One of the most persistent arguments for mass surveillance is that “there is no reasonable expectation of privacy on public roads.” Proponents argue that when the government hires companies like Flock to track citizens, no rights are violated because the tracking happens in public.

This reasoning is an annoying legal anachronism. It relies on a version of the law that died in 1967.

The government’s focus on the “public” nature of the road is exactly the “misleading” formulation the Supreme Court warned about in Katz v. United States, a case often erroneoulsy cited in support of the “public roads” argument.

In that case, the government argued that a phone booth was a public area and therefore not “constitutionally protected.” The Court rejected that geography-based argument as a distraction:

For the Fourth Amendment protects people, not places. […] This effort to decide whether or not a given ‘area,’ viewed in the abstract, is ‘constitutionally protected’ deflects attention from the problem presented by this case.

While the majority established the “people, not places” rule, Justice Harlan’s concurrence in Katz created the two-pronged test courts use today to determine if a search occurred:

1. Subjective: Did the person exhibit an actual expectation of privacy?
2. Objective: Is that expectation one that society recognizes as “reasonable”?

Crucially, Harlan noted that even in areas accessible to the public (like a telephone booth), an individual can still hold a constitutionally protected interest in their privacy.

In 2018, the Court addressed this again in Carpenter v. United States — Another case that often comes up in the Flock debate.

The government argued that location records weren’t protected because they were held by a third party (Sprint) and tracked movement in the public sphere.

The Court flatly disagreed, echoing the logic of the Katz Court when addressing both elements of that claim:

A person does not surrender all Fourth Amendment protection by venturing into the public sphere.

And,

[The “third-party business records”] distinction does not negate Carpenter’s anticipation of privacy in his physical location.

The Court made it clear: just because your privacy interests are “diminished” in public does not mean the Fourth Amendment “falls out of the picture entirely.”

That message in Carpenter echoed Katz: People, not places.^[1]

Whether the data is collected by a third-party contractor, obtained through commercial channels, or recorded on a public street, the legal line is consistent: The protection follows the person.

The argument that “there is no privacy on a public road” is a relic of pre-1967 thinking.