Whether a traffic stop based on a police officer’s mistaken understanding of the traffic laws violates the Fourth Amendment.

The driver in this case was stopped by the police because his car had only one working tail light, which the police mistakenly believed was a violation of North Carolina law. The ACLU's amicus brief, submitted with the National Association of Criminal Defense Lawyers and the Cato Institute, argues that a mistake of law can never supply the reasonable suspicion of wrongdoing that the Fourth Amendment requires in order to justify a traffic stop. Ignorance of the law is not an excuse for motorists, and it ought not to be an excuse for the police.

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