Skip Navigation

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

Summary: The proliferation of ALPR technology raises serious civil rights and civil liberties concerns. Courts, lawmakers, and technology vendors must take action.

Published: September 10, 2020
Cars in traffic
Spencer Platt/Getty

Amer­ic­ans drive. Accord­ing to one survey, 83 percent of U.S. adults drive a car at least several times a week. foot­note1_2e1jmwc 1 See Megan Brenan, “83% of U.S. Adults Drive Frequently; Fewer Enjoy It a Lot,” Gallup, July 9, 2018, In juris­dic­tions with limited or no public trans­port­a­tion, driv­ing may even rival cell phone use as a modern neces­sity. Cars connect people with work, love, school, prayer, and protest.

They also leave a data trail. Histor­ic­ally, it would have been virtu­ally impossible for law enforce­ment to routinely surveil all drivers. However, with the grow­ing use of auto­matic license plate read­ers (ALPRs), police can now receive alerts about a car’s move­ments in real time and review past move­ments at the touch of a button. ALPRs could prove valu­able in police invest­ig­a­tions and for non–law enforce­ment uses like help­ing govern­ment agen­cies to reduce traffic and curb envir­on­mental pollu­tion. But legal and policy devel­op­ments have failed to adequately address the risks posed by this highly invas­ive tech­no­logy. foot­note2_dejn1oe 2 The focus of this report is law enforce­ment use of ALPR. Although there is an import­ant discus­sion and analysis to be had regard­ing non–law enforce­ment applic­a­tions, such as ALPR use to help insti­tute conges­tion pricing or to auto­mate toll collec­tion, they are outside the scope of this report.

Recent events crys­tal­ize ongo­ing concerns. With Black Lives Matter demon­stra­tions taking place across the United States in the wake of the George Floyd and Breonna Taylor murders, law enforce­ment agen­cies large and small are deploy­ing their expans­ive surveil­lance arsen­als to monitor protest­ers. For many agen­cies, those surveil­lance tools include ALPRs, which have heightened relev­ance in local­it­ies where people must drive to protests, or if protests them­selves are occur­ring by car, as is increas­ingly happen­ing during the ongo­ing Covid-19 pandemic. foot­note3_kp91ln3 3 See, e.g., Katie Schoo­lov, “As Protests over the Killing of George Floyd Continue, Here’s How Police Use Power­ful Surveil­lance Tech to Track Them,” CNBC, June 18, 2020,­ful-surveil­lance-tech-to-track-protest­ors.html; Benjamin Wofford, “The Genius of Protest­ing in Car Cara­vans,” Wash­ing­to­nian, June 1, 2020, https://www.wash­ing­to­­ing-in-car-cara­vans/; Caroline Haskins and Ryan Mac, “Here Are the Minneapolis Police’s Tools to Identify Protest­ers,” BuzzFeed News, May 29, 2020, https://www.buzzfeed­­haskins1/george-floyd-protests-surveil­lance-tech­no­logy; and Cath­er­ine E. Shoi­chet, “They Can’t March in the Streets. So They’re Protest­ing in Their Cars Instead,” CNN, April 14, 2020,

The pandemic adds an addi­tional dimen­sion for consid­er­a­tion, as states look for creat­ive ways to control the virus’s spread. With car travel expec­ted to increase as states begin slowly loosen­ing restric­tions, ALPRs may play a larger role in law enforce­ment. foot­note4_qddm11d 4 See Haixia Wang, “41 Percent of Amer­ic­ans Say First Trip Will Be by Car within 100 Miles: Skift Research Travel Tracker,” Skift, May 1, 2020,­ic­ans-say-first-trip-will-be-by-car-within-100-miles-skift-research-travel-tracker. States such as Rhode Island have already direc­ted law enforce­ment to look for New York license plates in order to identify people who should be direc­ted to self-quar­ant­ine. foot­note5_yz7hyen 5 Prashant Gopal and Brian K. Sulli­van, “Rhode Island Police to Hunt Down New York­ers Seek­ing Refuge,” Bloomberg, March 27, 2020,–03–27/rhode-island-police-to-hunt-down-new-york­ers-seek­ing-refuge. Law enforce­ment agen­cies may look to auto­mate this process by using ALPR devices to alert officers any time an out-of-state license crosses into their local­it­ies.

This white paper explains how ALPR tech­no­logy works, focus­ing on its use by law enforce­ment agen­cies. It then analyzes both the legal and policy land­scapes, includ­ing how courts have ruled on the use of ALPRs, and how they can be expec­ted to rule in the future. Next, it outlines a series of concerns, ranging from high error rates to the impact on civil liber­ties and civil rights. Finally, it concludes with a set of recom­mend­a­tions for law enforce­ment, lawmakers, and tech­no­logy vendors to enhance trans­par­ency and account­ab­il­ity and mitig­ate the impact of this tech­no­logy on indi­vidu­als’ civil liber­ties and civil rights.

End Notes

How Do Automatic License Plate Readers Work?

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. Some versions can also snap pictures of a vehicle’s occu­pants and create unique vehicle IDs. foot­note1_d4×487i 1 Cath­er­ine Crump, You Are Being Tracked: How License Plate Read­ers Are Being Used to Record Amer­ic­ans’ Move­ments, Amer­ican Civil Liber­ties Union, New York, NY, July 2013, 4,­port-opt-v05.pdf; see also Dustin Slaughter, “Philly Police Admit They Disguised a Spy Truck as a Google Streetview Car,” Vice, May 12, 2016,; Sam Biddle, “Hacked Border Surveil­lance Firm Wants to Profile Drivers, Passen­gers, and Their ‘Likely Trip Purpose’ in New York City,” Inter­cept, July 9, 2019, https://thein­ter­­lance-percep­tics-new-york-city-drivers; and Lily Hay Newman, “New Traffic-Enforce­ment Tech Peers into Your Car and Counts Passen­gers,” Slate, April 28, 2015,­mated_vehicle_occu­pancy_detec­tion_looks_in_cars_counts_passen­gers_records.html. 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. foot­note2_qb63×1h 2 See, e.g., Crump, You Are Being Tracked, 5; Inter­na­tional Asso­ci­ation of Chiefs of Police, Privacy Impact Assess­ment Report for the Util­iz­a­tion of License Plate Read­ers, Alex­an­dria, VA, Septem­ber 2009, 2, 24–26,­­Click.aspx?fileticket=N%2BE2wvY%2F1QU%3D&tabid=87; and New York v. Davila, 901 N.Y.S.2d 787, 789 (2010) (describ­ing use of a hot list). They can also refer­ence vehicles that are listed in local and federal data­bases for reas­ons that may include unpaid park­ing tick­ets or inclu­sion in a gang data­base. foot­note3_jp4i­mas 3 See, e.g., “National Crime Inform­a­tion Center (NCIC),” Federal Bureau of Invest­ig­a­tion, Wash­ing­ton, DC, accessed August 26, 2020, (show­ing that NCIC data­base includes 21 files, includ­ing a Gang File and an Immig­ra­tion Viol­ator File); see also David J. Roberts and Meghann Casanova, Auto­mated License Plate Recog­ni­tion (ALRP) Systems: Policy and Oper­a­tional Guid­ance for Law Enforce­ment, Inter­na­tional Asso­ci­ation of Chiefs of Police, Alex­an­dria, VA, Septem­ber 2012, 6, 22,­es1/nij/grants/239604.pdf. These quer­ies happen auto­mat­ic­ally, though officers can also query plates manu­ally. foot­note4_3exy9e8 4 Roberts and Casanova, Auto­mated License Plate Recog­ni­tion Systems, 10.

