Research In Action

Since our last update on marijuana and driving, a few notable studies have been published. Here are the highlights: 

• A recent CDC report found that among the 12 million US residents who reported driving under the influence of marijuana in 2018, 9.2% were 16- to 20-year-olds, the second highest age group. While this prevalence is lower than those reporting driving under the influence of alcohol, we know that the influence of marijuana, especially when in combination with alcohol (which was found in nearly 10% of injured drivers in this age range presenting to level 1 trauma centers in other studies), can impair judgment and reaction time, increasing the already high risk for unsafe driving behaviors among this age group.
• A simulator driving study found that when compared to young adults who had never used marijuana, sober cannabis users with a history of marijuana use before age 16 had increased risky driving behaviors, including more crashes, driving at higher speeds, and cruising through more red lights. While this study could not conclude whether the marijuana use itself impacted the development of the brain to increase impulsive behaviors, or whether pre-existing impulsivity as a teenager was associated with starting cannabis use, there is evidence from other studies that teenagers who use marijuana have poorer performance on cognition tests. For instance, this study found that teens who used marijuana had lower scores in the areas of working memory, memory recall, and perceptual reasoning. These differences were seen long-term and were not reversed when cannabis use was discontinued. These differences were also seen independent of any effects from alcohol.
• A report from AAA Foundation for Safety found that five years after the legalization of marijuana in Washington state, the proportion of drivers in fatal crashes who were Δ-9-tetrahydrocannabinol (THC, the psychoactive ingredient of marijuana) positive remained about twice the level found prior to the legalization of marijuana. In 2017, the proportion of drivers in all fatal crashes (21%) was higher than in any other year in the 10-year period examined. This is consistent with an Insurance Institute for Highway Safety report in 2018 that found in states where marijuana was legalized as compared to nearby states where marijuana was not legalized, a 6% increase in motor vehicle crashes occurred.
• Finally, while we in the child injury prevention space tend to think about marijuana use and driving as it impacts teen drivers, we can't forget about child passengers. One study found that the presence of a child passenger in the car did not seem to predict or impact whether the adult driver tested positive for THC. In this study, 14% of adult drivers with a child passenger tested positive for THC (similarly,17% of adult drivers who did not have a child passenger tested positive for THC). The study did not measure the level of driver impairment, and drivers with a child passenger were less likely to report moderate to severe marijuana use. Still, this is obviously concerning given the potential impairment THC could cause for those behind the wheel. 

Learn more about marijuana use and driving.