Arizona prosecutors cannot charge pot users for driving under the influence of marijuana simply because traces of the drug remain in their bloodstream, without evidence of actual impairment, the state Supreme Court ruled Tuesday.

The 4-1 decision by the Arizona Supreme Court comes as states are increasingly moving toward liberalizing marijuana rules after voter initiatives in Colorado and Washington state in 2012 that legalized the drug for recreational purposes.

Many more states, including Arizona, allow medicinal marijuana and are grappling with how to prevent driving under the influence of pot, which can remain in the body even after users are no longer impaired.

In Tuesday's ruling, the Arizona Supreme Court found in favor of a driver, Hrach Shilgevorkyan, who was pulled over in Maricopa County and later admitted to smoking marijuana the night before. A blood test revealed he had the cannabis metabolite Carboxy-THC in his system.

That chemical does not cause impairment. Nevertheless, Maricopa County prosecutors charged Shilgevorkyan with driving under the influence in line with the state's zero-tolerance rules on drug-impaired driving. The Supreme Court said that was incorrect.

"Because the legislature intended to prevent impaired driving, we hold that the 'metabolite' reference in [state law] is limited to any of a proscribed substance's metabolites that are capable of causing impairment," Justice Robert Brutinel wrote in the court's majority opinion.

Maricopa County Attorney Bill Montgomery issued a strongly worded statement criticizing the opinion, saying the legislature's intent was clear in the law. However, his office does not plan to appeal the ruling to the U.S. Supreme Court, spokesman Jerry Cobb said.

The Arizona Supreme Court ruling mentioned the legality of medical cannabis in the state, finding that prosecuting drivers who are not impaired but have a marijuana metabolite in their system would "criminalize otherwise legal conduct."