Abstract
Objectives. We examined prevalence, trends, and correlates of driving or riding after use of drugs or alcohol among US high school seniors from 2001 to 2011.
Methods. Data come from Monitoring the Future, an annual survey of nationally representative samples of high school seniors. We used logistic regressions with data from more than 22 000 respondents to examine multivariate associations with demographic and lifestyle factors.
Results. Large numbers of US high school seniors put themselves and others at great risk of harm by driving after using marijuana or other illicit drugs or drinking alcohol or by riding in a vehicle whose driver had used marijuana, other illicit drugs, or alcohol. Driving after drinking has declined in recent years, but driving after use of marijuana has increased. A higher percentage of students reported driving after using marijuana than after having 5 or more alcoholic drinks. Risky driving and riding behaviors differed little between demographic subgroups but considerably according to lifestyle factors.
Conclusions. Stronger efforts are needed to combat adolescent driving under the influence of illicit drugs.
Motor vehicle crashes are a leading cause of mortality and morbidity among American youths.1 Alcohol is often a factor in these crashes, and alcohol-impaired driving has long been a focus of attention. In recent years, rates of driving under the influence of alcohol among American youths have declined, but are still unacceptably high.2 Impaired driving caused by use of substances other than alcohol has become an issue of increased concern. The National Institute on Drug Abuse commissioned a white paper on drugged-driving research,3 and a recent National Drug Control Strategy includes a goal of reducing drugged driving in the United States by 10% by the year 2015.4 Specifically, the Office of National Drug Control Policy aims to make preventing drugged driving a national priority on par with preventing drunk driving.5
The issue of drugs and driving has been of interest to the federal government for some time. The National Highway Traffic Safety Administration commissioned Drug Use and Highway Safety, a 101-page report published in 1971.6 In 1977, the National Institute on Drug Abuse issued its 11th research monograph, Drugs and Driving.7 The institute addressed the role of marijuana in driving in a 1980 research monograph.8
The National Highway Traffic Safety Administration conducted national roadside surveys of alcohol use by drivers in 1973, 1986, 1996, and 2007, but the 2007 survey was the first to include assessment of drug use.9 The 2007 survey found a dramatic decline in the percentage of nighttime drivers with blood alcohol concentrations above the legal limit of 0.08% to 2.2%, down from 7.5% in 1973.10 Among nighttime drivers, 16.3% tested positive for drugs, most often marijuana (8.6%). In another study by the traffic safety agency, data from the Fatal Accident Reporting System showed a rise in drug involvement in motor vehicle crashes.11 A recent systematic review and meta-analysis of the role of a comprehensive list of illicit or prescribed drugs in motor vehicle crashes found that most of the drugs were associated with increased risk.12 Other studies have reported on the role of marijuana in crashes.13,14 A recent systematic review of the association between marijuana use and risk of motor vehicle collision concluded that “acute cannabis consumption is associated with an increased risk of a motor vehicle crash, especially for fatal collisions.”15(p1)
Young drivers are particularly likely to be involved in motor vehicle crashes, so it is important to monitor their drug, drinking, and driving behaviors. We previously studied prevalence and trends in these behaviors among American high school seniors from 2001 to 2006 and concluded that impaired driving by youths remained a problem needing serious attention despite some modest progress in recent years.16
We analyzed data from 2001 through 2011 to answer the following questions about high school seniors:
What changes are taking place in the percentage who drive after using marijuana, using other illicit drugs, drinking any alcohol, or having 5 or more drinks?
What changes are taking place in the percentage who ride in a vehicle whose driver has used marijuana, used other illicit drugs, drunk any alcohol, or had 5 or more drinks?
What demographic and psychosocial characteristics are associated with these behaviors?
What percentage of individuals who report driving after using marijuana also report driving after heavy drinking?
What percentage of individuals who report driving after using marijuana are involved in accidents?
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METHODS
Our data came from the Monitoring the Future project, which has conducted annual surveys of nationally representative samples of American high school seniors since 1975. Survey procedures are described in detail elsewhere.17 Nationally representative samples of about 17 000 12th-grade students, located in about 135 schools, were selected each year through a multistage scientific sampling procedure. Confidential, self-completed questionnaires were administered during school hours, usually in a regularly scheduled class period, by professional interviewers employed by the University of Michigan. The questions on driving or riding after drinking or using drugs were included on only 1 of 6 forms, distributed in a random sequence within the classroom, so responses to these questions came from a random one sixth of the total sample of students. Questions on driving or riding in a motor vehicle after use of marijuana or other illicit drugs were added to the study in 2001, so we analyzed data from 2001 to 2011. Student response rates averaged 82% (range = 79%–85%); the great majority of nonresponse was attributable to absenteeism.
Driving and riding behaviors were assessed by the following questions:
During the last two weeks, how many times have you driven a car, truck, or motorcycle after … drinking alcohol? … having five or more drinks in a row? … smoking marijuana? … using other illicit drugs?
A second set of questions asked:
During the last two weeks, how many times (if any) have you been a passenger in a car … when the driver had been drinking? … when you think the driver had 5 or more drinks? … when the driver had been smoking marijuana? … when the driver had been using other illicit drugs?
Response categories were none, once, twice, 3 to 5 times, 6 to 9 times, and 10 or more times. We collapsed these into binary values (0 = none; 1 ≥ once). The questions did not assume that behaviors were mutually exclusive. For example, an individual could have driven after smoking marijuana and drinking heavily on the same occasion.
Two items asked about tickets or warnings for moving violations and being in an accident while driving:
Within the LAST 12 MONTHS how many times, if any, have you received a ticket (OR been stopped and warned) for moving violations, such as speeding, running a stop light, or improper passing?
During the LAST 12 MONTHS, how many accidents have you had while you were driving (whether or not you were responsible)?
Respondents were instructed not to include bumps or scratches in parking lots.
All demographic and lifestyle measures except geographic region and population density were obtained by self-report. Number of parents in the household indicated whether the respondent lived with zero, 1, or 2 parents or guardians. Parental education, a proxy for socioeconomic position (SEP), was derived from an average of 2 items (1 missing response allowed) about the amount of education achieved by parents (responses were completed grade school or less, some high school, completed high school, some college, completed college, and graduate or professional school after college). Religious commitment (high, medium, or low) was an average of 2 items (1 missing response allowed) assessing the importance of religion (responses were not important, a little important, pretty important, and very important) and frequency of attendance at religious services (never, rarely, 1–2 times/month, or ≥ 1 time/week). These 2 items were not asked of students in schools located in California, because of state regulations; thus we assigned all California students as missing data on this measure and treated this as a separate category.
Grade point average was the average during high school. Truancy was an average (categorized as none, low, medium, or high) of the frequency of skipping classes or whole days of school during the past 4 weeks. Evenings out was the number of evenings out for fun and recreation in a typical week (responses were 30). Miles driven was the number of miles the respondent reported driving a car, truck, or motorcycle in an average week (we categorized responses as 0, 1–50, 51–100, and > 100). All of these measures of lifestyle factors have been used extensively in other studies. More details on their psychometric properties, particularly construct validity, are provided elsewhere.18 We assigned geographic region according to US Census classifications of states into 4 regions: Northeast, Midwest, South, and West. We categorized population density according to US Census statistical areas: large metropolitan, other metropolitan, and nonmetropolitan.
We chose all of these variables because of their inherent interest (e.g., gender) or because they have demonstrated associations with drug or alcohol use.19,20 We weighted the data to adjust for differential probabilities of sample selection. We conducted logistic regressions for the multivariate analyses; these regressions took appropriate account of the complex sample