In addi­tion to check­ing data in real time, many cities and agen­cies retain plate inform­a­tion for future use, some­times indef­in­itely. foot­note5_4qb5cf9 5 Roberts and Casanova, Auto­mated License Plate Recog­ni­tion Systems, 8–9. 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. foot­note6_uhxezod 6 Crump, You Are Being Tracked, 5–6; Dave Maass and Beryl Lipton, “Data Driven: What Is ALPR?” Elec­tronic Fron­tier Found­a­tion, San Fran­cisco, CA, Novem­ber 15, 2018, These tools may cost little or noth­ing for police, often because the drivers them­selves shoulder the cost of the tech­no­logy through a fee charged on top of traffic ticket costs. foot­note7_tt8znpt 7 Eric Markow­itz, “Pay This Fee, or Go to Jail: How License Plate Scan­ner Vigil­ant Solu­tions Makes [sic] Money in Texas,” Inter­na­tional Busi­ness Times, Febru­ary 3, 2016,­ner-vigil­ant-solu­tions-makes-money-texas-2290835. Notably, drivers in some juris­dic­tions can be jailed for fail­ure to pay the private company’s fee. foot­note8_6znx8hi 8 Markow­itz, “Pay This Fee, or Go to Jail.”

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. foot­note9_9aygam6 9 See Tanvi Misra, “Who’s Track­ing Your License Plate?” Bloomberg CityLab, Decem­ber 6, 2018,­mated-license-plate-read­ers-privacy-data-secur­ity-police/576904. Accord­ing to the latest avail­able numbers from the Depart­ment of Justice’s Bureau of Justice Stat­ist­ics, 93 percent of police depart­ments in cities with popu­la­tions of 1 million or more use their own ALPR systems, some of which can scan nearly 2,000 license plates per minute. foot­note10_op27kb0 10 Brian A. Reaves, Local Police Depart­ments, 2013: Equip­ment and Tech­no­logy, Bureau of Justice Stat­ist­ics, U.S. Depart­ment of Justice, Wash­ing­ton, DC, July 2015, 4, In cities with popu­la­tions of 100,000 or more, 75 percent of police depart­ments use ALPR systems. foot­note11_inzjzir 11 Reaves, Local Police Depart­ments, 4. In some of the largest U.S. cities, millions of license plates are scanned over the course of a year. foot­note12_suesiei 12 See, e.g., Dave Maass and Beryl Lipton, “Data Driven: Explore How Cops Are Collect­ing and Shar­ing Our Travel Patterns Using Auto­mated License Plate Read­ers,” Elec­tronic Fron­tier Found­a­tion, San Fran­cisco, CA, Novem­ber 15, 2018,­mated-license-plate-reader-data­set (find­ing that between 2016 and 2017, the Los Angeles County Sher­iff’s Depart­ment scanned 234.36 million license plates with a 0.22 percent hit rate, the San Bern­ardino County Sher­iff’s Depart­ment scanned 162.69 million license plates with a 0.06 percent hit rate, and the Sacra­mento Police Depart­ment scanned 116.23 million license plates with a 0.1 percent hit rate). Accord­ing to a 2020 Cali­for­nia state auditor report, the Los Angeles Police Depart­ment (LAPD) alone has accu­mu­lated more than 320 million license plate scans, and the Sacra­mento Police Depart­ment recor­ded up to 1.7 million scans in just one week. foot­note13_q4ou6tb 13 Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers: To Better Protect Indi­vidu­als’ Privacy, Law Enforce­ment Must Increase Its Safe­guards for the Data It Collects, Sacra­mento, CA, Febru­ary 2020, 12,–118.pdf. 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. foot­note14_4ho0een 14 See Cal. Civ. Code § 1798.29; Neb. Rev. Stat. § 60–3206; Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 2 (find­ing that the LAPD “has not developed an ALPR policy at all”).

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. This process creates a revolving door of license plate scans from law enforce­ment to Vigil­ant Solu­tions back to law enforce­ment agen­cies. foot­note15_7miy3yt 15 See Vasudha Talla, “Docu­ments Reveal ICE Using Driver Loca­tion Data from Local Police for Deport­a­tions,” Amer­ican Civil Liber­ties Union, New York, NY, March 13, 2019,­rants-rights/ice-and-border-patrol-abuses/docu­ments-reveal-ice-using-driver-loca­tion-data.

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. foot­note16_m9h7pl4 16 See “Elec­tronic Toll Collec­tions: Easy System Integ­ra­tion for Vehicle Recog­ni­tion,” Percep­tics, Farragut, TN, accessed August 26, 2020, https://www.percep­­tronic-toll-collec­tion/; Alexis Rivas, “Auto­matic License Plate Scan­ners Get Green Light for Pollu­tion Research in San Diego,” NBC San Diego, Novem­ber 22, 2019, https://www.nbcsan­­matic-license-plate-scan­ners-approved-for-pollu­tion-emis­sions-research-san-diego-barrio-logan-national-city/2178630/; “Solu­tions,” Digital Recog­ni­tion Network, Fort Worth, TX, accessed August 26, 2020,­tions; and Sam Dean, “Neigh­bors Are Using These Smart Cameras to Track Strangers’ Cars — and Yours,” Los Angeles Times, Septem­ber 13, 2019,­ness/story/2019–09–12/flock-safety-license-plate-read­ers-los-angeles. 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. foot­note17_o90xep1 17 Dean, “Neigh­bors Are Using These Smart Cameras to Track Strangers’ Cars.” 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.

End Notes

What Does the Law Say?

