Driving Under the Influence of Alcohol, Marijuana, and Alcohol and Marijuana Combined Among Persons Aged 16–25 Years — United States, 2002–2014

Alejandro Azofeifa, DDS1; Margaret E. Mattson, PhD1; Rob Lyerla, PhD1

Motor vehicle accidents are the leading cause of death among youths and young adults aged 16–25 years in the United States (1). The prevalence of drinking and driving among high school students aged 16–19 years has declined by 54%, from 22.3% in 1991 to 10.3% in 2011 (2). However, the prevalence of weekend nighttime driving under the influence of marijuana (based on biochemical assays) among drivers aged ≥16 years has increased by 48%, from 8.6% in 2007 to 12.6% in 2013–2014 (3). Use of marijuana alone and in combination with alcohol has been shown to impair driving abilities (4–9). This report provides the most recent self-reported national estimates of driving under the influence of alcohol, marijuana, and alcohol and marijuana combined among persons aged 16–25 years, using data from the Substance Abuse and Mental Health Services Administration (SAMHSA) National Survey on Drug Use and Health (NSDUH) from 2002–2014. Prevalence data on driving under the influence of both substances were examined for two age groups (16–20 years and 21–25 years) and by sex and race/ethnicity. During 2002–2014, the prevalence of driving under the influence of alcohol alone significantly declined by 59% among persons aged 16–20 years (from 16.2% in 2002 to 6.6% in 2014; p<0.001) and 38% among persons 21–25 years (from 29.1% in 2002 to 18.1% in 2014; p<0.001). In addition, the prevalence of driving under the influence of alcohol and marijuana combined significantly declined by 39%, from 2.3% in 2002 to 1.4% in 2014 (p<0.001) among persons aged 16–20 years and from 3.1% in 2002 to 1.9% in 2014 (p<0.001) among persons aged 21–25 years. The prevalence of driving under the influence of marijuana alone declined 18%, from 3.8% in 2002 to 3.1% in 2014 (p = 0.05) only among persons aged 16–20 years. Effective public safety interventions,* such as minimum legal drinking age laws, prohibition of driving with any alcohol level >0 for persons aged <21 years, targeted mass media campaigns, roadside testing (e.g., sobriety checkpoints), and graduated driver licensing programs (10) have contributed to the decline in driving under the influence of alcohol in this population. These or similar interventions might be useful to prevent driving under the influence of other substances, such as marijuana alone or combined with other substances.

NSDUH collects annual information about the use of illicit drugs,† alcohol, and tobacco among the noninstitutionalized U.S. civilian population aged ≥12 years via household face-to-face interviews, using a computer-assisted personal interviewing system.§ Unweighted sample sizes for 2002–2014 survey cycles included 383,700 respondents aged 16–25 years. Alcohol use was defined as a report of drinking an alcoholic beverage within the past 12 months. Marijuana use was defined as a report of using marijuana ("pot" or "grass") or hashish ("hash") within the past 12 months. Driving under the influence of alcohol alone was defined as an affirmative response to the question, "During the past 12 months, have you driven a vehicle while you were under the influence of alcohol only?" Driving under the influence of marijuana only was defined as an affirmative response to the survey question, "During the past 12 months, have you driven a vehicle while you were under the influence of illegal drugs only?" (restricted to respondents who reported past-year marijuana use and no other illicit drug use). Driving under the influence of alcohol and marijuana was defined as an affirmative response to the question, "During the past 12 months, have you driven a vehicle while you were under the influence of a combination of alcohol and illegal drugs used together?" (restricted to respondents who reported past-year marijuana use and no other illicit drug use). Respondents who reported past-year marijuana use and did not report the use of any other illegal drugs during the past year, and who reported driving under the influence of drugs in the past year were considered to have driven under the influence of marijuana in the past year. Data on driving under the influence of alcohol alone, marijuana alone, and alcohol and marijuana combined were examined by sex, age, and race/ethnicity. Age was categorized by age of eligibility to drive a motor vehicle (16–20 years) and by legally permitted drinking age (21–25 years). Data were weighted to provide nationally representative estimates. Logistic regression analysis was used to examine temporal trends from 2002–2014 survey cycles; p-values of <0.05 were considered statistically significant.

Overall, in 2014, the reported prevalence of driving under the influence of alcohol alone was greater than that of marijuana alone or alcohol and marijuana combined, and when stratified by sex, age group, and race/ethnicity (Table). During 2002–2014, the reported prevalence of driving under the influence of alcohol alone among persons aged 16–20 years and 21–25 years declined from 16.2% to 6.6% and from 29.1% to 18.1%, respectively (p<0.001 for trend) (Figure 1). In addition, the reported prevalence of driving under the influence of alcohol and marijuana combined among persons aged 16–20 years and 21–25 years declined from 2.3% to 1.4% and 3.1% to 1.9%, respectively (p<0.001 for trend) (Figure 1). Reported prevalence of driving under the influence of marijuana alone did not change significantly during 2002–2014 in either age group. The reported prevalence of driving under the influence of alcohol alone increased with age, from 1.5% among persons aged 16 years to 18.1% among persons aged 21 years (Figure 2).

