GRAND RAPIDS, Mich. — Two western Michigan prosecutors are casting a wary eye on legislation that sets a threshold for marijuana impairment in drivers.

The senate bill comes just months after a special commission recommended that Michigan lawmakers not set limits for how much THC a person can have in their system before they are considered too impaired to drive.

“They decided that the science wasn’t settled enough in order to make that determination,’’ Muskegon County Prosecutor DJ Hilson said, referring to the recommendation by the Impaired Driving Safety Commission.

The panel was created to research the level of ingested medical marijuana/THC that may affect safe driving. In a report sent to state lawmakers, members said there is no scientifically-supported level of THC tied to impaired driving.

Against that backdrop, state Sen. Betty Jean Alexander, D-Detroit, introduced a bill that makes it illegal to drive with a THC level of five or more nanograms per milliliter of blood. Violators would face jail, community service and fines.

Colorado, Montana and Washington have set their limits at five nanograms. In Nevada and Ohio, the limit is two nanograms. In Pennsylvania, the limit is one nanogram.

Hilson, who is winding down his tenure as president of the Prosecuting Attorneys Association of Michigan, said a state-imposed limit for driving on THC may not accurately reflect the driver’s level of impairment.

“What makes you so certain that five nanograms is the appropriate level when a panel of experts made the determination that the science isn’t settled and we can’t make a recommendation,’’ he said. “What five nanograms is to you might be different for me. Maybe it’s one nanogram for me.’’

Kent County Prosecutor Chris Becker said police have to rely on other information, such as a motorist’s behavior and driving ability, to establish impairment from marijuana. 

“Juries look for that number in a marijuana case, but we don’t have it,’’ Becker said. “We’ve got to show that they were impaired because they smoked marijuana.  We’ve got to show that impairment. It’s not zero tolerance.’’

The 18-page senate bill, introduced in late May, has no co-sponsors. The bill has been referred to the senate Committee on Judiciary and Public Safety. 

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