License Plate Scanners on Police Cars: How ALPR Technology Tracks Vehicles and Impacts Privacy

Automated license plate readers (ALPRs), often referred to as license plate scanners, are sophisticated surveillance systems increasingly mounted on police cars and various infrastructures. These high-speed, computer-controlled camera systems are designed to automatically capture license plate numbers of every vehicle they encounter. Equipped with GPS and time-stamping capabilities, these scanners record the location, date, and time each license plate is scanned. This data, frequently including photographs of the vehicle and sometimes even occupants, is then transmitted to central servers, creating a vast network of vehicle movement data.

Law enforcement agencies utilize this collected information for various purposes, as vendors like Vigilant Solutions highlight. ALPR data can reconstruct a vehicle’s past locations, determine its presence at crime scenes, identify travel patterns, and potentially link vehicles of interest. Agencies often share this information across a wide network, as reported by The Desert Sun, expanding the reach and impact of this surveillance technology.

However, the widespread use of license plate scanners on police cars raises significant privacy concerns. Aggregated ALPR data can create a detailed and intimate portrait of a driver’s life, potentially chilling activities protected by the First Amendment. The technology can inadvertently target individuals visiting sensitive locations such as healthcare facilities, immigration support centers, gun stores, labor union offices, protest sites, or religious institutions.

Given that drivers are legally obligated to display license plates, the constant tracking and recording of movements by automatic license plate readers, even for law-abiding citizens, is a matter of growing concern. This article delves into the workings, applications, and implications of license plate scanners on police cars, exploring their role in modern law enforcement and the crucial privacy considerations they present.

Understanding License Plate Scanner Technology: How ALPR Systems Work

License plate scanners on police cars, and in broader applications, can be categorized into three main types, each serving unique surveillance purposes.

Mobile ALPR Cameras: Eyes on the Road

Mobile automated license plate reader on a police car.

Mobile ALPR systems are commonly mounted on police patrol cars, turning these vehicles into rolling data collection units. These license plate scanners operate continuously throughout a police officer’s shift, capturing license plate data as the car moves through city streets. The primary function of mobile ALPRs is to gather data from vehicles in motion, but they are also highly effective at scanning parked cars. Driving through parking lots, a patrol car equipped with a license plate scanner can rapidly collect data from hundreds of vehicles in a short period.

Mobile ALPRs facilitate a practice known as “gridding,” where police systematically drive through every block of a neighborhood, scanning and recording license plates to gather intelligence on residents. Furthermore, private companies like Vigilant Solutions also deploy mobile ALPRs to collect data, which is then sold to law enforcement and other entities, creating a commercial aspect to license plate surveillance.

Stationary or Fixed ALPR Cameras: Permanent Surveillance Points

Stationary automated license plate reader cameras at an intersection.

Stationary, or fixed, license plate scanners are installed in permanent locations like traffic lights, utility poles, building entrances, or freeway ramps. These cameras are designed to monitor vehicles passing by their fixed vantage point. While generally capturing vehicles in motion, the strategic placement of multiple stationary ALPR cameras along roads allows for tracking vehicle direction and speed.

A network of stationary license plate scanners can enable real-time vehicle tracking. Moreover, long-term data storage from these fixed cameras reveals patterns of movement, potentially indicating where a driver lives or works based on repeated scans in specific locations. Smaller towns sometimes utilize stationary ALPRs at town entrances and exits, effectively creating digital borders where every vehicle entering or leaving is documented. Police may even camouflage these cameras as ordinary objects, such as traffic cones or even cacti, to maintain covert surveillance. Fixed ALPR cameras are frequently integrated with red-light and speed enforcement systems and are also used for automated toll collection on highways and bridges.

ALPR Trailers: Deployable Surveillance Units

ALPR trailer used by law enforcement.

