Automatic License Plate Readers: Legal Status, Policy Recommendations for Law Enforcement

Excerpts from Article by Ángel Díaz and Rachel Levinson-Waldman

from Brennan Center for Justice of New York University Law School

Auto­matic license plate read­ers use a combin­a­tion of cameras and computer soft­ware to indis­crim­in­ately scan the license plates of every car passing by. The read­ers, which can be moun­ted on station­ary poles, moving police cruis­ers, and even hand­held devices, log the time and date of each scan, the vehicle’s GPS coordin­ates, and pictures of the car…  The devices send the data to ALPR soft­ware, which can compare each plate against a desig­nated “hot list.” Such lists can include stolen cars and cars asso­ci­ated with AMBER Alerts for abduc­ted chil­dren…  These quer­ies happen auto­mat­ic­ally, though officers can also query plates manu­ally.

In addi­tion to check­ing data in real time, many cities and agen­cies retain plate inform­a­tion for future use, some­times indef­in­itely. This data can be used to plot a partic­u­lar vehicle’s vari­ous loca­tions or to identify all the cars at a given loca­tion, and it can even be analyzed to predict routes and future loca­tions of a vehicle or set of vehicles. 

Law enforce­ment use of ALPRs is rapidly expand­ing, with tens of thou­sands of read­ers in use through­out the United States; one survey indic­ates that in 2016 and 2017 alone, 173 law enforce­ment agen­cies collect­ively scanned 2.5 billion license plates. 

Despite this expans­ive data collec­tion effort, many depart­ments have not developed a policy to govern the use of ALPR tech­no­logy, or provided privacy protec­tions. While states such as Cali­for­nia and Nebraska have passed laws requir­ing their depart­ments to estab­lish ALPR policies, not all depart­ments have complied.

Law enforce­ment use of ALPR data is not limited to reads captured by depart­ments’ own devices; many depart­ments have contracts with vendors that grant them access to private data­bases contain­ing scans from private ALPRs and from other local and federal law enforce­ment agen­cies. For example, Vigil­ant Solu­tions (owned by Motorola Solu­tions), a lead­ing provider of ALPR data to police based in Liver­more, Cali­for­nia, sells access to its data­base of more than 5 billion license plate scans collec­ted across the coun­try, includ­ing 1.5 billion reads provided by law enforce­ment agen­cies. 

Moreover, access to ALPR tools and data is not limited to law enforce­ment. For example, govern­ment agen­cies use license plate read­ers to auto­mate toll collec­tion and for pollu­tion research; busi­nesses analyze ALPR loca­tion data when assess­ing loan applic­a­tions to help verify an applic­ant’s listed home address or to detect commer­cial use of vehicles when analyz­ing insur­ance claims; and private indi­vidu­als and neigh­bor­hood asso­ci­ations can buy ALPRs for home and neigh­bor­hood secur­ity purposes. These private actors can main­tain their own hot lists of flagged license plate numbers and can share any data they collect with law enforce­ment at their discre­tion. Simil­arly, public agen­cies that collect and store ALPR data for non–law enforce­ment purposes may hold onto a data­set that proves allur­ing for police depart­ments.

What Does the Law Say?

The U.S. Consti­tu­tion’s Fourth Amend­ment protects people from unreas­on­able searches and seizures. Accord­ing to the U.S. Supreme Court, the Amend­ment’s purpose “is to safe­guard the privacy and secur­ity of indi­vidu­als against arbit­rary inva­sions by govern­ment offi­cials.” Until the late 1960s, the Supreme Court ruled that Fourth Amend­ment protec­tions only applied to searches and seizures of tangible prop­erty. But in 1967, the Court expan­ded Fourth Amend­ment protec­tions, hold­ing in Katz v. U.S. (1967) that “the Fourth Amend­ment protects people, not places.” Specific­ally, the govern­ment was now prohib­ited from intrud­ing upon a person’s “reas­on­able expect­a­tion of privacy.” In other words, if an indi­vidual seeks to keep some­thing private, and that expect­a­tion of privacy is “one that soci­ety is prepared to recog­nize as reas­on­able,” the Fourth Amend­ment is triggered, and the govern­ment gener­ally must obtain a warrant suppor­ted by prob­able cause before conduct­ing a search. This approach seeks to protect the “priva­cies of life” from “arbit­rary power, ” and to “place obstacles in the way of a too permeat­ing police surveil­lance.” 

By contrast, the Court has not required a warrant or other heightened stand­ard for police officers to take pictures of indi­vidual license plates and compare them against a law enforce­ment data­base. Its reas­on­ing has been twofold. First, due to “the pervas­ive regu­la­tion of vehicles capable of trav­el­ing on the public high­ways,” there is no expect­a­tion of privacy in the content of license plates.   

