Discussion

Although 4.7% of the U.S. population aged ≥16 years reported driving under the influence of marijuana and 0.9% reported driving under the influence of illicit drugs other than marijuana, these estimates are lower than the 8.0% (20.5 million) who reported driving under the influence of alcohol in 2018 (NSDUH, unpublished data, 2019). The highest prevalence of driving under the influence of marijuana was among persons aged 21–25 years. The second highest was among the youngest drivers (those aged 16–20 years), who already have a heightened crash risk because of inexperience¶; thus, their substance use is of special concern. In a study of injured drivers aged 16–20 years evaluated at level 1 trauma centers in Arizona during 2008–2014 (3), 10% of tested drivers were simultaneously positive for both alcohol and tetrahydrocannabinol, the main psychoactive component of marijuana. Data from the 2018 NSDUH indicate a high prevalence (34.8%) of past-year marijuana use among young adults aged 18–25 years (4). Studies have reported that marijuana use among teenagers and young adults might alter perception, judgement, short-term memory, and cognitive abilities (5). Given these findings, states could consider developing, implementing, and evaluating targeted strategies to reduce marijuana use and potential subsequent impaired driving, especially among teenagers and young adults.

Research has determined that co-use of marijuana or illicit drugs with alcohol increases the risk for driving impairment (5,6). The use of these substances has been associated with impairment of psychomotor and cognitive functions while driving (6,7). In addition, previous research has demonstrated evidence of a statistical association between marijuana use and increased risk for motor vehicle crashes; however, methodologic limitations of studies limit inference of causation (8). Scientific studies have been unable to link blood tetrahydrocannabinol levels to driving impairment (8), and the effects of marijuana in drivers likely varies by dose, potency of the product consumed, means of consumption (e.g., smoking, eating, or vaping), length of use, and co-use of other substances, including alcohol. Additional data are needed to clarify the contribution of drug and polysubstance use to impaired driving prevalence and the resulting crashes, injuries, and deaths.

A national roadside survey using biochemical specimens among drivers aged ≥16 years found that during 2013–2014, the percentages of weekend nighttime drivers who tested positive for alcohol, marijuana (i.e., tetrahydrocannabinol) and illicit drugs were 8.3%, 12.6%, and 15.1%, respectively (9), although a positive test does not necessarily imply impairment. Collecting and testing biologic specimens (e.g., blood or oral fluids) currently required to test for drugs has challenges, including, in some circumstances, the need for a judge to order collection and testing (which can delay roadside testing, thus allowing drug levels to drop with time); variation in substances tested and methodology used by different toxicology laboratories; and the current state of development of oral fluid testing. The increased use of marijuana and some illicit drugs in the United States (4) along with the results of this report, point to the need for rapid and sensitive assessment tools to ascertain the presence of and impairment by marijuana and other illicit drugs. In addition, adoption and application of standards for toxicology testing and support for laboratories to implement recommendations are needed to improve understanding of the prevalence of drug- and polysubstance-impaired driving (10).

The findings in this report are subject to at least five limitations. First, because NSDUH data are self-reported, they are subject to recall and social desirability biases. Second, variations in laws and regulations among states and counties regarding marijuana could have resulted in negative responses to the NSDUH substance use survey questions for fear of legal consequences, leading to an underestimation of the prevalence of the use and driving under the influence in some jurisdictions. Third, the NSDUH questions are not limited to driving under the influence of marijuana only or each illegal substance only; therefore, persons might be driving under the influence of more than one substance at a given time. Fourth, self-reported data are subject to the respondents’ interpretations of being under the influence of a drug. Finally, NSDUH does not assess whether all respondents drive; therefore, reported percentages of impaired drivers might be underestimated.

Impaired driving is a serious public health concern that needs to be addressed to safeguard the health and safety of all who use the road, including drivers, passengers, pedestrians, bicyclists, and motorcyclists. Collaboration among public health, transportation safety, law enforcement, and federal and state officials is needed for the development, evaluation, and further implementation of strategies to prevent alcohol-, drug-, and polysubstance-impaired driving (2). In addition, standardized testing for alcohol and drugs among impaired drivers and drivers involved in fatal crashes could advance understanding of drug- and polysubstance-impaired driving and assist states and communities with targeted prevention efforts.