Driving After Drug or Alcohol Use

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• CHAPTER 1: INTRODUCTION
  • Summary of NHSDA Methodology
  • Limitations of the Data
  • Overview of the Report

CHAPTER 1: INTRODUCTION

Drug-impaired driving has been identified as a major threat to public health, but there are limited data on the prevalence and nature of drug use and driving. In an effort to address the lack of available data, the Substance Abuse and Mental Health Services Administration (SAMHSA), in conjunction with the National Highway Traffic Safety Administration (NHTSA), developed and implemented a special module of questions in the 1996 National Household Survey on Drug Abuse (NHSDA). The Driving Behaviors Module was designed to gather information about the prevalence and patterns of substance use among U.S. drivers.

An earlier study conducted by the NHTSA (1992b) examined the incidence and role of drug use and driving in fatal traffic crashes. Using blood specimens, police reports, coroner/medical examiner reports, and reports from the Fatality Analysis Reporting System [ Previously called the Fatal Accident Reporting System.] (FARS), the study examined the role of alcohol and 43 other drugs in the deaths of 1,882 drivers of cars, trucks, and motorcycles in seven states from 1990 to 1991. Nearly 60 percent of the blood specimens examined tested positive for at least one substance. Among drivers who tested positive for only one substance, alcohol was the most common substance. The majority of drivers were legally intoxicated with blood alcohol concentration (BAC) levels greater than .10 g/dl. Even though alcohol was by far the most prevalent substance found, cannabis and cocaine were detected in about six percent of the sample. Additionally, among drivers who tested positive for multiple substances, multiple drug use without alcohol use was extremely rare.

The 1992 NHTSA study also examined the demographic characteristics of drivers who tested positive for drug use. Drivers age 15-24 were more likely to test positive for cannabis and cocaine, with or without alcohol, than drivers age 25 and older. Additionally, male drivers were over-represented among those who tested positive for alcohol and every drug group except the benzodiazepines-only group (i.e., minor tranquilizers), where females predominated.

Studies on driving following drug use are limited; therefore, this report makes a significant contribution to our knowledge of the prevalence and patterns of drug use and driving. The primary focus of this report is driving following drug use, however, data on drinking and driving are also presented for comparison purposes. The data presented in this report were obtained in face-to-face interviews with 11,847 NHSDA respondents age 16 and older, representing over 166 million U.S. drivers, who reported that they had driven a car or other motor vehicle in the 12 months prior to the interview, as well as whether or not they had driven within two hours of drug and/or alcohol use.

Summary of NHSDA Methodology

The NHSDA is the primary source of statistical information on the use of tobacco, alcohol, and illicit drugs in the United States. Conducted by the Federal Government since 1971, the NHSDA is administered to a representative sample of the U.S. population age 12 and older at their place of residence. The NHSDA has been directed by the Substance Abuse and Mental Health Services Administration (SAMHSA) of the U.S. Department of Health and Human Services since October 1992. The main goal of the survey is to estimate and monitor trends in the prevalence of substance use in the United States.

The NHSDA sample consists of residents of households, persons living in non-institutional group quarters (e.g., shelters, dormitories, rooming houses), and civilians living on military bases. The sample excludes active military personnel, U.S. citizens living abroad, residents of institutional settings (e.g., prisons and hospitals), and homeless persons not living in a shelter at the time of the survey.

The household interview takes approximately one hour to complete and incorporates procedures designed to maximize truthful responses to potentially sensitive questions about illicit drug use (e.g., the use of self-administered answer sheets). Data are collected on the recency and frequency of use of various licit and illicit drugs, opinions about drugs, problems associated with drug use, and drug treatment experiences. In addition to detailed information about substance use, the NHSDA also collects basic demographic information on employment, race/ethnicity, age, education, income, marital status, health status, mental problems, health insurance, utilization of health services, and access to health care. A more detailed description of the NHSDA methodology is included in Appendix C.

In some years, agencies have co-sponsored the NHSDA to support the collection of data on special topics. The 1996 NHSDA included a special module of supplemental questions on driving behaviors in conjunction with substance use. The Driving Behaviors Module, funded by the National Highway Traffic Safety Administration, collected detailed information on the occurrence of driving within two hours of drug and/or alcohol use. The data collected in the module include: substance use and driving patterns, perceived ability to drive safely and likelihood of being stopped by the police when driving following substance use, as well as reasons for driving within two hours of substance use. A copy of the module is included in Appendix E.

Limitations of the Data

The data presented in this report have several important limitations. First, the data are entirely self-reported, including the main analysis variable (i.e., whether or not the respondent drove after drug and/or alcohol use). Therefore, behavioral estimations may be inaccurate or biased, particularly for substance use behaviors. A second limitation of the data is that the Driving Behaviors Module addresses the use of only the following drugs: marijuana/ hashish,cocaine, tranquilizers, sedatives, and stimulants. For tranquilizers, sedatives, and stimulants, over-the-counter use for medical reasons and use "for the experience or feeling caused" are included; however, driving following either type of use would constitute drug-impaired driving. In addition, some of the more detailed data are available only for driving after marijuana/hashish or cocaine use; however, use of these drugs is generally more common. Third, there are no data on the quantity of each drug used before driving; however, level of impairment following alcohol use is inferred from blood alcohol concentration (BAC) level estimates. A final limitation of the data is the relatively rare occurrence of driving after drug use. In some cases, due to small sample sizes, statistical precision for the behavioral estimates was inadequate [ These estimates are indicated with an asterisk in the tables. ] or statistical significance at the .05 level was not achieved. Only the statistically significant findings are discussed in the text. These, as well as other limitations of the data, are discussed in further detail in Appendix C.

Overview of the Report

The format of this report is primarily graphical, with text presented to highlight statistically significant findings. However, when two estimates are found to be significantly different it does not necessarily imply that the difference is large or meaningful. Rather, statistical significance means that one can conclude, with a small risk of error, that the two estimates would be found to be different if the survey were replicated with different samples drawn from the same population, using the same sampling procedures. That is, the differences cannot be attributed solely to sampling error.

All of the data in this report reflect behaviors that occurred during the 12 month period immediately preceding the NHSDA interviews. Chapter 2 begins by examining the prevalence of driving after substance use in various demographic groups. Chapter 2 also contains the following sections:

·Section A—Comparison of the demographic characteristics of respondents who drove within two hours of drug use, with or without alcohol, to those of respondents who drove following alcohol use only.

·Section B—Description of the driving patterns of respondents who drove within two hours of marijuana use.

·Section C—Description of the perceptions of respondents who drove within two hours of marijuana use.

·Section D—Description of the substance use patterns of respondents who drove within two hours of marijuana use.

·Section E—Examination of the drinking patterns and blood alcohol concentration (BAC) level estimates of respondents who drove within two hours of alcohol use.

Chapter 3 summarizes the findings of this report, providing a demographic profile of NHSDA respondents who reported driving following drug use. Chapter 3 also provides implications for policy and suggestions for future research. The appendices contain detailed data tables, tables of the standard errors for the estimates, a detailed description of the NHSDA methodology, the procedure used for calculating the BAC level estimates, and the Driving Behaviors Module.

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This page was last updated on June 16, 2008.

SAMHSA, an agency in the Department of Health and Human Services, is the Federal Government's lead agency for improving the quality and availability of substance abuse prevention, addiction treatment, and mental health services in the United States.