Cite as: 517 U. S. 806 (1996)
Opinion of the Court
fractions by plainclothes police in unmarked vehicles; such investigation only minimally advances the government's interest in traffic safety, and may indeed retard it by producing motorist confusion and alarm—a view said to be supported by the Metropolitan Police Department's own regulations generally prohibiting this practice. And as for the Fourth Amendment interests of the individuals concerned, petitioners point out that our cases acknowledge that even ordinary traffic stops entail "a possibly unsettling show of authority"; that they at best "interfere with freedom of movement, are inconvenient, and consume time" and at worst "may create substantial anxiety," Prouse, 440 U. S., at 657. That anxiety is likely to be even more pronounced when the stop is conducted by plainclothes officers in unmarked cars.
It is of course true that in principle every Fourth Amendment case, since it turns upon a "reasonableness" determination, involves a balancing of all relevant factors. With rare exceptions not applicable here, however, the result of that balancing is not in doubt where the search or seizure is based upon probable cause. That is why petitioners must rely upon cases like Prouse to provide examples of actual "balancing" analysis. There, the police action in question was a random traffic stop for the purpose of checking a motorist's license and vehicle registration, a practice that—like the practices at issue in the inventory search and administrative inspection cases upon which petitioners rely in making their "pretext" claim—involves police intrusion without the probable cause that is its traditional justification. Our opinion in Prouse expressly distinguished the case from a stop based on precisely what is at issue here: "probable cause to believe that a driver is violating any one of the multitude of applicable traffic and equipment regulations." Id., at 661. It noted approvingly that "[t]he foremost method of enforcing traffic and vehicle safety regulations . . . is acting upon observed violations," id., at 659, which afford the " 'quantum of individualized suspicion' " necessary to ensure that police
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