. Fixed ALPR systems are often installed at intersections to monitor vehicle traffic.

Stationary, or fixed, license plate scanner cameras are permanently installed in specific locations. Common placements include traffic signals, utility poles, building entrances, and freeway off-ramps. These cameras are designed to capture license plates of vehicles as they pass by within their field of view.

When multiple stationary license plate scanner cameras are strategically positioned along a roadway, the collected data can reveal not only the direction of travel but also the speed of a vehicle. With a network of these cameras, law enforcement can achieve real-time vehicle tracking. Furthermore, long-term data storage enables the analysis of historical travel patterns, potentially revealing where a driver lives or works based on frequently recorded locations. Some smaller municipalities have even implemented license plate scanner cars at town entrances and exits, effectively creating a digital border where every vehicle entering or leaving is documented. In some instances, police may camouflage ALPRs as ordinary objects, such as traffic cones or even cacti, to maintain covert surveillance.

License plate scanner cars are frequently integrated with other traffic management systems, such as automated red-light and speed enforcement cameras, and are also used for electronic toll collection on roads and bridges.

Mobile ALPR Cameras

Mobile license plate scanner mounted on a police patrol car. Mobile ALPR systems expand surveillance capabilities by being deployed on vehicles.

Mobile license plate scanner cameras are typically mounted on police patrol vehicles, transforming them into license plate scanner cars. This mobile configuration allows law enforcement officers to gather license plate data while driving their regular patrols throughout a city or region. In most cases, these systems are activated at the beginning of a shift and remain operational until the shift concludes, ensuring continuous data collection.

Beyond capturing plates of moving vehicles, mobile ALPR cameras are particularly effective at recording license plates of parked cars. For example, a patrol car equipped with a license plate scanner car system can systematically drive through parking lots, rapidly collecting data from hundreds of vehicles in a matter of minutes.

Law enforcement also utilizes license plate scanner cars for a practice known as “gridding.” This involves systematically driving an ALPR-equipped vehicle along every street in a neighborhood to compile comprehensive intelligence on residents and their vehicle presence.

Furthermore, private companies like Vigilant Solutions operate license plate scanner cars to collect plate data on a large scale. This privately gathered data is then sold to law enforcement agencies and other entities, expanding the reach of vehicle surveillance beyond public sector operations.

ALPR Trailers

ALPR trailer deployed by the Walnut Creek Police Department. Trailer-based license plate scanner cars provide flexible and temporary surveillance in targeted areas.

ALPR technology is also available in trailer-mounted configurations, creating mobile license plate scanner car trailers that police can deploy to specific locations for extended periods. These ALPR trailers function similarly to fixed ALPR systems, collecting data without requiring permanent infrastructure installation. The Drug Enforcement Administration (DEA) has previously acquired these systems, often disguised as speed enforcement trailers, to monitor vehicle movements in areas along the U.S.-Mexico border. License plate scanner car trailers and vehicles have also been strategically positioned by police near gun shows and political rallies for targeted surveillance.

ALPR Databases

The vast amounts of data collected by license plate scanner cars are typically stored in databases for extended periods, often for as long as five years. These databases may be managed directly by police departments or, frequently, by private companies like Vigilant Solutions and Flock Safety. Law enforcement agencies that do not operate their own ALPR systems can access data collected by other agencies through regional sharing networks and commercial platforms operated by these private vendors. Notably, several companies operate independent ALPR databases outside of law enforcement, contracting with individuals to equip private vehicles with cameras for data collection. This commercially gathered data is then sold to various businesses, such as insurance companies, and is also available to law enforcement agencies through subscription services.

Hotlists

Law enforcement agencies frequently utilize “hotlists” within their ALPR systems. These hotlists are pre-loaded lists of license plates that the system actively searches for, such as plates associated with stolen vehicles or outstanding warrants. Police officers can also create and add to these hotlists. When a license plate scanner car detects a plate that matches an entry on a hotlist, the system instantly sends an alert to the officer in the patrol car (for mobile readers) or to the relevant agency (for fixed readers). Some hotlists include plates linked to low-level misdemeanors and even traffic violations. In some instances, agencies use these hotlists to generate revenue by targeting vehicles with unpaid citations.

What Kinds of Data a License Plate Scanner Car Collects

License plate scanner cars capture a range of data points beyond just the license plate number. Each scan includes location data and a precise timestamp of when and where the plate was recorded. Advanced systems can also identify the make and model of the vehicle. These systems are capable of processing thousands of license plates per minute, highlighting the scale of data collection. One prominent vendor boasts a dataset exceeding 6.5 billion scans, growing by an estimated 120 million data points each month.

By combining data points from multiple license plate scanner cars, it is possible to determine the direction and speed of a vehicle through triangulation. Aggregated over time, this data can reveal a detailed history of a vehicle’s movements. Sophisticated algorithms applied to this data can identify regular travel patterns and even predict future locations of a driver. The data also reveals all vehicles that have visited a specific location under surveillance.

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

In addition to license plate data, the images captured by license plate scanner cars can reveal visual information about the vehicle itself, including its make and model, and potentially even details such as vehicle color, physical damage, and bumper stickers. The images may also capture views of the vehicle’s occupants, including drivers and passengers, and the immediate surroundings, even capturing individuals entering or exiting the vehicle. Some ALPR systems create comprehensive “vehicle fingerprints” that combine license plate data with these additional visual characteristics.

How Law Enforcement Uses License Plate Scanner Car Technology

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A time-lapse visualization of the data collected by Oakland Police Department license plate scanner cars.

License plate scanner car technology gathers data indiscriminately, collecting information on millions of ordinary individuals. By mapping vehicle locations and times and tracing past movements, police can use stored data to construct detailed profiles of drivers’ lives, identifying past behavior patterns and potentially predicting future actions. This is all accomplished despite the fact that the vast majority of individuals whose license plate data is collected are not suspected of any criminal activity. Before ALPR technology, law enforcement officers had to manually record license plates, which imposed practical limitations on data collection and required officers to make selective choices about which vehicles to track. License plate scanner cars remove these limitations, enabling the tracking of all vehicles and facilitating faster, broader data collection with reduced manpower.

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

Law enforcement agencies primarily utilize license plate scanner car data for two main purposes: real-time investigations and historical investigations.

Real-time Investigations

By adding a license plate to a “hotlist,” law enforcement can leverage license plate scanner cars to automatically identify and track specific vehicles in real-time. License plates are commonly added to hotlists when a vehicle is reported stolen or associated with an outstanding warrant. Officers may also add a plate number if a vehicle has been observed at a crime scene, if the owner is a suspect in an investigation, or if the vehicle is believed to be linked to gang activity. Hotlists may also include vehicles associated with less serious offenses.

Historical Investigations

Since license plate scanner cars collect data on all vehicles, not just those on hotlists, officers can search and analyze historical data using a full or partial license plate number, or even a physical address. For instance, in the case of a convenience store robbery, an officer could input the store’s location to identify vehicles that were present nearby at the time of the incident. The officer can then investigate these plate numbers to uncover other locations where these vehicles have been recorded by license plate scanner cars, potentially establishing patterns or connections.

Training materials and policies in some jurisdictions advise officers that a hotlist alert alone may not be sufficient grounds for a vehicle stop. Officers are typically instructed to visually confirm the plate number to ensure accuracy. Failures in manual confirmation, combined with occasional machine errors, have resulted in wrongful stops.

Law enforcement agencies often cite successes such as recovering stolen vehicles and locating abducted children as positive outcomes of license plate scanner car technology. However, ALPR data has also been employed for mass enforcement of minor offenses, such as identifying uninsured vehicles or tracking individuals with unpaid court fees.

Data retention policies for license plate scanner car data vary significantly among agencies, ranging from short periods of days to several years. Notably, private companies may retain data indefinitely.

Who Sells License Plate Scanner Car Technology

Vigilant Solutions (a subsidiary of Motorola Solutions) and Flock Safety are leading vendors in the license plate scanner car technology market in the United States. Other prominent vendors include Rekor, Elsag, Axon, Perceptics, and Jenoptik.

Vigilant Solutions, through its sister company Digital Recognition Network, gains access to privately collected ALPR data through partnerships with vehicle repossession companies, who passively gather ALPR data with their own license plate scanner cars. Flock Safety has established similar partnerships with numerous homeowners associations, which provide data to law enforcement. Both Vigilant Solutions and Flock Safety facilitate data sharing among law enforcement agencies nationwide.

