Is it a violation of the Constitution for a police officer to act on a misunderstanding of the law? That’s what the Supreme Court asked in its first case of the new term.

The story of Heien v. North Carolina begins in 2009, when Nicholas Heien and a friend were pulled over by a sheriff’s deputy in Mt. Airy for a broken taillight. The officer believed North Carolina law required two working lights.

Heien nearly escaped the stop with a warning citation, but the officer asked for consent to search the car. A surprise in retrospect, Heien agreed. The search turned up a bag of cocaine.

After losing in trial court, Heien appealed the decision, aided by the discovery that North Carolina law actually suggests only one working taillight is needed. Specifically, the law says all vehicles “shall be equipped with a stop lamp on the rear of the vehicle” [emphasis added]. He argued that the deputy should be held to the same standard that citizens must follow—namely, that ignorance of the law is no excuse.

The intermediate appeals court in North Carolina agreed with Heien, finding that “an officer’s mistaken belief that a defendant has committed a traffic violation is not an objectively reasonable justification for a traffic stop.”

But the state Supreme Court reversed, pointing to another statute that refers to “rear lamps” [emphasis added] and concluding that the officer’s “mistake of law was objectively reasonable” and ultimately constitutional. The bag of cocaine was ruled admissible. Heien then turned to the nation’s court of last resort.

The Supreme Court has long excused reasonable “mistakes of fact” in permitting a search or seizure. This case asks if “mistakes of law” should be treated the same way.

Oral arguments yesterday morning—captured by court artist Art Lien—showed little indication of the ultimate outcome, especially because the Justices spent a good deal of time debating exactly what they would need to rule on. Is it merely a question of Fourth Amendment rights? Or is it also a question of applying a remedy for a violation of those rights?

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Still, the Justices were very much engaged with the official question at hand—namely, “whether a police officer’s mistake of law can be provide the individualized suspicion that the Fourth Amendment requires to justify a traffic stop.”

Justice Samuel Alito pressed Stanford law professor Jeffrey Fisher to define the “reasonableness,” or lack thereof, demonstrated by the deputy.

“I don’t mean to ask this in the context of any other body of the Court’s law, just in the common-sense understanding of the term,” he said. “Even if an attorney sat down and read the relevant North Carolina statutes, do you think it would be reasonable for that attorney to conclude that you have to have two functioning brake lights and not just one?”

Justice Sonia Sotomayor pointed out that the deputy was involved in “criminal interdiction” when he spotted Heien’s car. Indeed, the deputy had admitted in trial that the broken taillight was merely a pretext for pulling the car over to investigate criminal suspicion.

“So how many citizens have been stopped for one brake light [and] asked to have their car searched?” she asked Robert Montgomery, North Carolina assistant attorney general. “And is that something we as a society should be encouraging?”

Justice Elena Kagan suggested the deputy might have read the statute too broadly. “We want [officers] to enforce the law fairly and as written—and not to push every statute to the furthest it could go without being found utterly unreasonable,” she said.

A ruling could be announced as soon as January.

Nicandro Iannacci is a web strategist at the National Constitution Center.

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