The U.S. Consti­tu­tion’s Fourth Amend­ment protects people from unreas­on­able searches and seizures. foot­note1_8w9whmk 1 State law may provide addi­tional protec­tions that go further than the guar­an­tees of the Fourth Amend­ment. See, e.g., Elec­tronic Commu­nic­a­tions Privacy Act, Cal. Penal Code § 1546. 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.” foot­note2_grid9jh 2 Carpenter v. United States, 138 S. Ct. 2206, 2213 (2018) (quot­ing Camara v. Muni­cipal Court of City and County of San Fran­cisco, 387 U.S. 523, 528 (1967)). Until the late 1960s, the Supreme Court ruled that Fourth Amend­ment protec­tions only applied to searches and seizures of tangible prop­erty. foot­note3_jtk548e 3 See Olmstead v. United States, 277 U.S. 438, 464–66 (1928). But in 1967, the Court expan­ded Fourth Amend­ment protec­tions, hold­ing in Katz v. U.S. (1967) that “the Fourth Amend­ment protects people, not places.” foot­note4_32gjkdx 4 Katz v. United States, 389 U.S. 347, 351 (1967). 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. foot­note5_j4ehk76 5 See Smith v. Mary­land, 442 U.S. 735, 740 (1979) (quot­ing Katz, 389 U.S. at 361). 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.” foot­note6_ny069fb 6 Carpenter, 138 S. Ct. at 2214 (quot­ing Boyd v. United States, 116 U.S. 616, 630 (1886); and United States v. Di Re, 332 U.S. 581, 595 (1948)).

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. foot­note7_9yab3cr 7 Cali­for­nia v. Carney, 471 U.S. 386, 392 (1985); see also New York v. Class, 475 U.S. 106, 113 (1986) (“Auto­mo­biles are justi­fi­ably the subject of pervas­ive regu­la­tion by the State. Every oper­ator of a motor vehicle must expect that the State, in enfor­cing its regu­la­tions, will intrude to some extent upon that oper­at­or’s privacy.”). 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. foot­note8_7eiqps2 8 See U.S. v. Knotts, 460 U.S. 276, 281 (1983); see also Card­well v. Lewis, 417 U.S. 583, 590 (1974) (plur­al­ity opin­ion); Town of Wood­worth, 132 So.3d 422 (La. App. 2013); State v. Davis, 239 P.3d 1002 (Ore. App. 2010); State v. Myrick, 659 A.2d 976; Davila, 901 N.Y.S.2d 787; U.S. v. Diaz-Castaneda, 494 F.3d 1146 (9th Cir. 2007); U.S. v. Ellison, 462 F.3d 557 (6th Cir. 2006); Olabi­siomo­tosho v. City of Hous­ton, 185 F.3d 521, 529 (5th Cir. 1999); United States v. Walraven, 892 F.2d 972, 974 (10th Cir. 1989); and United States v. Matthews, 615 F.2d 1279 (10th Cir. 1980).

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.” foot­note9_l4l5api 9 Delaware v. Prouse, 400 U.S. 648, 662 (1979). 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. foot­note10_c2e6fyn 10 See Knotts, 460 U.S. at 283–84.

More recently, the Court’s applic­a­tion of the Fourth Amend­ment has evolved signi­fic­antly in response to tech­no­lo­gical “innov­a­tions in surveil­lance tools.” foot­note11_kirk4fd 11 Carpenter, 138 S. Ct. at 2214. 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.” foot­note12_4whx90r 12 Kyllo v. United States, 533 U.S. 27, 35 (2001). 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). foot­note13_b3idxw9 13 Riley v. Cali­for­nia, 134 S. Ct. 2473 (2014); United States v. Jones, 132 S. Ct. 945 (2012); Carpenter, 138 S. Ct. 2206.

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.’” foot­note14_kok63tt 14 Jones, 132 S. Ct. at 956 (Soto­mayor, J., concur­ring) (quot­ing United States v. Cuevas-Perez, 640 F.3d 272, 285 (7th Cir. 2011) (Flaum, J., concur­ring)). 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). foot­note15_3jw6wiu 15 Carpenter, 138 S. Ct. at 2223. 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. foot­note16_k72w9h3 16 Carpenter, 138 S. Ct. at 2223.

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. foot­note17_0rjy6z2 17 Carpenter, 138 S. Ct. at 2218. Just as with CSLI, auto­matic license plate read­ers enable data collec­tion that is “detailed, encyc­lo­pedic, and effort­lessly compiled.” foot­note18_keg76a7 18 Carpenter, 138 S. Ct. at 2216. 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. foot­note19_8bsrlgz 19 Carpenter, 138 S. Ct. at 2216. 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. foot­note20_xlj88r2 20 See, e.g., Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 1 (find­ing that “99.9 percent of the 320 million images Los Angeles stores are for vehicles that were not on a hot list when the image was made”). 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. foot­note21_0egrs5y 21 Carpenter, 138 S. Ct. at 2210. 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.” foot­note22_ftdnppg 22 Carpenter, 138 S. Ct. at 2218–19 (quot­ing Kyllo, 533 U.S. at 36). 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. foot­note23_upcgh9b 23 Carpenter, 138 S. Ct. at 2219–20. 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. foot­note24_tijeu0t 24 See United States v. Miller, 425 U.S. 435, 443 (1976) (“The Fourth Amend­ment does not prohibit the obtain­ing of inform­a­tion revealed to a third party and conveyed by him to Govern­ment author­it­ies, even if the inform­a­tion is revealed on the assump­tion that it will be used only for a limited purpose and the confid­ence placed in the third party will not be betrayed”). 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.” foot­note25_89ean2q 25 Carpenter, 138 S. Ct. at 2220. Histor­ical ALPR data simil­arly chron­icles the move­ments of all vehicles, regard­less of the registered owner’s connec­tion to a suspec­ted crime.

The Carpenter Court also reasoned that indi­vidu­als do not truly volun­tar­ily share their loca­tion data with wire­less carri­ers; instead, the data is auto­mat­ic­ally collec­ted simply by possess­ing a cell phone — a device the Court described as “indis­pens­able to parti­cip­a­tion in modern soci­ety” — and by connect­ing to a mobile network. foot­note26_g41y­glw 26 Carpenter, 138 S. Ct. at 2210. Simil­arly, a major­ity of Amer­ic­ans rely on driv­ing in order to fully parti­cip­ate in soci­ety, and their move­ments are logged by ALPRs by virtue of simply driv­ing and park­ing on public roads. Just as the only way to avoid gener­at­ing CSLI would be to turn off a mobile device, the only way to avoid ALPR data collec­tion would be to give up driv­ing alto­gether or to keep a vehicle away from the range of a license plate reader — an impossible task in many places. foot­note27_3ws43r9 27 See Depart­ment of Home­land Secur­ity, “Privacy Impact Assess­ment for CBP License Plate Reader Tech­no­logy,” Wash­ing­ton, DC, July 6, 2020, 8,­a­tion/dhscbp­pia-049-cbp-license-plate-reader-tech­no­logy (“The only way to opt out of such surveil­lance is to avoid the impacted area, which may pose signi­fic­ant hard­ships and be gener­ally unreal­istic”). Carpenter thus suggests that the third-party doctrine is equally inap­plic­able to histor­ical loca­tion data collec­ted by ALPR devices.

Although the Supreme Court has not yet addressed whether police access to histor­ical ALPR data requires a warrant, appeals courts have begun hear­ing chal­lenges to warrant­less ALPR data­base searches. However, courts appear reluct­ant to embrace a bright-line rule that extends Carpenter to ALPR searches. The result has been a series of one-off decisions that seek to avoid direct engage­ment with the fore­see­able prolif­er­a­tion of ALPR data.

