Legal limits for driving on pot not backed by science, study shows

May 11, 2016 by Mary Wisniewski, Chicago Tribune
cannabis
Cannabis indica. Credit: Wikipedia

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 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 12 states have laws that forbid any level of marijuana in the system while driving.

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 - 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 ," 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 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 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.

Explore further: Fatal road crashes involving marijuana double after state legalizes drug

Related Stories

Fatal road crashes involving marijuana double after state legalizes drug

May 10, 2016
Fatal crashes involving drivers who recently used marijuana doubled in Washington after the state legalized the drug, according to the latest research by the AAA Foundation for Traffic Safety. New research also shows that ...

Scientific basis for laws on marijuana, driving questioned

May 10, 2016
Motorists are being convicted of driving under the influence of marijuana based on arbitrary state standards that have no connection to whether the driver was actually impaired, says a study by the nation's largest auto club.

Marijuana use involved in more fatal accidents in Colorado

May 15, 2014
The proportion of marijuana-positive drivers involved in fatal motor vehicle crashes in Colorado has increased dramatically since the commercialization of medical marijuana in the middle of 2009, according to a study by University ...

Researchers study how marijuana impacts driving

June 24, 2015
A new study conducted at the University of Iowa's National Advanced Driving Simulator has found drivers who use alcohol and marijuana together weave more on a virtual roadway than drivers who use either substance independently. ...

Marijuana's hazy contribution to highway deaths

September 2, 2014
Public officials and safety advocates worry there will be more drivers high on pot and a big increase in traffic deaths as states liberalize marijuana laws.

Recommended for you

India launches 'Modicare', world's biggest health scheme

September 23, 2018
Prime Minister Narendra Modi on Sunday launched the world's biggest health insurance scheme, promising free coverage for half a billion of India's poorest citizens ahead of national elections next year.

Patient-centered visual aid helps physicians discuss risks, treatments with parents

September 21, 2018
A series of illustrations and charts designed as decision aids for parents of children with minor head injuries helped them communicate with emergency medicine physicians and make informed decisions about their child's care, ...

Alcohol responsible for one in 20 deaths worldwide: WHO

September 21, 2018
Alcohol kills three million people worldwide each year—more than AIDS, violence and road accidents combined, the World Health Organization said Friday, adding that men are particularly at risk.

Smart pills dumb down medical care, experts warn

September 21, 2018
Enthusiasm for an emerging digital health tool, the smart pill, is on the rise but researchers at the University of Illinois at Chicago have published a paper in the American Journal of Bioethics that cautions health care ...

Crunched for time? High-intensity exercise = same cell benefits in fewer minutes

September 20, 2018
A few minutes of high-intensity interval or sprinting exercise may be as effective as much longer exercise sessions in spurring beneficial improvements in mitochondrial function, according to new research. The small study ...

China's doctor shortage prompts rush for AI health care

September 20, 2018
Qu Jianguo, 64, had a futuristic medical visit in Shanghai as he put his wrist through an automated pulse-taking machine and received the result within two minutes on a mobile phone—without a doctor present.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.