Which case is the first significant U.S. Supreme Court decision involving motor vehicles?

• A. Mapp v. Ohio
• B. Katz v. United States
• C. Carroll v. U.S.
• D. Terry v. Ohio

Answer: (C)

The main idea here is how the Fourth Amendment applies to motor vehicles. The first major Supreme Court ruling on this topic is Carroll v. United States. It established the automobile exception: when there is probable cause to believe a vehicle contains contraband or evidence, police may search the car without a warrant because cars can be moved quickly and evidence could be lost or hidden if they had to wait for a warrant. The decision also reflects the understood lower expectation of privacy in vehicles compared with homes, given that a car is on public roads and readily movable.

In Carroll, the Court held that the combination of probable cause and the car’s mobility justifies a warrantless search of the vehicle, and the evidence found can be admissible. This creates a practical, situation-based rule: the urgency created by a moving vehicle and the potential for evidence to disappear makes a warrantless search reasonable when probable cause exists.

Other cases you might encounter on Fourth Amendment topics deal with different issues—such as the exclusionary rule, privacy expectations in other contexts, or stop-and-frisk—so they aren’t about automobile searches in the same way Carroll is.