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.