Study: Marijuana’s Role In Crash Risk Often Overstated

Oslo, Norway: Studies estimating the likelihood of THC-positive drivers being involved in motor vehicle accidents compared to drug-free drivers often fail to adequately control for potentially confounding variables, according to a literature review published online ahead of print in the journal Addiction.

Norwegian researchers reviewed more than 20 driving culpability studies, as well as two meta-analyses, published between the years 1982 and 2015. They found that some studies possessed methodological shortcomings that led to authors’ overestimating the association between marijuana and crash risk.

“Higher estimates from earlier meta-reviews were found to be largely driven by methodological issue,” they determined, “in particular the use of … data without adjustment for known confounders” such as age and gender.

Following adjustments, researchers concluded, “[A]cute cannabis intoxication is related to a statistically significant risk increase of low to moderate magnitude [odds ratio between 1.2 and 1.4].”

By contrast, a 2015 case-control study by the US National Highway Traffic Safety Administration reported that driving with legal amounts of booze in one’s system is associated with a nearly four-fold increased crash risk (odds ratio = 3.93).

For more information, please contact Paul Armentano, NORML Deputy Director, at: Full text of the study, “The effects of cannabis intoxication on motor vehicle collision revisited and revised,” appears in Addiction. NORML has additional information regarding cannabis’ impact on psychomotor performance and motor vehicle accident risk here: