Tracking Traffic

You Think No One Knows Where You Are Driving? Think Again.

Written by Pierre Grosdidier

A picture of Traffic and Traffic Camera

Automatic license plate readers, or ALPRs, are high-speed cameras that snap pictures of license plates and transcribe and save their numbers in databases with time stamps and geolocation data.

ALPRs identify toll road violators, trace stolen cars, and even locate forgetful
drivers’ cars in garages.

In Commonwealth v. McCarthy, the Supreme Judicial Court of Massachusetts recently opined on whether police use of ALPRs constituted a search under the Fourth Amendment or under the Massachusetts Declaration of Rights.1 State police used ALPRs to track vehicular traffic through the only two bridges over the Cape Cod Canal. The system retained records for a year and provided real-time alerts for designated plate numbers.

Detectives used these ALPRs to track Jason J. McCarthy, a suspected heroin trafficker. The police arrested him following an ALPR alert, and he moved to suppress the ALPR data. The court anchored its analysis in the tension that exists in the U.S. Supreme Court’s landmark Katz v. United States decision, namely that even though what a person knowingly and publicly exposes is not subject to Fourth Amendment protection, “[a] person does not surrender all Fourth Amendment protection by venturing into the public sphere.”2

Under this analysis, the observation of a car’s license plate is not a search because the plate is “knowingly exposed.” But, tracking a person’s movements with a GPS device attached to the person’s vehicle constitutes a search because it provides a detailed account of the person’s life. The difference between these extremes “is not one of degree but of kind.”3 Even though a person might expose individual movements to the public, say by attending church service, the person does not do so knowingly in the aggregate and retains a reasonable expectation of privacyin the mosaic of information that modernsurveillance technology can create.

In this case, the question turned on whether ALPRs produced a sufficiently detailed mosaic of the suspect’s movements to offend constitutional rights.4 The court first stressed that the question was not the amount of data that prosecutors sought to admit into evidence, but the amount of data that they had collected, or had sought to collect. This information was not in the record, however. The court observed that “enough cameras in enough locations” tracking vehicles and storing data for a year would constitute a constitutionally protected search. The same was true of ALPR real-time alerts, which, when sufficiently deployed, would act like cellphone “ping(s).”5

The court decided the case narrowly, following the precept that “Fourth Amendment [and art. 14] cases must be decided on the facts of each case, not by extravagant generalizations.”6 It held that four cameras at the ends of two bridges only gave authorities limited information regarding the suspect’s movements, and did not allow them to constantly track his whereabouts so as to reveal “the privacies of life.”7 The cameras in this case, therefore, did not offend the Fourth Amendment and Massachusetts’ Article 14. The court declined to indicate under what circumstances ALPRs would step over the line. TBJ


1.  142 N.E.3d 1090, 1095 (Mass. 2020). The Fourth Amendment protects individuals from unreasonable searches.

2.  Id. at 1100 (quoting Katz v. United States, 389 U.S. 347, 351 (1967); Carpenter v. United States --- U.S. ---, 138 S. Ct. 2206, 2217 (2018)).

3.  Id. at 1103 (quoting United States v. Maynard, 615 F.3d 544, 562 (D.C. Cir. 2010), aff’d in part sub nom.).

4.  Id.

5.  Id. at 1105.

6.  Id. (quoting Dow Chem. Co. v. United States, 476 U.S. 227, 238 n.5 (1986)) (bracketed term in original).

7.  Id. at 1106 (quoting Carpenter, 138 S. Ct. at 2217).

Headshot of Pierre GrosdidierPierre Grosdidier is an attorney in Houston. He belongs to the first group of attorneys certified in construction law by the Texas Board of Legal Specialization in 2017. His practice also includes data privacy and unauthorized computer access issues and litigation. Prior to practicing law, Grosdidier worked in the process control industry. He holds a Ph.D. from Caltech and a J.D. from the University of Texas School of Law. Grosdidier is a member of the State Bar of Texas, an AAA Panelist, a registered P.E. in Texas (inactive), a member of the Texas Bar Foundation, an American Bar Foundation fellow, and the State Bar of Texas Computer & Technology Section treasurer for 2020-2021.

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