Beardsley, Jensen and Von Wald
The Fourth Amendment's prohibition against unreasonable searches and seizures applies to people driving their car. Two circumstances justify law enforcement stopping a vehicle to search it: (1) if the officer has a "reasonable suspicion" that criminal activity (of any kind) may be afoot; or (2) the officer has "probable cause" to believe that a traffic violation has occurred.
What constitutes "reasonable suspicion" that criminal activity is afoot? The South Dakota Supreme Court says it cannot be a product of "whim, caprice, or idle curiosity, but ... is enough that the stop is based upon ‘specific and articulable facts, which taken together with rational inferences from those facts, reasonably warrant the intrusion." Once reasonable suspicion arises, the officer may stop the vehicle to confirm or discount that suspicion.
How about "probable cause" to believe that a traffic violation has occurred? According to our Court, "probable cause exists when the facts ‘would lead a reasonable and prudent person to believe it fairly probable that a crime has been committed." The activity being witnessed must be a crime. If it is not– if the officer is mistaken about the law - the stop is illegal and evidence obtained during a search will be suppressed.
There was no crime in State of South Dakota v. Wright, 2010 S.D.91. The officer believed the driver he overtook and passed had an obligation to dim his lights after the officer passed him. After explaining that there is no such obligation in the pertinent statute (the obligation to dim lights exists only when passing or meeting another vehicle, not when being passed), the Court held that the officer's mistake of law meant he had no right to stop the vehicle so his search was unconstitutional. The evidence the officer found – marijuana - should be suppressed.
The Wright decision is helpful if you're interested in the issue of when law enforcement can stop vehicles driving down the road.
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