Threats Posed by License Plate Scanner Cars

License plate scanner car technology is a powerful surveillance tool with the potential to significantly infringe upon individual privacy and civil liberties, impacting entire communities.

There have been documented cases of abuse of this technology by law enforcement agencies. For example, police in New York City drove down a street and electronically recorded the license plates of all vehicles parked near a mosque. Police in Birmingham, UK, targeted a Muslim community with license plate scanner car surveillance while misrepresenting the nature of the project to the public. EFF analysis of Oakland Police Department ALPR data revealed that license plate scanner cars were disproportionately deployed in low-income communities and communities of color.

Moreover, misuse of law enforcement databases, including license plate information, by individual officers is a recurring concern. In one instance, a Washington, D.C. police officer pleaded guilty to extortion after accessing license plate data of vehicles near a gay bar and using the information to blackmail vehicle owners. More recently, a police lieutenant in Kechi, Kansas, was arrested for allegedly using a Flock Safety ALPR database to stalk his estranged wife.

Beyond intentional misuse, license plate scanner cars are susceptible to misreading plates, which can lead to serious consequences. In 2009, a San Francisco city worker, Denise Green, was subjected to a high-risk traffic stop, handcuffed at gunpoint, and forced to her knees because her vehicle was mistakenly identified as stolen due to a license plate reader error. Her experience led to a U.S. Ninth Circuit Court of Appeals ruling that technology alone cannot justify 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 a license plate scanner car system incorrectly flagged their vehicle as stolen.

The accumulation of ALPR data over extended periods increases its invasiveness and vulnerability to misuse and data breaches. Even U.S. Customs and Border Protection, a well-resourced federal agency, experienced a data breach at its ALPR vendor, Perceptics, resulting in sensitive data being leaked online. Implementing sensible data retention limits, clear policies governing data access within agencies, and robust audit and control processes are crucial steps to mitigate these risks. From a privacy perspective, the most effective safeguard would be for police to not retain any data when a scanned vehicle does not match a hotlist.

License plate scanner cars can also be used to target vulnerable populations, such as immigrant communities and individuals seeking or providing reproductive healthcare services.

EFF’s Work on ALPR

The EFF has been actively engaged in investigating and addressing the privacy threats associated with license plate scanner car technology since 2012 through public records requests, litigation, and legislative advocacy.

ALPR Litigation

EFF, in collaboration with the ACLU of Southern California, filed lawsuits against the Los Angeles County Sheriff’s Department and the Los Angeles Police Department after these agencies refused to release ALPR data, claiming it was exempt from the California Public Records Act as investigative records. This argument was challenged, with legal experts noting the “Orwellian” implication that all residents were under investigation. In 2017, the California Supreme Court ruled in favor of EFF and ACLU, directing the case back to the Superior Court.

EFF and the ACLU also represented local activists in a 2021 lawsuit against the Marin County Sheriff’s Office, Lagleva v. Marin County Sheriff. The lawsuit addressed the sheriff’s office’s practice of sharing ALPR data with out-of-state agencies, including Immigration & Customs Enforcement (ICE), violating California laws regulating ALPR use and prohibiting the sharing of criminal justice data for immigration enforcement. A settlement was reached where the sheriff’s office agreed to cease data sharing outside of California.

Beyond California, EFF has submitted amicus briefs in legal challenges to excessive ALPR data collection and use in Virginia, Massachusetts, and Nevada.

ALPR Accountability and Transparency

In 2015, the California legislature enacted S.B. 34, legislation mandating data protection measures for ALPR users, including access logs, public meetings prior to ALPR program implementation, usage and privacy policies, and access log maintenance. The law also restricts public agencies from selling, sharing, or transferring ALPR data except to other public agencies.

EFF has coordinated volunteer efforts to collect ALPR policies from agencies across California and to highlight agencies failing to comply with S.B. 34. Through projects like Data Driven and Data Driven 2, EFF has independently filed public records requests with 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 report substantiated many of EFF’s concerns, revealing inadequate agency policies, non-compliance with the law, and overly broad data sharing practices.