For example, in U.S. v. Yang (2020), the Ninth Circuit ruled that the defend­ant did not have stand­ing to chal­lenge govern­ment quer­ies of a private ALPR data­base for records of his rental car travels when he kept the vehicle past the contract due date in viol­a­tion of company policy. foot­note28_js2k5i4 28 United States v. Yang, No. 18–10341, 2020 WL 2110973, at *6–7 (9th Cir. May 4, 2020),­store/opin­ions/2020/05/04/18–10341.pdf. This ruling now compels defend­ants in the Ninth Circuit to prove that they had a suffi­ciently close rela­tion­ship with the prop­erty that was searched before the court will address their Fourth Amend­ment rights.

And in Common­wealth v. McCarthy (2020), the Massachu­setts Supreme Judi­cial Court ruled that while wide­spread use of ALPR devices can implic­ate a person’s Fourth Amend­ment privacy interest in the whole of their move­ments, the limited surveil­lance under­taken in that case did not viol­ate the defend­ant’s reas­on­able expect­a­tion of privacy. foot­note29_b640djb 29 Common­wealth v. McCarthy, 484 Mass. 493, 494 (2020),­setts/supreme-court/2020-sjc-12750.pdf?ts=1587124946. The case involved police officers’ use of ALPR hot list noti­fic­a­tions to track the defend­ant’s move­ments as he traveled across two bridges over the course of two months. foot­note30_npa85n4 30 McCarthy, 484 Mass. at 495. The court applied what is commonly referred to as the “mosaic theory,” where the long-term surveil­lance of a person’s public move­ments trig­gers a privacy interest that could be absent with only limited or isol­ated monit­or­ing. foot­note31_6fx7ckc 31 McCarthy, 484 Mass. at 502. The court acknow­ledged that “with enough cameras in enough loca­tions, the historic loca­tion data from an ALPR system in Massachu­setts would invade a reas­on­able expect­a­tion of privacy and consti­tute a search for consti­tu­tional purposes.” foot­note32_yn1ez9d 32 McCarthy, 484 Mass. at 506. Four ALPR devices on two bridges did not, however, rise to this level, accord­ing to the ruling. foot­note33_a4eor2n 33 McCarthy, 484 Mass. at 508–9. Lower courts will continue to face the dilemma of how to rule on the specific use of ALPRs in a given case while taking into account the Supreme Court’s admon­i­tion that courts must consider the logical evol­u­tion of these systems of surveil­lance. foot­note34_r4ir­bz7 34 See Carpenter, 138 S. Ct. at 2210 (“At any rate, the rule the Court adopts ‘must take account of more soph­ist­ic­ated systems that are already in use or in devel­op­ment,’ . . . and the accur­acy of CSLI is rapidly approach­ing GPS-level preci­sion.” (quot­ing Kyllo, 533 U.S. at 36)).

Separ­ate from Fourth Amend­ment consid­er­a­tions, courts have also considered how ALPR tech­no­logy may viol­ate privacy protec­tions under state law. For example, the Virginia Supreme Court is currently hear­ing an appeal seek­ing to reopen the substant­ive issue of whether the Fair­fax County Police Depart­ment’s use of an ALPR system to pass­ively track the move­ments of cars that were not on a hot list viol­ates the state’s Govern­ment Data Collec­tion and Dissem­in­a­tion Prac­tices Act. foot­note35_so0y434 35 See Peti­tion for Appeal, Neal v. Fair­fax County Police Depart­ment, Case No. CL-2015–5908, 295 Va. 334 (August 29, 2019),­OWOzfH. This act requires, among other things, that inform­a­tion not be collec­ted unless the need for it has been clearly estab­lished ahead of its collec­tion — a stand­ard that indis­crim­in­ate collec­tion of ALPR data cannot meet. foot­note36_wqg03md 36 Govern­ment Data Collec­tion and Dissem­in­a­tion Prac­tices Act, VA Code Ann. § 2.2–3800(C)(7). If the trial court’s ruling is upheld, the Fair­fax County Police will be required to purge ALPR data that is not linked to a crim­inal invest­ig­a­tion and to stop using ALPRs to pass­ively collect data on people who are not suspec­ted of crim­inal activ­ity. foot­note37_dd9en5d 37 Peti­tion for Appeal, Neal v. Fair­fax County Police Depart­ment, 4.

ALPRs are relev­ant to more than privacy. Courts have also considered whether an ALPR hit provides suffi­cient justi­fic­a­tion for a police officer to stop a car. In Kansas v. Glover (2020), the Supreme Court ruled that a license plate search indic­at­ing that a car’s registered owner has had his or her license revoked gives police reas­on­able suspi­cion to perform a traffic stop in the absence of inform­a­tion suggest­ing that someone other than the owner is driv­ing the vehicle. foot­note38_qxh11py 38 Kansas v. Glover, 140 S. Ct. 1183 (2020). Several state courts reached the same conclu­sion. foot­note39_7sqwmoc 39 See, e.g., Hernan­dez-Lopez v. State, 319 Ga. App. 662 (2013) (hold­ing that an ALPR alert indic­at­ing that a car’s registered owner had an outstand­ing warrant gave officers reas­on­able suspi­cion to justify a traffic stop); see also Hill v. State, 321 Ga. App. 817 (2013) (hold­ing that an ALPR alert from a device moun­ted on a police cruiser gave an officer reas­on­able suspi­cion to justify a traffic stop); and Traft v. Common­wealth, 539 S.W.3d 647 (Ky. 2018) (hold­ing that an ALPR alert indic­at­ing an active bench warrant gave an officer reas­on­able suspi­cion to justify a traffic stop).

An ALPR hit is not always a suffi­cient basis for a stop, however. foot­note40_xzrkax8 40 Green v. City and County of San Fran­cisco, 751 F.3d 1039, 1045 (9th Cir. 2014) (ruling that “an uncon­firmed hit on the ALPR does not, alone, form the reas­on­able suspi­cion neces­sary to support an invest­ig­at­ory deten­tion”). In 2014, the U.S. Court of Appeals for the Ninth Circuit considered an erro­neous ALPR alert that led to a traffic stop in which a woman was detained and held at gunpoint. foot­note41_zuukkn5 41 Green, 751 F.3d 1039 at 1041. Unlike in Glover, where an officer manu­ally searched a license plate number and confirmed that the truck he observed matched the vehicle in the data­base, in Green v. City and County of San Fran­cisco (2014), an ALPR device moun­ted on an officer’s cruiser malfunc­tioned and returned a hit for a differ­ent vehicle and license plate number than the plaintiff’s car. foot­note42_3yi64qf 42 Green, 751 F.3d 1039 at 1046. An officer radi­oed in a descrip­tion of the plaintiff’s vehicle, along with the incor­rect license plate number picked up by the ALPR device. foot­note43_u1z5cwt 43 Green, 751 F.3d 1039 at 1042. A second officer iden­ti­fied the plaintiff’s car, but did not attempt to confirm whether the radi­oed license plate number matched the plates on the plaintiff’s car. The Ninth Circuit ruled that the case could proceed on the ques­tion of whether the second officer should have taken addi­tional steps to inde­pend­ently confirm whether the ALPR device had iden­ti­fied the right car and license plate number before initi­at­ing a traffic stop. foot­note44_6cdpyt3 44 Green, 751 F.3d 1039 at 1043. The Green decision, which analyzed an ALPR system that “frequently” makes mistakes, may suggest that there are situ­ations in which reli­ance on an ALPR hit remains insuf­fi­cient to justify a traffic stop.

