Fact Check: Does Pot Impair Driving?

Fact Check: Does Pot Impair Driving
Fact Check: Does Pot Impair Driving

Impaired driving: even the term tends to cause a knee-jerk internal reaction for most people. Rightfully so; drunk driving is a factor in up to 30 percent of all vehicular accidents across the United States.
Lately, the conversation is changing to focus on pot and its effects. With impending legalization of marijuana in many locations, it’s time that studies, conclusions and consequences of driving under the influence of pot become standard.
One such recent study focused on the effect of using either alcohol or marijuana as well as the two taken together. The conclusions unfortunately leave more questions than answers.


• The study was conducted by researchers at the University of Iowa. It was also sponsored by the Highway Traffic Safety Administration, the National Institute of Drug Abuse, and the Office of National Drug Control Policy. All three organizations worked together to compare results in an attempt to identify a more concrete answer.
• Researchers used The University of Iowa’s National Advanced Driving Simulator (NADS) to test just how well drivers coped when under the influence of marijuana. Participants were split up into four categories: Sober drivers, Drivers who used only cannabis, Drivers who used only alcohol and Drivers who used both alcohol and cannabis
• The study included a total of 13 people, almost evenly split between men and women, who were at least 21 years of age, but no older than 37. All used marijuana and alcohol occasionally (no more than three times per week, even regarding casual single-drink use).
• Drivers in all four categories were assessed for weaving within and outside of the lane. Alcohol-only participants showed an expected and marked increase in verging into other lanes, weaving the most severely of all groups.
• Drivers who inhaled only cannabis also showed some signs of impairment, weaving within their own lane. However, they did not weave into other lanes like the alcohol-only group.
• Drivers who used cannabis products were blood tested for concentration during testing. Those who had a rating of at least 13.1 ug/L of THC showed the most impairment. The level of weaving (used to judge overall impairment) was similar to that of drivers with blood concentrations of alcohol at the national legal limit (0.08 by breathalyzer).
• Surprisingly, the study also told researchers something else: breathalyzer testing may be effectively useless for testing for cannabis impairment. U of I’s research fellow, Andrew Spurgin, explained. “Everyone wants a Breathalyzer which works for alcohol because alcohol is metabolized in the lungs. But for cannabis this isn’t as simple due to THC’s metabolic and chemical properties.”
Ultimately, the study shows that using large amounts of cannabis may, in fact, be as impairing as alcohol. Many questions remain. There’s a legal limit for driving under the influence of alcohol. The same needs to be established for THC. Then there’s the really key issue of needing an on site testing device for THC, equivalent to the breathalyzer for alcohol. Clearly, no clear conclusion can be drawn at this time. Much more research is needed.