License plate scanners are also available as towable trailers, offering a mobile yet temporary surveillance solution. Police can deploy these ALPR trailers to specific locations for extended periods, collecting data much like fixed ALPRs but without the need for permanent installation. The Drug Enforcement Administration (DEA) has utilized ALPR trailers, sometimes disguised as speed enforcement trailers, to monitor vehicles in areas near the U.S.-Mexico border. Law enforcement has also deployed ALPR trailers or ALPR-equipped vehicles near gun shows and political rallies, indicating their use in monitoring specific events and gatherings.

ALPR Data: Databases, Hotlists, and Information Collected

The data captured by license plate scanners is typically stored in databases for extended periods, often up to five years. These databases may be managed by police departments themselves or by private companies like Vigilant Solutions or Flock Safety. Law enforcement agencies lacking their own ALPR infrastructure can access data collected by others through regional sharing systems and commercial networks. Notably, several private companies operate independent ALPR databases, contracting with individuals to install cameras on private vehicles and selling this data to various clients, including insurance companies and, through subscriptions, to law enforcement.

License plate scanners collect not only the license plate number but also location data, and precise date and time stamps for each scan. Advanced systems can also capture vehicle make and model. These systems are incredibly efficient, capable of scanning thousands of plates per minute. One vendor boasts a dataset exceeding 6.5 billion scans, growing by 120 million data points monthly.

By triangulating data from multiple scans, ALPR systems can determine vehicle direction and speed. Aggregated over time, this data reveals a vehicle’s complete travel history, identifying regular patterns and even predicting future movements. The data also documents all vehicles visiting specific locations. While driver names are not directly collected, law enforcement can cross-reference license plate numbers with other databases to identify vehicle owners.

In addition to license plate data, captured photographs may include images of the vehicle, vehicle occupants, the surrounding environment, and even people entering or exiting vehicles. Some systems create “vehicle fingerprints,” incorporating details like vehicle color, make, model, damage, and bumper stickers.

To enhance real-time law enforcement capabilities, agencies use “hotlists”—pre-loaded lists of license plates of interest, such as stolen vehicles or those linked to outstanding warrants. Police officers can also create custom hotlists. When a license plate scanner detects a plate on a hotlist, an immediate alert is sent to officers, enabling rapid response. However, some hotlists include minor offenses, and agencies sometimes use them for revenue generation by targeting drivers with unpaid citations.

How Law Enforcement Uses License Plate Scanner Technology

A time-lapse visualization of license plate reader data collection by police vehicles in Oakland.

License plate scanners on police cars and in other deployments gather data indiscriminately, collecting information on vast numbers of individuals. By tracking vehicle locations and times, police can construct detailed histories of drivers’ movements, potentially predicting future behavior, even though most individuals whose data is collected are not suspected of any crime. Before ALPR technology, manual license plate collection was limited, requiring officers to make choices about which vehicles to track. ALPRs remove these limitations, enabling comprehensive and continuous surveillance with reduced manpower.

A 2021 EFF report analyzing data from 63 California law enforcement agencies revealed that only 0.05% of ALPR data was relevant to immediate public safety concerns at the point of capture. Law enforcement primarily utilizes license plate readers for two main purposes:

Real-Time Investigations

By adding license plates to a hotlist, police can use ALPRs for immediate vehicle identification and tracking. Hotlists commonly include plates of stolen vehicles, vehicles associated with warrants, or those linked to crime scenes or suspects. However, hotlists may also include plates related to minor offenses.

Historical Investigations

ALPR systems retain data from all scanned vehicles, not just those on hotlists. This historical data allows officers to search using a plate number, partial plate, or location to analyze past vehicle movements. For instance, police can identify vehicles near a crime scene at a specific time by searching ALPR records for that location. They can then track those vehicles’ movements at other times and locations.

It’s important to note that while hotlist alerts can prompt police action, protocols often require officers to visually confirm plate matches to avoid errors. Machine errors and failures in manual verification have led to wrongful stops. While law enforcement emphasizes ALPRs’ role in recovering stolen vehicles and locating abducted children, the technology is also used for broader enforcement of less serious offenses, such as tracking uninsured drivers or individuals with overdue court fees. Data retention policies vary, with some agencies storing data for days and others for years, while private entities may retain data indefinitely.