End Notes

Policy Concerns

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. This may explain why at least 16 states have passed laws regu­lat­ing the use of ALPRs or the use of data collec­ted by the devices. foot­note1_jqxy107 1 “Auto­mated License Plate Read­ers: State Stat­utes,” National Confer­ence of State Legis­latures, Wash­ing­ton, DC, updated June 23, 2020,­com­mu­nic­a­tions-and-inform­a­tion-tech­no­logy/state-stat­utes-regu­lat­ing-the-use-of-auto­mated-license-plate-read­ers-alpr-or-alpr-data.aspx. Some prohibit the use of ALPRs except for limited public safety purposes, whereas others estab­lish controls govern­ing their use, includ­ing mandat­ory privacy policies, limits on data reten­tion, express limits on the types of invest­ig­a­tions in which they can be used, and mandat­ory audits. foot­note2_j9gr473 2 See, e.g., Ark. Code Ann. §§ 12–12–1803 (prohib­it­ing use of ALPRs by indi­vidu­als, part­ner­ships, corpor­a­tions, asso­ci­ations, or state agen­cies, and limits use by law enforce­ment); Neb. Rev. Stat. §§ 60–3206 (requir­ing any state agency using ALPRs to adopt and post a privacy policy); Mont. Code Ann. §§ 46–5–118 (estab­lish­ing a ninety-day limit on the reten­tion of license plate data collec­ted by ALPRs); 23 Vt. Stat. Ann. §§ 1607–07 (“Deploy­ment of ALPR equip­ment by Vermont law enforce­ment agen­cies is inten­ded to provide access to law enforce­ment reports of wanted or stolen vehicles and wanted persons and to further other legit­im­ate law enforce­ment purposes.”); and Md. Public Safety Code § 3–509(b)(1)(2)(c)(2)(ii) (estab­lish­ing mandat­ory audits of ALPR systems). These regu­la­tions high­light many of the concerns around ALPRs listed below and predict many of the recom­mend­a­tions that follow.

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. foot­note7_87ix4t8 7 Jonathan Dienst et al., “Suspect in NY Hanukkah Stabbings Researched Hitler, Had 2 Knives in Car: Offi­cial,” NBC4 New York, Decem­ber 30, 2019,; Carly Moore, “License Plate Read­ers Used in Find­ing Miss­ing Tennessee Girl,” KKCO NBC11 News, August 19, 2016,; and Matt Pierce, “How Tech­no­logy Helped Crack the Kansas City High­way Shooter Case,” Los Angeles Times, April 20, 2014,­now/la-na-nn-kansas-city-high­way-shooter-20140419-story.html. 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 concerns: An extremely small percent­age of cars scanned by ALPRs — gener­ally far below 1 percent — are connec­ted to any crime or wrong­do­ing. foot­note8_glx4asj 8 See, e.g., Maass and Lipton, “Data Driven: Explore How Cops Are Collect­ing and Shar­ing Our Travel Patterns”; see also Crump, You Are Being Tracked, 13–15 (noting that “only 0.2 percent of reads [in Mary­land] were asso­ci­ated with any crime, wrong­do­ing, minor regis­tra­tion prob­lem, or even suspi­cion of a prob­lem,” and 0.05 percent of plate reads by the Minnesota State Patrol led to cita­tions or arrests). 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. foot­note9_5nza­pz6 9 Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 1. Never­the­less, many juris­dic­tions keep the scans “just in case,” stor­ing the data for anywhere from 90 days to two years or even indef­in­itely. foot­note10_qbg11ig 10 See, e.g., Crump, You Are Being Tracked, 8–9 (noting that a 2012 survey of 40 police depart­ments found that 5 percent did not retain ALPR reads; 18 percent retained data for either 30 days or less or for between two and six months; and 13 percent retained ALPR reads indef­in­itely); Seattle Police Depart­ment, 2018 Surveil­lance Impact Report: Auto­mated License Plate Recog­ni­tion (ALPR) (Patrol), Seattle Inform­a­tion Tech­no­logy, Seattle, WA, Janu­ary 31, 2019, 21,­ments/Depart­ments/Tech/Privacy/SPD%20ALPR%20(Patrol)%20-%20Fi­nal%20SIR.pdf (noting that data is retained for 90 days); San Fran­cisco Police Depart­ment, “Auto­mated License Place Recog­ni­tion Vehicles,” Depart­ment Bulletin, San Fran­cisco, CA, Septem­ber 22, 2010, https://cdn.muck­­B20DGO20­POLICIES.pdf (noting that plate scan inform­a­tion is retained for two years); Los Angeles County Sher­iff’s Depart­ment, “Auto­mated License Plate Recog­ni­tion (ALPR) Privacy Policy,” Los Angeles, CA, accessed August 26, 2020,­vacy­Po­licy.pdf (noting that plate scan inform­a­tion is retained for two years); and Shawn Musgrave, “Under Current Policy, Boston Police Can Keep Scanned License Plate Data Indef­in­itely,” Muck­rock, Octo­ber 1, 2012, https://www.muck­ These scans, over time, can reveal indi­vidu­als’ move­ments and help create detailed pictures of their private lives. foot­note11_0sqjnbh 11 See Depart­ment of Home­land Secur­ity, “Privacy Impact Assess­ment for CBP License Plate Reader Tech­no­logy,” 10 (“[A]LPR data from third party sources may, in the aggreg­ate, reveal inform­a­tion about an indi­vidu­al’s travel over time, or provide details about an indi­vidu­al’s private life, lead­ing to privacy concerns or implic­at­ing consti­tu­tion­ally-protec­ted freedoms.”),­a­tions/privacy-pia-cbp049a-cbplpr­tech­no­logy-july2020.pdf.

