- Court
- U.S. Supreme Court
- Decided
- Citation
- 460 U.S. 276 (1983)
- Docket
- No. 81-1802
- Binding in
- Federal (nationwide)
- Status
- Held
What the court held, in plain English
Police using a radio beeper to follow a car on public roads did not conduct a Fourth Amendment search, because a person traveling on public roads has no reasonable expectation of privacy in those movements. The Court left open whether “dragnet-type” surveillance would be treated differently.
The question
Did police conduct a Fourth Amendment search when they used a radio transmitter (a “beeper”) hidden in a container to track a vehicle’s movements on public roads?
The facts
Minnesota officers placed a beeper inside a container of chloroform that was sold to a suspect. They followed the vehicle carrying it, using both visual surveillance and the beeper signal. When they lost contact, a helicopter-mounted monitor picked up the signal again at a cabin in Wisconsin owned by Knotts. Officers then got a search warrant for the cabin and found a drug laboratory.
What the court held
In an opinion by Justice Rehnquist, the Court reversed the Eighth Circuit and held that monitoring the beeper was neither a search nor a seizure. The Court reasoned that “a person travelling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another,” and that the beeper revealed nothing officers couldn’t have seen by following the car.
The Court also addressed the argument that its ruling would allow round-the-clock surveillance of anyone. It responded that “if such dragnet-type law enforcement practices as respondent envisions should eventually occur, there will be time enough then to determine whether different constitutional principles may be applicable.”
Justices Brennan, Blackmun and Stevens wrote separate opinions concurring in the judgment.
Why it matters for license plate readers
Knotts is the starting point for the argument that recording a vehicle on public roads isn’t a search. It’s also the source of the “dragnet” reservation that later cases, and challenges to ALPR networks, draw on: a single observation on a public road is one thing, while continuous, large-scale collection may raise different questions. See is a license plate reader a Fourth Amendment search?
What it didn’t decide
- Whether “dragnet-type” surveillance would require different constitutional rules. The Court expressly left this for a later case.
- Anything about license plate readers. The case involved a single beeper and a single trip.
Later history
Knotts has not been overruled. The Supreme Court discussed it in United States v. Jones (2012) and Carpenter v. United States (2018), and Massachusetts’s high court discussed it in Commonwealth v. McCarthy (2020).
One ruling doesn’t settle every case. A decision binds courts within its jurisdiction, and outcomes depend on the facts. See how we summarize cases in our editorial standards.