Delaware v. Prouse (1979)

In Delaware v. Prouse, 440 U.S. 648 (1979), the U.S. Supreme Court ruled that evidence obtained from discretionary automobile spot checks without probable cause is violative of the Fourth Amendment.

The case began in New Castle, Delaware, on November 30, 1976. At 7:20 p.m., a city police officer, cruising the streets and admittedly filling time until the end of his shift, randomly stopped an automobile driven by William Prouse. The officer later testified that he witnessed no sign of erratic driving, excessive speeding, or equipment violation. The sole purpose of the stop was to conduct a spot check of the driver's license and automobile registration. As the officer approached Prouse's car, he smelled marijuana and saw a bag of it in plain view on the floor of the car. The officer arrested Prouse and charged him with illegal possession of a controlled substance. At trial, Prouse's attorney was granted a motion to suppress the evidence after contending the officer lacked probable cause to justify the search and seizure. The Delaware Supreme Court affirmed the lower court's ruling, holding the officer's actions constituted an unreasonable search and seizure in violation of the Fourth and Fourteenth Amendments.

In an 8–1 decision, the U.S. Supreme Court affirmed the lower courts' ruling. Writing for the majority, Justice Byron White noted that, given the low probability of identifying illegal drivers through random stops, the state's interest in using unbridled, discretionary spot checks to ensure highway safety does not justify the intrusion or outweigh drivers' right to privacy. However, White added that the state might employ other, less intrusive, spot checks, such as questioning all traffic at roadblock stops. Thus, the majority believed that automobile safety stops must not be arbitrary, but may pass constitutional muster, provided authorities establish and follow proper protocol.

Justice William Rehnquist, the lone dissenter, contended the majority ruling was illogical and inconsistent. He argued that it elevated “the adage ‘misery loves company’ to a novel role in Fourth Amendment jurisprudence.”

Based on Prouse, many state and local law enforcement authorities subsequently developed and implemented protocol for roadblock-type stops. Such mass, suspicionless stops were later upheld in Michigan Department of State Police v. Sitz (1990).