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. foot­note12_0jm7y6b 12 Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 18. 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. foot­note13_hkjatdx 13 See Ali Winston, “License-Plate Read­ers Let Police Collect Millions of Records on Drivers,” Reveal News (Center for Invest­ig­at­ive Report­ing, Berke­ley, CA), June 26, 2013, https://www.reveal­­ers-let-police-collect-millions-of-records-on-drivers. 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. An ACLU invest­ig­a­tion found that more than 80 local police depart­ments had set up their LEARN settings to share ALPR data with U.S. Immig­ra­tions and Customs Enforce­ment (ICE), even though the prac­tice may viol­ate local privacy laws or sanc­tu­ary policies. foot­note14_kylq4×5 14 See Neb. Rev. Stat. § 60–3206. 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. In one instance, the Cali­for­nia state auditor found that despite efforts to limit data shar­ing with ICE, confus­ing vendor settings had left three differ­ent ICE agen­cies with access to ALPR data from Marin County Sher­iff’s Office, frus­trat­ing compli­ance with a Cali­for­nia law that places controls on local police cooper­a­tion with immig­ra­tion author­it­ies. foot­note15_9f988w1 15 Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 26. Customs and Border Protec­tion also receives ALPR data from commer­cial vendors, includ­ing inform­a­tion from across the United States, “outside of the border zone in which CBP activ­it­ies take place.” foot­note16_yfupxju 16 Depart­ment of Home­land Secur­ity, “Privacy Impact Assess­ment for CBP License Plate Reader Tech­no­logy,” 6.

With public agen­cies seek­ing to collect ALPR data for uses such as toll collec­tion or envir­on­mental analysis, there are concerns that the inform­a­tion being collec­ted may be inten­tion­ally or unin­ten­tion­ally shared with law enforce­ment agen­cies. Without policies govern­ing the type of data that is collec­ted, stored, and shared, these govern­ment data sets may create a fric­tion­less data shar­ing oppor­tun­ity that frus­trates attempts to limit law enforce­ment access to ALPR data. For example, in San Diego, law enforce­ment officers regu­larly access smart street­light foot­age — in some cases, to surveil Black Lives Matter protests — even though the street­lights project was origin­ally inten­ded to assist city plan­ners and app developers. foot­note17_ixjqnj5 17 Jesse Marx, “Police Used Smart Street­light Foot­age to Invest­ig­ate Protest­ers,” Voice of San Diego, June 29, 2020, https://www.voiceof­san­­ment/police-used-smart-street­light-foot­age-to-invest­ig­ate-protest­ers/.

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. foot­note18_acglqia 18 See, e.g., “Privacy Policy for Flock Safety,” Flock Safety Group, Inc. (website), Atlanta, GA, updated Febru­ary 22, 2017, https://www.flock­ (“We may also access, use, preserve and/or disclose your personal inform­a­tion or Record­ings to law enforce­ment author­it­ies, govern­ment offi­cials, and/or third parties, if legally required to do so or if we have a good faith belief that such access, use, preser­va­tion or disclos­ure is reas­on­ably neces­sary to: (a) comply with a legal process or request; (b) enforce our Terms of Service, includ­ing invest­ig­a­tion of any poten­tial viol­a­tion thereof; (c) detect, prevent or other­wise address secur­ity, fraud or tech­nical issues; or (d) protect the rights, prop­erty or safety of Flock, its users, a third party, or the public as required or permit­ted by law.”). 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.

  • Layer­ing ALPR data with other surveil­lance systems: License plate read­ers can be used along­side other kinds of tech­no­lo­gies to facil­it­ate even more wide­spread surveil­lance. The New York City Police Depart­ment (NYPD) Domain Aware­ness System, for instance, can track the move­ments of cars and people using its 20,000 secur­ity cameras and license plate read­ers (along with face and object detec­tion tech­no­logy). foot­note19_m6pyaly 19 Rocco Para­s­can­dola and Tina Moore, “NYPD Unveils New $40 Million Super Computer System that Uses Data from Network of Cameras, License Plate Read­ers and Crime Reports,” NY Daily News, August 8, 2012,­ers-crime-reports-article-1.1132135; see also “A Conver­sa­tion with Jessica Tisch ’08: Revo­lu­tion­iz­ing Police Tech with the NYPD,” Harvard Law Today, July 17, 2019,­sa­tion-with-jessica-tisch-08 (claim­ing that the NYPD has access to 20,000 CCTV cameras). Several police depart­ments, includ­ing those in Chicago, Detroit, and Memphis, incor­por­ate ALPR tech­no­logy into Real Time Crime Centers (RTCCs) that combine license plate data with surveil­lance foot­age from thou­sands of police, school, and traffic cameras, gunshot detec­tion systems, and social media monit­or­ing. foot­note20_91uc9y5 20 Carol Robin­son, “Birm­ing­ham Police Will Fight Crime with All-Seeing Live Tech­no­logy,”, May 28, 2019,­ing­ham/2019/05/birm­ing­ham-police-will-fight-crime-with-all-seeing-live-tech­no­logy.html. The expans­ive and ongo­ing monit­or­ing that is facil­it­ated through these integ­rated systems may be incom­pat­ible with consti­tu­tional freedoms, includ­ing the right to free assembly and the right to privacy. These burdens hinder the collect­ive organ­iz­ing neces­sary to hold the govern­ment account­able to the will of the people.
  • Lack of trans­par­ency and access controls: While ALPRs are increas­ingly ubiquit­ous, many police depart­ments do not actively main­tain use policies, and there are insuf­fi­cient controls to protect against misuse. foot­note21_g5wn­hoc 21 See, e.g., Shawn Musgrave, “Massachu­setts Police Lack Policies for License Plate Scan­ners,” Muck­rock, April 10, 2013, https://www.muck­­ners-use-across-massachu­setts/. A 2016 invest­ig­a­tion by the Asso­ci­ated Press found that police officers across the coun­try abuse confid­en­tial data­bases to spy on love interests, journ­al­ists, busi­ness asso­ci­ates, and others. foot­note22_m4lcpeq 22 Sadie Gurman, “AP: Across US, Police Officers Abuse Confid­en­tial Data­bases,” Asso­ci­ated Press, Septem­ber 27, 2016,­en­tial-data­bases. For example, the invest­ig­a­tion repor­ted officers stalk­ing ex-girl­friends, look­ing up the addresses of crushes, and in one case, running searches on a journ­al­ist who wrote a series of stor­ies crit­ical of the depart­ment. ALPR data­bases could easily be put to similar use. foot­note23_hzw5q57 23 Gurman, “Police Officers Abuse Confid­en­tial Data­bases.” Police depart­ments exacer­bate this prob­lem when they fail to imple­ment access and monit­or­ing safe­guards to ensure that ALPR data is only accessed on a “need to know” basis, and that access is appro­pri­ately logged and monitored to protect against misuse. Instead, some depart­ments auto­mat­ic­ally install ALPR soft­ware on every computer assigned to staff, even when their posi­tion does not require access to this kind of inform­a­tion. foot­note24_tifs599 24 See, e.g., Auditor of the State of Cali­for­nia, Auto­mated License Plate Read­ers, 33. There is a further lack of trans­par­ency in many juris­dic­tions where vendor contracts prohibit police depart­ments from disclos­ing their use of surveil­lance systems to the public. foot­note25_sy96wf9 25 See, e.g., Cyrus Farivar, “FBI Really Does­n’t Want Anyone to Know About 'Stin­gray’ Use by Local Cops,” ArsTech­nica, Febru­ary 10, 2015, https://arstech­­gray-use-by-local-cops/.
  • Dispar­ate impact concerns: ALPRs can also be deployed to target communit­ies of color or other vulner­able popu­la­tions. The NYPD has used license plate read­ers as part of its wide­spread surveil­lance of Muslim communit­ies in the New York and New Jersey area. foot­note26_jrcdz8x 26 Adam Gold­man and Matt Apuzzo, “NYPD Defends Tactics over Mosque Spying; Records Reveal New Details on Muslim Surveil­lance,” Huff­ing­ton Post, Febru­ary 25, 2012, http://www.huff­ing­ton­; see also Hassan v. City of New York, 804 F.3d 277, 285–87 (3d Cir. 2015) (list­ing factual alleg­a­tions by plaintiffs alleging extens­ive, targeted surveil­lance of Muslim community); Diala Shamas and Nermeen Arastu, Mapping Muslims: NYPD Spying and Its Impact on Amer­ican Muslims, Muslim Amer­ican Civil Liber­ties Coali­tion, Creat­ing Law Enforce­ment Account­ab­il­ity and Respons­ib­il­ity, and Asian Amer­ican Legal Defense and Educa­tion Fund, New York, NY, 2013, 14,­ics/clin­ics/immig­ra­tion/clear/Mapping-Muslims.pdf. And an invest­ig­a­tion of license plate read­ers in Oakland, Cali­for­nia, found that they were located predom­in­antly in Black and Latino neigh­bor­hoods, despite the fact that auto­mobile crimes and offenses predom­in­antly occurred else­where. foot­note27_6024fp7 27 Dave Maass and Jeremy Gillula, What You Can Learn from Oakland’s Raw ALPR Data, Elec­tronic Fron­tier Found­a­tion, San Fran­cisco, CA, Janu­ary 21, 2015, Even the place­ment of ALPRs in “high crime” neigh­bor­hoods will likely reflect a history of biased and select­ive enforce­ment that has already led to the over-poli­cing of communit­ies of color. The guise of neut­ral surveil­lance will only rein­force these prac­tices and main­tain the attend­ant poten­tial for deadly police encoun­ters.