Second, long­stand­ing preced­ent holds that drivers on public roads cannot expect their move­ments to be kept private from the police since they could be observed by any member of the public (though, as discussed below, this presump­tion is begin­ning to shift). In keep­ing with these doctrines, courts have regu­larly held that law enforce­ment officers may, at their discre­tion and without any suspi­cion of crim­inal activ­ity, perform at least an initial check of a license plate against a law enforce­ment data­base. 

Even so, there have long been hints that the track­ing of vehicles’ move­ments could, under some circum­stances, trig­ger Fourth Amend­ment concerns. As far back as 1979, the Supreme Court declared that “an indi­vidual oper­at­ing or trav­el­ing in an auto­mobile does not lose all reas­on­able expect­a­tion of privacy simply because the auto­mobile and its use are subject to govern­ment regu­la­tion.” Simil­arly, when the Court analyzed the use of beeper tech­no­logy in the 1980s, it distin­guished limited monit­or­ing from “twenty-four hour surveil­lance of any citizen in the coun­try,” reserving the ques­tion of whether such “drag­net type law enforce­ment prac­tices” merit the applic­a­tion of

differ­ent consti­tu­tional prin­ciples. 

The Court’s applic­a­tion of the Fourth Amend­ment has evolved signi­fic­antly in response to tech­no­lo­gical “innov­a­tions in surveil­lance tools.”   In Kyllo v. U.S. (2001), for instance, the Supreme Court held that police need a warrant before they can use a thermal imager to detect heat coming from a garage. By doing so, the Court rejec­ted a return to a “mech­an­ical inter­pret­a­tion” of the Fourth Amend­ment, under which the Consti­tu­tion would have protec­ted only against phys­ical intru­sions into a person’s private space, hold­ing instead that it was neces­sary to ensure that people were not left “at the mercy of advan­cing tech­no­logy.” Over time, the Court has ruled that law enforce­ment must obtain a warrant before search­ing a suspect’s cell phone during an arrest (even though it had previ­ously allowed warrant­less searches incid­ent to arrest), before installing a GPS tracker on an auto­mobile for long-term monit­or­ing (despite preced­ent suggest­ing that vehicu­lar move­ments are not private), and before obtain­ing histor­ical cell-site loca­tion inform­a­tion reveal­ing an indi­vidu­al’s daily move­ments (although third-party inform­a­tion can normally be obtained without a warrant). 

The reas­on­ing in these cases is instruct­ive. Take U.S. v. Jones (2012), in which the Supreme Court held that the police need a warrant in order to install a GPS track­ing device on a car and use it for exten­ded surveil­lance. In her concur­rence, Justice Sonia Soto­mayor observed that inex­pens­ive loca­tion track­ing “makes avail­able at a relat­ively low cost such a substan­tial quantum of intim­ate inform­a­tion about any person whom the Govern­ment, in its unfettered discre­tion, chooses to track” that it “may ‘alter the rela­tion­ship between citizen and govern­ment in a way that is inim­ical to demo­cratic soci­ety.’”   

Similar themes run through the Court’s decision in Carpenter v. U.S. (2018), which holds that police must get a warrant before they can obtain histor­ical inform­a­tion from cell phone providers about the loca­tion of indi­vidu­als’ mobile phones (known as cell-site loca­tion inform­a­tion, or CSLI).   The Court observed that this inform­a­tion could be used to track the minu­tiae of people’s daily lives. It reasoned that the “depth, breadth, and compre­hens­ive reach” of this data, along with “the ines­cap­able and auto­matic nature of its collec­tion” by virtue of simply carry­ing a cell phone, neces­sit­ate a warrant suppor­ted by prob­able cause.

While the Carpenter decision narrowly addresses the use of histor­ical CSLI, it provides an import­ant frame­work for analyz­ing reas­on­able expect­a­tions of privacy in the digital age. Loca­tion track­ing via ALPR data­bases raises many of the same concerns outlined in Carpenter; an applic­a­tion of its frame­work should lead courts to conclude that police must first obtain a warrant before search­ing histor­ical loca­tion inform­a­tion from ALPR data­bases.

Specific­ally, first, Carpenter instructs courts to consider the capa­city of a tech­no­logy to enable ongo­ing surveil­lance that would have been unima­gin­able before the digital age.   Just as with CSLI, auto­matic license plate read­ers enable data collec­tion that is “detailed, encyc­lo­pedic, and effort­lessly compiled.”   

A person’s phone is constantly creat­ing records simply by being powered on and connect­ing to the network. Simil­arly, ALPRs auto­mat­ic­ally collect inform­a­tion about every car that passes within their range. But while a person might turn off their cell phone while they travel, it may be almost impossible to avoid trav­el­ing some roads without expos­ing one’s vehicle to ALPRs.

