Setting a THC Impaired Driving Limit is Not as Easy As Alcohol, Commission Member Says

Dec 5, 2018

Recreational marijuana becomes legal in Michigan Thursday. Some law enforcement officers worry this will mean more people will drive drug-impaired.


So how much marijuana is too much to drive and how can drivers and police know when they are over the limit?

A commission started under Governor Snyder is charged with answering  questions like these.

The Impaired Driving Safety Commission is charged with recommending a THC limit that would make driving illegal. THC is the main psychoactive ingredient in marijuana. Dr. Carol Flannagan is a traffic safety expert and professor at the University of Michigan Transportation Research Institute, and a commission member. She says setting a THC standard is not as simple as it is with alcohol.

“What we’ve read suggests that because THC and other most other drugs – cocaine - are fat soluble rather than water soluble. They don’t metabolize the same way into blood. They don’t metabolize the same way as alcohol. They don’t clear out of your body the same way. They don’t test in your blood or in breath the same as alcohol,” says Flannagan.

According to the Governors Highway Safety Association, a non-profit that represents highway safety offices that administer federal grant money to address highway safety issues, 7 states have laws with THC limits of 1 nanogram to 5 nanograms as it relates to impaired driving.

“It slows down your reaction time and limits your ability to deal with sudden changes," says Flannagan.

Flanngagn says the commission is in early talks and has not made a decision on whether to recommend a THC limit in Michigan. Under current law, when police spot problematic driving they can prove impairment through a number of roadside and additional tests. A toxicology report showing THC was in a person’s system would be used as additional evidence. Whether or not a THC limit is set, police say their testing will still remain a crucial component in impaired driving enforcement.

“Clearly there needs to be a big push to educate people in the same way that people are educated about alcohol,” says Flannagan.

And that means driving the message home that it’s never ok to drive impaired.

The commission’s report is due by March of next year.