Some police depart­ments also incor­por­ate ALPR data into gang data­bases, which allows officers to track vehicles asso­ci­ated with suspec­ted gang members. foot­note28_o1usqgr 28 See, e.g., City of Chicago Office of Inspector General, Review of the Chicago Police Depart­ment’s “Gang Data­base,” Chicago, IL, April 2019, https://igch­­base-Review.pdf. These gang data­bases are notori­ously unre­li­able, as they rely on vague and often contra­dict­ory criteria for inclu­sion. foot­note29_3lcy9pu 29 See, e.g., Anita Chab­ria, “A Routine Police Stop Landed Him on Cali­for­ni­a’s Gang Data­base. Is It Racial Profil­ing?,” Los Angeles Times, May 9, 2019,­ics/la-pol-ca-cali­for­nia-gang-data­base-calgang-crim­inal-justice-reform-20190509-story.html. Gang data­bases, which contain tens of thou­sands of names, are almost over­whelm­ingly comprised of indi­vidu­als of color, and people frequently have no oppor­tun­ity to chal­lenge their inclu­sion. foot­note30_yau3njh 30 See Alice Speri, “New York Gang Data­base Expan­ded by 70 Percent Under Mayor Bill de Blasio,” Inter­cept, June 11, 2018, https://thein­ter­­base-expan­ded-by-70-percent-under-mayor-bill-de-blasio/. The LAPD suspen­ded use of Cali­for­ni­a’s statewide gang data­base after announ­cing audits and invest­ig­a­tions in response to alleg­a­tions that police fals­i­fied records and listed inno­cent people as gang members. foot­note31_xj7ko1s 31 Kristina Bravo, “LAPD Suspends Use of CalGang Data­base Months after Announ­cing Probe of Officers Accused of Falsi­fy­ing Inform­a­tion,” KTLA Los Angeles, Janu­ary 20, 2020,­base-months-after-announ­cing-probe-of-officers-accused-of-falsi­fy­ing-inform­a­tion/. Mean­while, an audit by Chica­go’s Office of Inspector General found that the city’s gang data­base contained incom­plete and conflict­ing data, with some entries rais­ing seri­ous concerns about how officers “perceive and treat the people with whom they inter­act.” foot­note32_2lc4xck 32 City of Chicago Office of Inspector General, Review of the Chicago Police Depart­ment’s “Gang Data­base,” 2, 6.

In the wake of nation­wide protests that followed the police killings of George Floyd and Breonna Taylor, public atten­tion has increas­ingly focused on the ongo­ing instances of police brutal­ity and racial bias in poli­cing. However, there is a risk that police depart­ments and legis­lat­ors may incor­rectly propose surveil­lance as a neut­ral altern­at­ive. Surveil­lance that dispro­por­tion­ately targets communit­ies of color carries a distinct and cogniz­able equal protec­tion harm: brand­ing them with a badge of inferi­or­ity. As one appel­late court wrote, “Our nation’s history teaches the uncom­fort­able lesson that those not on discrim­in­a­tion’s receiv­ing end can all too easily gloss over the ‘badge of inferi­or­ity’ inflic­ted by unequal treat­ment itself. Clos­ing our eyes to the real and ascer­tain­able harms of discrim­in­a­tion inev­it­ably leads to morn­ing-after regret.” foot­note33_qw33zjp 33 Hassan, 804 F.3d at 291.

  • 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. foot­note34_bdx0ot1 34 Mark Bowes, “Police Recor­ded License Plates at Obama Inaug­ur­a­tion,” Rich­mond Times-Dispatch, August 18, 2013, http://www.times­dis­–5e46–8336-d5f4ba076cb7.html. Simil­arly, police in Denver spied on anti-logging activ­ists and shared license plate inform­a­tion with the FBI’s Joint Terror­ism Task Force when the activ­ists held a train­ing on nonvi­ol­ence. foot­note35_ipgkxt0 35 Kirsten Atkins, “State­ment — Kirsten Atkins, Target of Illegal Spying,” Amer­ican Civil Liber­ties Union, New York, NY,­ment-kirsten-atkins-target-illegal-spying.

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. foot­note36_2mzwy9t 36 See, e.g., United States v. Jones, 565 U.S. 400, at 416 (“Aware­ness that the Govern­ment may be watch­ing chills asso­ci­ational and express­ive free­dom.”). 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.” foot­note37_4dfy34b 37 Shamas and Arastu, Mapping Muslims, 4. 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.” foot­note38_n2s2lf6 38 Inter­na­tional Asso­ci­ation of Chiefs of Police, Privacy Impact Assess­ment Report for the Util­iz­a­tion of License Plate Read­ers, 13. 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. foot­note39_jn5mtsr 39 See, e.g., Glenn Green­wald, “Govern­ment Harass­ing and Intim­id­at­ing Brad­ley Manning Support­ers,” Salon, Novem­ber 9, 2010,

End Notes


In light of these concerns, the need for a multi­fa­ceted response to the prolif­er­a­tion of ALPR use is over­due. This section contains a number of recom­mend­a­tions for poli­cy­makers, law enforce­ment agen­cies, and tech­no­logy vendors. These are inten­ded as start­ing points, as the circum­stances and imple­ment­a­tion of ALPR use reforms will vary by juris­dic­tion.