Vendors of ALPR Technology

Key vendors in the license plate scanner market include Vigilant Solutions (a Motorola Solutions subsidiary) and Flock Safety. Other providers are Rekor, Elsag, Axon, Perceptics, and Jenoptik. Vigilant Solutions, through its sister company Digital Recognition Network, accesses privately collected ALPR data from repossession companies. Flock Safety partners with homeowners associations to gather data. Both Vigilant and Flock facilitate data sharing among law enforcement agencies nationwide.

Threats Posed by ALPR: Privacy and Civil Liberties

License plate scanner technology presents significant threats to individual privacy and community rights. Abuse by law enforcement agencies has been documented, including instances of New York police recording plates near a mosque and Birmingham police targeting a Muslim community. EFF data from the Oakland Police Department revealed disproportionate ALPR deployment in low-income and minority communities.

Individual officers have also misused databases containing license plate information. A Washington, D.C. officer was convicted of extortion for blackmailing vehicle owners identified near a gay bar using license plate data. More recently, a Kechi, Kansas, officer was arrested for stalking his estranged wife using a Flock Safety ALPR database.

ALPR errors can also lead to severe consequences. The case of Denise Green in San Francisco, who was pulled over at gunpoint due to an ALPR misread, highlights the risks of technological errors. Similarly, in Aurora, Colorado, Black youths were wrongly detained due to an ALPR system error. While the Ninth Circuit Court of Appeals ruled against stops based solely on technology, this ruling is not universally applied, leaving individuals vulnerable to such errors.

Long-term storage of aggregated ALPR data increases privacy invasion risks and susceptibility to misuse and data breaches. Even well-funded agencies like Customs & Border Protection have experienced data breaches, as seen with the Perceptics hack. Reasonable data retention limits, access controls, and audits are crucial to mitigate these risks. Ideally, eliminating data retention for vehicles not on hotlists would offer stronger privacy protection.

License plate scanners can also be used to target immigrant communities and individuals seeking or providing reproductive healthcare, further underscoring the potential for misuse and the need for robust safeguards.

EFF’s Work on ALPR: Advocacy for Accountability and Transparency

The Electronic Frontier Foundation (EFF) has been actively addressing the privacy threats posed by license plate scanner technology since 2012, employing public records requests, litigation, and legislative advocacy.

ALPR Litigation: Challenging Surveillance Practices

EFF, alongside the ACLU of Southern California, sued the Los Angeles County Sheriff’s Department and the LAPD to access ALPR data. The agencies initially claimed exemption under the California Public Records Act, arguing the data was for investigative purposes—a claim that EFF challenged as overly broad. The California Supreme Court ruled in favor of EFF and ACLU in 2017, compelling the agencies to disclose the data.

In Lagleva v. Marin County Sheriff, EFF sued the Marin County Sheriff’s Office in 2021 for sharing ALPR data with out-of-state agencies, including ICE, violating California law. A subsequent settlement prohibited the sheriff from further out-of-state data sharing. EFF has also filed amicus briefs in cases in Virginia, Massachusetts, and Nevada, challenging excessive ALPR data collection and use.

ALPR Accountability and Transparency: Promoting Policy and Oversight

In 2015, California passed S.B. 34, requiring ALPR operators to implement data protection measures, maintain access logs, conduct public meetings before program initiation, and establish usage and privacy policies. The law also restricts data sharing to public agencies. EFF has actively monitored ALPR policy compliance in California and highlighted agencies failing to adhere to the law. Through projects like Data Driven and Data Driven 2, EFF has used public records requests to increase transparency around ALPR use. EFF’s advocacy led to a 2019 California state audit of law enforcement ALPR practices, which confirmed EFF’s concerns regarding policy gaps and data oversharing.