Second, the Carpenter Court considered the extent to which data collec­tion is indis­crim­in­ate, target­ing not only people under invest­ig­a­tion but a much broader segment of the popu­la­tion.  While ALPR scans provide a differ­ent level of pinpoint accur­acy than CSLI, they also indis­crim­in­ately collect data about every car that passes by a license plate reader, regard­less of the driver’s connec­tion to crim­inal activ­ity. In fact, the vast major­ity of scans capture inform­a­tion about drivers who are not suspec­ted of any wrong­do­ing.   The only limit­a­tions on this ongo­ing surveil­lance of all cars trav­el­ing a public road are the number of ALPRs and the data reten­tion policies main­tained

by police or third-party vendors.

Third, the Court considered the extent to which the long-term CSLI reten­tion allowed officers to effect­ively create a time machine of a person’s move­ments. Just as with histor­ical CSLI, the long-term reten­tion of plate data allows the police to retro­act­ively track every loca­tion where a partic­u­lar car was tagged by an ALPR device. To be sure, the current scope of ALPR devices does not match the scope of cell phone towers blanket­ing the coun­try, which makes a direct compar­ison diffi­cult. Nonethe­less, the current adop­tion rate of ALPRs suggests that this tech­no­logy will continue to expand its cover­age areas. In fact, the Carpenter Court ruled that lower courts “must take account of more soph­ist­ic­ated systems that are already in use or in devel­op­ment.” 

ALPR tech­no­logy is expand­ing at a rapid rate, with grow­ing data­bases contain­ing billions of license plate scans, and with govern­mental and private ALPR devices captur­ing larger swaths of cities. Courts should consider this fore­see­able future when confron­ted with nascent uses of ALPR that appear smal­ler in scale.

Finally, the Court ruled that an inter­pret­a­tion of the Fourth Amend­ment called the third-party doctrine is inap­plic­able to histor­ical CSLI. Under the third-party doctrine, indi­vidu­als do not have a reas­on­able expect­a­tion of privacy in inform­a­tion they are deemed to have volun­tar­ily handed over to third parties. The Carpenter Court found that while this doctrine is appro­pri­ate for limited disclos­ures such as bank records or a log of dialed tele­phone numbers, it should not apply to CSLI data, which can provide a “chron­icle of a person’s phys­ical pres­ence compiled every day, every moment, over several years.” Histor­ical ALPR data simil­arly chron­icles the move­ments of all vehicles, regard­less of the registered owner’s connec­tion to a suspec­ted crime.

In light of the wide satur­a­tion of license plate read­ers, it is crit­ical that the use of these devices be accur­ate, bias-free, and protect­ive of estab­lished legal values and consti­tu­tional rights. Unfor­tu­nately, publicly avail­able inform­a­tion suggests that this is not the case. 

High error rates

Errors can arise in at least two ways — inac­cur­ate hot lists and inac­cur­ate reads. If hot lists are not updated, an indi­vidual may be pulled over when, for instance, the system incor­rectly indic­ates that a license is suspen­ded when it has actu­ally been rein­stated. Inac­cur­ate reads are surpris­ingly common as well: one random­ized control trial in Vallejo, Cali­for­nia, found that 37 percent of all ALPR “hits” from fixed read­ers (such as those attached to a street light) and 35 percent from mobile ALPRs were misreads — an aston­ish­ingly high error rate. 

In several high-profile incid­ents, drivers have been pulled over because a reader read the numbers on their license plates wrong and erro­neously tagged the vehicles as stolen.  In one instance, a Color­ado woman and several chil­dren were detained and hand­cuffed face­down on the ground after an ALPR mistook their SUV for a stolen motor­cycle from a differ­ent state. In another, the chair of Oakland’s Privacy Advis­ory Commis­sion was mistakenly stopped and detained at gunpoint after his rental car’s license plate triggered an out-of-date hit signal­ing to police officers that the car had been stolen. Even in cases where a vehicle is accur­ately flagged, it may not convey accur­ate inform­a­tion about an indi­vidual. A car can be shared among family members, among friends, or as part of a carshare; this real­ity may place low-income indi­vidu­als, who are more likely to share cars, at greater risk of misid­en­ti­fic­a­tion.

To be sure, license plate read­ers have had some high-profile successes: a man accused of stabbing several people after break­ing into a rabbi’s home during a Hanukkah celeb­ra­tion was found in part due to an alert from an ALPR device; a Tennessee girl abduc­ted by her noncustodial father was recovered when a license plate camera spot­ted his car; and police were able to use inform­a­tion from a license plate reader to help halt a string of random shoot­ings on high­ways in Kansas City, Missouri. Despite these anec­dotal successes, there has not been a thor­ough assess­ment of the tool’s value. Any such assess­ment would require consid­er­a­tion of the ALPR’s addi­tional costs and bene­fits described here.