  • Adopt reten­tion limits and require warrants for search­ing histor­ical data: Plates that are scanned and do not match a hot list alert should be promptly discarded. Using ALPRs to record the move­ments of all vehicles in a muni­cip­al­ity goes far beyond the limited inform­a­tion that is revealed by an isol­ated capture of license plate data. It also goes beyond what an ordin­ary person could observe on a public road. Altern­at­ively, if plates are retained, the reten­tion period should be as brief as possible — on the scale of days, not months. These limits could be imposed both by depart­mental policies and by state law. If muni­cip­al­it­ies elect to permit reten­tion of license plate data to enable histor­ical searches, such searches should require a warrant suppor­ted by prob­able cause absent emer­gency situ­ations. ALPR data­bases collect expans­ive and sens­it­ive accounts of people’s move­ments regard­less of whether they are suspec­ted of crim­inal activ­ity in a manner that is not avail­able through tradi­tional surveil­lance. Aside from ceas­ing to drive alto­gether, it is exceed­ingly diffi­cult to avoid this type of surveil­lance — a condi­tion that is likely to become more pronounced as ALPR tech­no­logy contin­ues to expand.
  • Insti­tute a two-step scan­ning process: When it comes to officers indi­vidu­ally running plates, police depart­ments should tailor their scan­ning processes so that the first pass through a data­base will not yield protec­ted personal inform­a­tion, instead reveal­ing only regis­tra­tion inform­a­tion and pres­ence on a hot list. Only if that first inquiry reveals a “basis for further police action” should the officer be permit­ted to proceed to a second step, which would “allow access to the ‘per­sonal inform­a­tion’ of the registered owner, includ­ing name, address, social secur­ity number, and if avail­able, crim­inal record.” foot­note1_m3ys­m52 1 State v. Donis, 157 N.J. 44, 55 (1998). This simple process can help deter police use of these systems for purposes other than law enforce­ment.
  • Require veri­fic­a­tion of hot list data: Law enforce­ment agen­cies should enact policies that require both inde­pend­ent veri­fic­a­tion of the inform­a­tion yiel­ded from a hot list and real-time updat­ing of hot list data. These steps would help prevent erro­neous and poten­tially danger­ous stops based on incor­rect or outdated inform­a­tion.
  • Require trans­par­ency and invite community input into ALPR use and policies: The public should have an oppor­tun­ity to offer input into whether and how ALPRs are deployed. Given that a very small percent­age of license plates scanned will actu­ally be connec­ted to a crime, communit­ies may decide that money spent on ALPRs is better spent on other public safety needs, or that the privacy trade-offs are not worth­while. If ALPRs are purchased, mech­an­isms must be in place to soli­cit feed­back from inter­ested community members on the policies govern­ing their use. This public consulta­tion process should also inform the types of crimes that merit inclu­sion on an ALPR hot list. Draft and final policies should be easily avail­able to the public and must include the results of ongo­ing audits to detect and deter misuse of the system. ALPRs should not be deployed absent clear and enforce­able use policies. Except in emer­gency circum­stances, histor­ical ALPR searches should not be conduc­ted absent a warrant suppor­ted by prob­able cause.
  • Main­tain audit logs: Law enforce­ment use of ALPR data should be logged and stored in a format that permits audit­ing. First, a log should main­tain details about auto­mated ALPR alerts, includ­ing the reason for the alert, whether any inform­a­tion was auto­mat­ic­ally shared with other agen­cies, and the outcome of the alert. Second, logs should track every time an officer seeks to access histor­ical ALPR data as part of an invest­ig­a­tion. This log should specify the officer and the crime being invest­ig­ated and require evid­ence that a warrant has been obtained or specific­a­tion of the exigent circum­stances that mandate quicker access. If this func­tion­al­ity is not avail­able through vendor plat­forms, law enforce­ment should estab­lish internal access controls to ensure the same outcome.

Separ­ately, each police depart­ment should estab­lish a log that tracks and cata­logs all the ways they receive, store, and share ALPR data. This includes the license plate reads collec­ted by their own devices, as well as those provided by other law enforce­ment agen­cies, by private vendors, and volun­tar­ily by busi­nesses and indi­vidu­als. Many vendor plat­forms provide auto­mated meth­ods for track­ing and updat­ing author­ized data flows, but each depart­ment should appoint an appro­pri­ate office to lead their efforts to track and main­tain this log. When a depart­ment elects to share ALPR data with another law enforce­ment agency, the parties should enter into data shar­ing arrange­ments ensur­ing that policies regard­ing access control and reten­tion are at least as strict as those of the origin­at­ing agency. The receiv­ing agency should also commit to enter­ing into similar data shar­ing agree­ments for any down­stream data shar­ing. Without adequate steps to protect down­stream data shar­ing, even the most rigid policies will be insuf­fi­cient once data is shared with a depart­ment that does not main­tain the same level of protec­tion.

  • Conduct audits for dispar­ate impact: Law enforce­ment use of ALPRs should be peri­od­ic­ally audited in order to protect against dispar­ate impact on histor­ic­ally margin­al­ized communit­ies and consti­tu­tion­ally protec­ted activ­it­ies. These audits should eval­u­ate the times and loca­tions where ALPRs are used to ensure that they are not being used to dispro­por­tion­ately target partic­u­lar communit­ies or consti­tu­tion­ally protec­ted activ­it­ies such as protests. To facil­it­ate this process, law enforce­ment agen­cies must keep records that detail the loca­tions where ALPRs are deployed and the areas where histor­ical searches are being run. Audits should also assess the types of invest­ig­a­tions that merit a vehicle’s inclu­sion on a hot list to ensure that low-level offenses are not effect­ively being used to target vulner­able communit­ies. Audits should eval­u­ate the extent to which ALPR data is used with other surveil­lance tech­no­lo­gies — such as predict­ive poli­cing algorithms or inclu­sion in gang data­bases — in a manner that could dispro­por­tion­ately harm histor­ic­ally margin­al­ized groups or consti­tu­tion­ally protec­ted activ­ity.
  • Conduct audits to ensure effect­ive safe­guards: Every ALPR policy should include regu­lar audits to eval­u­ate safe­guard effect­ive­ness. These audits should ensure that ALPR data is only avail­able to employ­ees with a need to access the data, that their access is promptly termin­ated when no longer neces­sary, and that ALPR searches are appro­pri­ately limited to specific law enforce­ment invest­ig­a­tions. Ongo­ing over­sight of the use of ALPR data within law enforce­ment agen­cies is an essen­tial safe­guard to detect and prevent officers’ misuse of the system.

End Notes