THC is the active ingredient in marijuana; a nanogram is one-billionth of a gram. 

The debate is not new. A similar bill introduced two years ago did not advance. Prosecutors and criminal defense attorneys have joined ranks against setting a level for marijuana intoxication in motorists.

“The science says setting an arbitrary number just because it makes us feel good is not the way to go,’’ Becker said.

Defense attorney Bruce Alan Block agrees.

“It’s an arbitrary number; it’s a feel-good number. It is based on nothing,’’ Block said. “There is no correlation between a per se level of THC in your blood and impaired driving.’’

As it stands, law enforcement has to look for signs of marijuana impairment, such as bad driving and poor performance on field sobriety tests. 

"It's not relying on a number,'' Becker said. "You have to show signs of impairment. You have to show bad driving. You have to show field sobriety tests or there’s an accident.’’

Six states have set nanogram limits on THC blood content. Among them are Montana and Washington, which set their limits at five nanograms. In Pennsylvania, the limit is one nanogram.

Block says marijuana affects drivers differently. A person who regularly uses marijuana may be perfectly fine to drive with five or more nanograms of THC in their system. It's not necessarily the same for novice users.

“There could be people that test under five nanograms that are stoned out of their minds,’’ he said. “A lot of people know that that’s just a number that’s plucked out of the air."

The bill was introduced by Rep. Pamela Hornbeger of Macomb County. In a news release, she says it comes in response to the death last year of a 3-year-old girl in southeast Michigan.