Here is the crux of the majority opinion:
Reasonable suspicion arises from the combination of an officer's understanding of the facts and his understanding of the relevant law. The officer may be reasonably mistaken on either ground. Whether the facts turn out to be not what was thought, or the law turns out to be not what was thought, the result is the same: the facts are outside the scope of the law. There is no reason, under the text of the Fourth Amendment or our precedents, why this same result should be acceptable when reached by way of a reasonable mistake of fact, but not when reached by way of a similarly reasonable mistake of law.
. . .
Heien also contends that the reasons the Fourth Amendment allows some errors of fact do not extend to errors of law. Officers in the field must make factual assessments on the fly, Heien notes, and so deserve a margin of error. In Heien's view, no such margin is appropriate for questions of law: The statute here either requires one working brake light or two, and the answer does not turn on anything "an officer might suddenly confront in the field." Brief for Petitioner 21. But Heien's point does not consider the reality that an officer may "suddenly confront" a situation in the field as to which the application of a statute is unclear—however clear it may later become. A law prohibiting "vehicles" in the park either covers Segways or not, see A. Scalia & B. Garner, Reading Law: The Interpretation of Legal Texts 36-38 (2012), but an officer will nevertheless have to make a quick decision on the law the first time one whizzes by.
Contrary to the suggestion of Heien and amici, our decision does not discourage officers from learning the law. The Fourth Amendment tolerates only reasonablemistakes, and those mistakes—whether of fact or of law— must be objectivelyreasonable. We do not examine the subjective understanding of the particular officer involved. Cf. Whren v. United States, 517 U. S. 806, 813 (1996). And the inquiry is not as forgiving as the one employed in the distinct context of deciding whether an officer is entitled to qualified immunity for a constitutional or statutory violation. Thus, an officer can gain no Fourth Amendment advantage through a sloppy study of the laws he is dutybound to enforce.
Finally, Heien and amici point to the well-known maxim, "Ignorance of the law is no excuse," and contend that it is fundamentally unfair to let police officers get away with mistakes of law when the citizenry is accorded no such leeway. Though this argument has a certain rhetorical appeal, it misconceives the implication of the maxim. The true symmetry is this: Just as an individual generally cannot escape criminal liability based on a mistaken understanding of the law, so too the government cannot impose criminal liability based on a mistaken understanding of the law. If the law required two working brake lights, Heien could not escape a ticket by claiming he reasonably thought he needed only one; if the law required only one, Sergeant Darisse could not issue a valid ticket by claiming he reasonably thought drivers needed two. But just because mistakes of law cannot justify either the imposition or the avoidance of criminal liability, it does not follow that they cannot justify an investigatory stop. And Heien is not appealing a brake-light ticket; he is appealing a cocaine-trafficking conviction as to which there is no asserted mistake of fact or law.I blogged about this case back in November, and noted that this case could have significant implications for California's law of search and seizure. At the time, I was posting on the California Court of Appeal's opinion in People v. Campuzano. There, the Court held that a police officer's mistake of law rendered a stop unreasonable when the officer stopped a suspect who was riding his bicycle on the sidewalk. While the officer thought that riding a bicycle on the sidewalk was illegal if it was done in a commercial district, the Court concluded that the bicycle needed to be ridden in front of a business that was operational. Because the defendant in Campuzano was not riding his bicycle in front of an operational business, the officer's conclusion that the defendant had violated the law was mistaken, and the court held that this mistake of law rendered the stop unconstitutional.
After Heien, however, the decision in Campuzano is no longer the law in California. Article One, section 28(f)(2) of California's Constitution establishes that rules excluding evidence in criminal cases are to be only as broad as federal constitutional rules. The California Supreme Court affirmed this limitation in In re Lance W. This means that California's constitutional law of search and seizure cannot exclude a broader range of evidence than the United States Constitution.
Accordingly, the blanket statement by the California Court of Appeal in Campuzano that "[a] traffic stop based on a mistake of law is unreasonable and not subject to the good-faith exception," is no longer the law in California. Under Heien, as long as the officer's belief that the defendant has broken some law is reasonable, any ensuing traffic stop is also reasonable.
Questions remain regarding just how much of a mistake of law is permitted under the "reasonable mistake of law" exception. But I would not be surprised if the officer's mistake in Campuzano fell within this exception, and it will be interesting to see how lower courts define the scope of this new facet of Fourth Amendment doctrine.