Legal blood limits for marijuana are not an accurate way to measure whether someone was driving while impaired, and can lead to unsafe drivers going free while others are wrongfully convicted, according to a new study.

The study released Tuesday by the AAA Foundation for Traffic Safety found that drivers can have a low level of THC, the active ingredient in marijuana, in their blood and be unsafe behind the wheel, while others with relatively high levels may not be a hazard.

Marijuana is not metabolized in the system in the same way as alcohol. So while a person with a blood-alcohol level of .08 or higher is considered too drunk to drive, it’s not possible to say the same thing absent other evidence about a person testing at 5 nanograms per milliliter of blood of THC — the level used to find impairment by Colorado, Montana and Washington, the study found.

The difference matters, because Illinois and 11 other states have laws that forbid any level of marijuana in the system while driving. A pot decriminalization bill being considered in the Illinois legislature would raise the level to 5 ng/ml. The bill faces opposition from law enforcement and anti-pot advocates.

Efforts to legally measure marijuana impairment have become a major concern for lawmakers as more states move to legalize cannabis, either for medical use or adult recreational use. Four states have legalized pot for recreational use by adults, and 24 states — including Illinois, plus Washington, D.C. — allow medical use, according to the Marijuana Policy Project, a D.C.-based advocacy group.

“It’s an attempt to try to do an apples-to-apples comparison with blood alcohol concentration,” said Chris Lindsey, senior legislative analyst for the Marijuana Policy Project. He noted that the AAA findings echo earlier research. “They found out that these things can’t really be compared.”

Another problem is that high THC levels may drop before a test is administered, because the average time to collect blood from a suspect driver is often two hours, the AAA study found. Frequent pot users can exhibit high levels of the drug long after use, while levels can decline rapidly among occasional users, so it is difficult to develop fair guidelines, the study found.

Because of the problem in measuring whether someone is impaired with a blood test, AAA urged states to also look at behavioral and physiological evidence through field sobriety tests, such as seeing whether a driver has bloodshot eyes or is able to stand on one leg.

“That kind of testing has proved effective in court,” said J.T. Griffin, chief government affairs officer for Mothers Against Drunk Driving, or MADD.

He pointed to a 2015 study by the National Highway Traffic Safety Administration that found no big crash risk associated with people driving with marijuana in their system, but says more study is needed.

Alcohol remains the biggest drug problem on the highways, he said.

“We know that almost one-third of all traffic deaths are caused by alcohol,” Griffin said.

AAA released a second study Tuesday that showed fatal crashes involving drivers who recently used marijuana had doubled in Washington after that state legalized the drug in December 2012 — the percentage of drivers involved in fatal crashes who had used marijuana jumped to 17 percent from 8 percent between 2013 and 2014.

Most drivers who had THC in their systems also had alcohol or other drugs in their blood at the time of the crash, the study found. The study noted that the drivers who had THC in their blood were not necessarily impaired nor were they necessarily at fault in the crashes.

Beth Mosher, a spokeswoman for AAA Chicago, said the increase is nevertheless a “cause for concern,” which makes it more important to look for new ways to enforce impaired driving scientifically.

“Just because a drug is legal does not mean it is safe to use while operating a motor vehicle,” Mosher said.

[email protected]

___

(c)2016 the Chicago Tribune

Visit the Chicago Tribune at www.chicagotribune.com

Distributed by Tribune Content Agency, LLC.