Privacy and data secur­ity 

An extremely small percent­age of cars scanned by ALPRs — gener­ally far below 1 percent — are connec­ted to any crime or wrong­do­ing. For example, an audit found that 99.9 percent of the ALPR images stored by the LAPD are for vehicles not on a hot list at the time a license plate was scanned. Never­the­less, many juris­dic­tions keep the scans “just in case, ” stor­ing the data for anywhere from 90 days to two years or even indef­in­itely. These scans, over time, can reveal indi­vidu­als’ move­ments and help create detailed pictures of their private lives. In addi­tion to inform­a­tion gener­ated by ALPRs, police officers can also add to and store sens­it­ive inform­a­tion in the data­bases hous­ing license plate scans through open text fields and hot lists avail­able in the user inter­face. For example, the Cali­for­nia state auditor found that law enforce­ment can input inform­a­tion includ­ing personal inform­a­tion such as names, addresses, dates of birth, and phys­ical descrip­tions, and they can also store crim­inal justice inform­a­tion such as crim­inal charges and warrant inform­a­tion. License plate read­ers have also been known to capture private inform­a­tion, such as shots of chil­dren exit­ing a car in the drive­way of a home or activ­ity inside an open garage — inform­a­tion that surely should not be retained. This is inform­a­tion that goes far beyond the legit­im­ate need to find stolen cars or vehicles linked to AMBER Alerts. The ongo­ing stor­age of this wide array of sens­it­ive inform­a­tion also raises secur­ity concerns, as this inform­a­tion can be vulner­able to data breaches and hack­ing. The data secur­ity applied to ALPR data may not be commen­sur­ate with the sens­it­iv­ity of the data being held.

Data shar­ing concerns

Many vendors allow their law enforce­ment clients to share and receive ALPR data from other law enforce­ment agen­cies. For example, through Vigil­ant Solu­tions’ Law Enforce­ment Archival Report­ing Network (LEARN), police depart­ments can elect to auto­mat­ic­ally share their collec­tion of license plate reads with outside law enforce­ment part­ners that are also part of the network. These data shar­ing arrange­ments are not always made public or adequately tracked by police depart­ments, which can result in imper­miss­ible or unac­count­able shar­ing. Local laws and policies will have limited effect if they do not address auto­mated data shar­ing or if law enforce­ment cannot effect­ively control data flows in and out of their depart­ments. 

As more compan­ies sell ALPRs to homeown­ers, addi­tional data shar­ing concerns emerge. For example, this trend allows police officers to expand the reach of their surveil­lance systems by provid­ing them with access to private device feeds that may be outside the scope of law enforce­ment policies govern­ing their own equip­ment (if any exist at all). When police officers soli­cit data from private ALPR systems, the decision to share inform­a­tion is up to the indi­vidual homeowner or the private company provid­ing the service. While compan­ies may main­tain privacy policies that explain the situ­ations in which they share inform­a­tion with law enforce­ment, the policy only covers the company and the person who purchased the devices. The registered owners of vehicles tracked and logged by these private devices will not receive notice or an oppor­tun­ity to object to data shar­ing arrange­ments between police and private indi­vidu­als.

Impact on protec­ted First Amend­ment rights

Law enforce­ment agen­cies have a history of misus­ing license plate surveil­lance to monitor First Amend­ment–­pro­tec­ted activ­ity. During the 2008 pres­id­en­tial elec­tion, the Virginia State Police recor­ded the license plate numbers of attendees at polit­ical rallies for Barack Obama and Sarah Palin — and subsequently at Pres­id­ent Obama’s inaug­ur­a­tion — and kept the data for more than three years until it was purged follow­ing an opin­ion from the Virginia Attor­ney General warn­ing that ongo­ing reten­tion would viol­ate the state’s Govern­ment Data Collec­tion and Dissem­in­a­tion Prac­tices Act. 

Such surveil­lance — whether it involves the loca­tions of multiple cars that appear together in the same place, or of a single car at places like a mosque, synagogue, or rally — has a chilling effect on Amer­ic­ans’ First Amend­ment rights to freedoms of asso­ci­ation, reli­gion, and speech. 

An invest­ig­a­tion into an NYPD program that monitored mosque visit­ors’ license plates found that this surveil­lance “chilled consti­tu­tion­ally protec­ted rights — curtail­ing reli­gious prac­tice, censor­ing speech and stunt­ing polit­ical organ­iz­ing.” 

The Inter­na­tional Asso­ci­ation of Chiefs of Police has noted that ALPRs can cause people to “become more cautious in the exer­cise of their protec­ted rights of expres­sion, protest, asso­ci­ation, and polit­ical parti­cip­a­tion because they consider them­selves under constant surveil­lance.” And there is always the specter of more flag­rant abuse, such as putting a polit­ical oppon­ent’s license plate on a hot list and using it to keep track of that person’s where­abouts.