Count the brake lights.

Photographer: Logan Mock-Bunting/Getty Images

Even Ignorant Cops Have to Follow the Law

Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His books include “Cool War: The Future of Global Competition” and “Divided by God: America’s Church-State Problem -- and What We Should Do About It.”
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You don’t need to go to law school to know that ignorance of the law is no excuse -- for you. But what about the cops? This morally significant question opens the U.S. Supreme Court term today, as the justices consider whether to admit evidence obtained after a traffic stop that was based on an erroneous but reasonable interpretation of the law.

The police usually know the law pretty well, so the problem rarely arises. When it does, however, it implicates the very basis of due process and the question of what makes our legal system just.

The case, Heien v. North Carolina, involves Nicholas Heien, who has been cycling between being the unluckiest and the luckiest drug trafficker alive. In April 2009, a sergeant from the Surry County, North Carolina, sheriff’s department pulled over a car whose right rear brake light had failed. The sergeant told the driver that he would get off with a warning. But something triggered his suspicion, and he asked for permission to search the vehicle. The driver said it was fine with him, but suggested that the officer ask Heien, the passenger, because it was his car. Heien agreed -- and a search revealed a plastic bag full of cocaine.

Thus far Heien was unlucky -- and a little dumb. His stroke of luck came on appeal. In a burst of creativity, his lawyer argued that, under North Carolina law, you don’t need two brake lights: One will do the trick. Support came from the half-century old North Carolina code, which says that you can’t operate a motor vehicle “unless it shall be equipped with a stop lamp on the rear of the vehicle.” The singular “a,” Heien’s lawyer claimed, proved that the stop was unlawful.

A North Carolina appellate court, apparently influenced by Justice Antonin Scalia’s habit of reading the text of statutes literally, held that Heien was right. Having just one brake light doesn’t violate North Carolina law. Then came the big leap. The state appeals court held that, because the initial traffic stop was unlawful, anything found in the search pursuant to that stop must be excluded from evidence. Heien was a free man -- and the luckiest person in North Carolina.

Not so fast, said the North Carolina Supreme Court. It didn't displace the lower court’s interpretation of statute. Instead the court held that even if the police had acted erroneously, their erroneous interpretation of the law to require two functioning brake lights was a reasonable mistake. Heien couldn’t be prosecuted for a traffic violation. But the results of the search could be admitted against him. Heien was back in jail.

Here’s where the law gets interesting. When it comes to unlawful searches, the U.S. Supreme Court has held that if the police make a reasonable error of fact, the search may be admitted. Imagine that the police mistakenly thought a driver’s brake lights were out when in fact they were working but obscured by thick layer of mud. Assuming the police reasonably thought the lights were out when they stopped the car, a search pursuant to the stop would be admissible even though, as a matter of fact, there were no grounds for a traffic violation.

The North Carolina Supreme Court took the view that a reasonable mistake of law by the police should be no different from a reasonable mistake of fact. The police must be able to do their job. As one of the North Carolina justices noted, most people in the state surely believed two brake lights were required. On the face of it, it seems absurd to let Heien go on a technicality -- especially when it was reasonable for the police to believe that the law required two brake lights. A number of other state supreme courts and federal appellate courts agree with this approach.

Yet there are also courts that take the opposite view, and the split between the different approaches is why the Supreme Court took the case. The court should decide the case for Heien, for a straightforward reason: A mistake of fact is not the same thing as a mistake of law. Police are fallible humans, and it’s appropriate to respect their reasonable factual errors. But when it comes to the law, the police should operate under the exact same presumption as ordinary citizens.

Because civilians are presumed to know the law, and punished even if they don’t, the police should operate under the reciprocal assumption. They should be presumed to know the law, and the government should lose the case if it turns out that the police didn’t.

It isn’t only that what’s sauce for the goose is sauce for the gander. The presumption that we all know the law plays a crucial role in the ideal of due process, specifically the notion that no one should be punished without prior notice of the law he has violated. Sometimes called the “principle of legality,” the idea is that there should be no punishment without law -- nulla poena sine lege, if you’re into law Latin.

The obvious problem with this bedrock of a just legal system is that, in reality, you don’t always know what the law is. Our answer, morally tenuous but practically necessary, is that it’s your responsibility as a citizen to know the law. The government is therefore justified in punishing you if you violate it.

The police must similarly be presumed to know the law. Without this presumption, there would be a perverse incentive for the police to maintain ignorance so as to rely later on the reasonableness of their mistaken beliefs. To treat the police as free of the same obligation to inform themselves that we impose on private citizens would undercut the principle that people cannot be punished without prior notice of the illegality of their conduct. The police could make arrests without illegality, and people could go to prison as a consequence, even if not for the conduct that led to the arrest in the first place.

Nicholas Heien should go free -- not as a reward for his conduct, but as an exemplar of how the rule of law should work in a free society.

This column does not necessarily reflect the opinion of Bloomberg View's editorial board or Bloomberg LP, its owners and investors.

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Noah Feldman at

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Stacey Shick at