Discussion

During 2002–2014, the prevalence of driving under the influence of alcohol alone and alcohol and marijuana combined significantly declined among persons aged 16–20 years and 21–25 years. Data from 2014 show that underage (<21 years) drinking and driving does occur at age 16 years and that percentages of persons who report driving under the influence of alcohol increase as age increases, peaking at around the minimum legal drinking age (21 years). Because driving under the influence of alcohol, marijuana, or a combination of alcohol and marijuana has been shown to impair some driving abilities (4–9), additional prevention efforts are needed to further reduce driving under the influence of both substances. Effective strategies to reduce alcohol-impaired driving recommended by the Community Preventive Services Task Force can also be relevant to marijuana impaired driving.

Despite the decline in reported driving under the influence of alcohol alone and alcohol and marijuana combined from 2002 to 2014, data from the 2014 NSDUH¶ indicate that 60% of young adults aged 18–25 years used alcohol during the past month, 38% engaged in binge drinking,** and 20% had used marijuana. Marijuana is the illicit drug most frequently used in this age group. Furthermore, the 2013–2014 National Roadside Survey showed that the prevalence of driving under the influence of marijuana has increased 48% among weekend nighttime drivers aged ≥16 years (3). Differences in the findings reported here and those from the National Roadside Survey might be attributable to survey self-reporting bias; what is detected and tested by road law enforcement and what is perceived as driving impairment ("being under the influence") by a survey respondent could be different. Also, the National Roadside Survey might have overestimated the proportion of impaired drivers because it tested for marijuana's psychoactive substances, delta-9-tetrahydrocannabinol (THC) and 11-hydroxy-delta-9-tetrahydrocannabinol, in oral fluids and blood levels. Some psychoactive substances might remain detectable for long periods of time after impairment is no longer present. In addition, the National Roadside Survey only includes weekend nighttime drivers aged ≥16 years. Differences also could represent greater detection of alcohol and drug-positive drivers during weekend nighttime periods (3).

Alcohol and marijuana combined have cognitive and psychomotor effects that might impair driving abilities (4–9). The effects of driving under the influence of both substances on individual persons depend on many factors, including amount consumed or smoked, body mass index, absorption into the bloodstream, age, sex, and alcohol or marijuana use habits and frequency. Road testing for alcohol is commonly implemented and used by law enforcement; however, because no standard measurement to determine marijuana-related driving impairment currently exists, road testing is challenging and practices vary by state. Given the prevalence of alcohol use, binge drinking, and marijuana use among persons aged 18–25 years, additional education, prevention efforts, and additional road safety measures (e.g., sobriety checkpoints, ignition interlock, improved field testing for THC levels, and standards for determining driving impairment) focused on younger adults might be needed to ensure safety among drivers, vehicle occupants, and pedestrians.

The findings in this report are subject to at least four limitations. First, data are self-reported and are subject to recall and social desirability bias, and individual perception of driving impairment. Second, only respondents who reported past-year marijuana use, did not report the use of any other illegal drugs in the past year, and reported driving under the influence of drugs in the past year were coded in the survey as having driven under the influence of marijuana in the past year. Therefore, the estimates of driving under the influence of marijuana alone and combined with alcohol do not include the 35.9% of all marijuana users who reported using some other illicit drug in the past year, and as a result, the estimated number of persons who self-reported driving under the influence of marijuana in the past year likely was underestimated. Third, given the differences in marijuana legislation among states, some marijuana users could possibly have responded negatively to NSDUH's original question, which might have contributed to underestimation of driving under the influence of marijuana. Finally, currently no level of consumption to determine impairment of driving while under the influence of marijuana exists; therefore, self-reported data are subject to various interpretations of impairment (i.e., being under the influence) among individual users, and likely represent a conservative estimate.

Youth and young adult driving under the influence of any psychoactive substance is an important public health problem that needs the attention of parents, public health officials, law enforcement, and federal and state officials. In addition, alcohol and marijuana initiation might coincide with youths' first driving experiences. Therefore, additional research and surveillance data are needed to better understand the magnitude of the impact of driving under the influence of psychoactive substances, especially marijuana, to ensure public road safety.

Acknowledgments

Laura J. Sherman, PhD, Division of Evaluation, Analysis and Quality, Center for Behavior Health Statistics and Quality, SAMHSA; Stephanie Barnett, Michael Penne, MPH, Jeremy Porter, RTI International, North Carolina.

1Division of Evaluation, Analysis and Quality, Center for Behavior Health Statistics and Quality, Substance Abuse and Mental Health Services Administration.

Corresponding author: Alejandro Azofeifa, Alejandro.Azofeifa@samhsa.hhs.gov, 240-276-1282.

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