A special commission is advising Michigan lawmakers not to set limits on how much THC a person can have in their system before they’re deemed too impaired to drive.

The six-member commission appointed by former Republican Governor Rick Snyder has studied the possibility of setting a legal limit on delta-9-tetrahydrocannabinol (THC) -- the active mind-altering chemical in marijuana -- for the past year.

Last week the Impaired Driving Safety Commission sent its report to state lawmakers, which recommended they not set a numerical impairment limit for THC.

After reviewing several studies, the commission found there is no scientifically supported threshold for THC that correlates to impaired driving.

“It’s something that people want to compare it to alcohol,” said defense attorney Barton Morris, who founded Cannabis Legal Group in Royal Oak. “It simply doesn’t exist.”

Blood levels of THC can be low when people are very impaired -- and conversely, chronic users of marijuana with high levels of THC in their blood can not be impaired, according to the commission’s report. With multiple ways of consuming marijuana -- from smoking it to eating infused edibles -- the time it takes to feel its effects vary widely and from person to person.

Studies have found that people impaired by cannabis typically drive slower, leave more space between cars and take fewer risks than when they drive sober, according to the commission’s report.

The Impaired Driving Safety Commission included Michigan State Police Director Col. Kriste Etue, Dr. William Morrone, Norbert Kaminski of Michigan State University, University of Michigan professor Carol Flannagan, forensic toxicologist Nicholas Fillinger and medical marijuana patient and activist Margeaux Bruner.

“The commission was very accurate,” Morris said. “There cannot be a THC concentration in blood that reasonably equates to impaired driving.”

Six states have established per se limits for THC impairment. Colorado, Montana and Washington have set theirs at 5 nanograms per milliliter (ng/mL). Nevada and Ohio have limits of 2 ng/ML, and Pennsylvania has a legal limit of 1 ng/ML.

Without a legal limit for marijuana -- like the 0.08 percent blood alcohol content -- Michigan prosecutors are left to the facts of the case, said D.J. Hilson, president of the Prosecuting Attorney Association of Michigan.

"The science is unsettled in that area. I think most of us prosecutors appreciate that because of that they decided not to recommend a level; as we really have with the change in the law we're going to be focused on the facts of the case," Hilson said.

A positive test result for THC does not mean a driver can automatically be charged with driving under the influence, Morris said.

“Now it’s just a factor that must be taken into consideration,” Morris said, explaining a driver’s ability to control the car, field sobriety tests and the police officer’s observations are also factors for prosecutors.

The Impaired Driving Safety Commission recommended that more police officers be trained on how to identify drugs and drugged driving, and to continue a planned expansion of roadside drug tests.

A trial run of the roadside saliva tests in five counties last year found most of the drivers police suspected were impaired were under the influence of marijuana. A statistical analysis of the roadside tests found that they performed well compared to the subsequent blood tests.

Just as the results from breathalyzer tests for alcohol cannot be used in court, the roadside saliva tests for marijuana and other drugs also cannot be used to charge a driver with a crime, Hilson said.

-- Amy Biolchini is the marijuana beat reporter for MLive. Contact her with questions, tips or comments at abiolch1@mlive.com. Read more from MLive about medical and recreational marijuana.