LANSING, MI -- Medical marijuana patients have some protection from prosecution for driving under the

influence after the Michigan Supreme Court issued a unanimous ruling late this

afternoon holding the Michigan Medical Marihuana Act pre-empts the

zero-tolerance standard of the Michigan Vehicle Code for driving after having used the drug.

The ruling, signed by all seven justices, came in an eight-page decision released to the public via Twitter in the case of Rodney Lee Koon, who was stopped for driving 83 mph in a 55 mph zone in 2010 in Grand Traverse County.

During the traffic stop, Koon informed the officer he was a registered medical marijuana patient, showed the officer a marijuana pipe and admitted he had smoked the drug five to six hours prior to the traffic stop.

Koon then submitted to a blood test which showed he had THC, the active compound in marijuana, in his bloodstream despite having used the drug several hours before.

Koon was subsequently charged with operating a motor vehicle while under the influence of a Schedule 1 controlled substance, and argued at trial that the zero-tolerance provision of the MVC was inconsistent with the MMMA. Koon was successful at the district and circuit court levels, but a 2012 decision from the Michigan Court of Appeals concluded that the zero-tolerance provision was valid.

The Supreme Court, however, overturned the appeals court decision, reasoning that the MMMA provides registered patients with immunity from prosecution for medical use, which the statute defines to include "internal possession." The high court noted in its decision, however, that while the MMMA prohibits driving while under the influence of marijuana, it does not define "under the influence."

"While we need not set exact parameters of when a person is 'under the influence,' we conclude that it contemplates something more than having any amount of marijuana in one's system and requires some effect on the person," the court wrote.

The court also delivered a suggestion to the legislature that implementing a "legal limit" for marijuana intoxication would resolve similar cases. "It goes almost without saying that the MMMA is an imperfect statute, the interpretation of which has repeatedly required this Court's intervention," the court wrote. "Indeed, this case could have been easily resolved if the MMMA had provided a definition of 'under the influence.'"

The court remanded the case back to the trial court for further proceedings in accordance with the opinion.

Matt Newburg, one of the attorneys who represented Koon throughout the case, said he was unsure if prosecutors would choose to retry the case. "I don't know what the prosecutors will do," Newburg said. "What I do know is that prosecutions regarding driving and medical marijuana patients just got a lot harder to prove."

Brian Smith is the statewide education and courts reporter for MLive. You can email him at bsmith11@mlive.com and follow him on Twitter.