The materials reviewed here are those dealing with the drug-crash problem created by various groups of drivers. The scope of the review included foreign as well as U.S. literature with a direct bearing on highway safety. The review covers the period 1980 to the present and addresses the following four major areas of research:
- detection and measurement of drugs,
- experimental research,
- epidemiologic research, and
- countermeasures for dealing with drug-impaired driving.
The review emphasizes controlled substances to include marijuana, benzodiazepines, non-benzodiazepine sedative and hypnotic drugs, and others such as amphetamines, cocaine, hallucinogens, and narcotic drugs. However, research related to any other drugs having the potential to significantly impair driving is also included in the review. Applicable research conducted in foreign countries, and documented in the English language, is included.
Specific topical areas examined for literature are:
- Types of drugs that have been addressed in the scientific literature pertinent to traffic safety, and the general nature of their biokinetics, their measurement, and their acute and chronic effects on the human body.
- Effects of these types of drugs on behaviors related to driving-related performance and to driving performance.
- Drug usage and patterns of usage in the general driving age population.
- The presence of drugs in various types in crashes, that is:
- Fatal crashes,
- Non-fatal crashes,
- Crashes involving pedestrians and bicyclists.
- The crash risk created by drug usage -- the question of causality.
- Characteristics of people who use drugs and drive, including:
- Biographical variables - for example, age, sex, race, ethnicity, income;
- Driving variables - for example, where, when, types of vehicles, traffic law violations, trips;
- Drug usage variables - for example, where, when, frequency, problem users;
- Other variables - for example, psychosocial factors, medical conditions.
- Approaches to dealing with the drug-impaired driving problem, and the effectiveness of those approaches, for example:
- Public Information and Education,
- Alternative transportation.
- Methodologic problems in all of the above areas of research.
- Future research needs in all of the above areas.
Major issues considered when reviewing a document were:
- Subjects and subject selection. Whether the study's subjects were representative of the general population of drivers, were a special group selected perhaps as a matter of convenience, or were selected in some manner that makes it difficult to determine exactly who they were and what group they represented.
- Nature of the data used in the study. Whether the data used in the study were valid for quantifying the study variables, that is, which kinds of specimens (blood, urine, other) were analyzed for which drugs; how were the specimens collected and preserved; how were they analyzed; and how were the results classified?
- Findings of the study. Of major concern in this respect is whether the findings are consistent with the study design, the data acquired and used in the study, and the analysis of the data. Inappropriate interpretations of a study's results have to be explicitly identified, even though we did not necessarily discard an otherwise useful study because its findings were somehow flawed.
- Nature and appropriateness of the statistical techniques. Whether the study contains a clear explanation of the statistical techniques, and whether the techniques described were appropriate for detecting practically meaningful differences between groups that were compared in the study.
- Drugs as a causal factor. If a clinical approach was used, whether the procedure used led to a plausible conclusion as to whether drugs could have caused the crashes studied. If an experimental approach were used, issues of the comparability of the crash group and the comparison non-crash group will arise, along with issues related to the method used for determining any relative risk factors developed from such an analysis. Of major concern will be the issue of whether drug presence among drivers studied is being extrapolated inappropriately to mean causation.
SOURCES OF LITERATURE
Literature sources included collections and individual documents that have not been placed in traditional collections. Types of repositories that were contacted include:
- Specialized libraries of highway safety literature maintained by such organizations as NHTSA, The University of Michigan Transportation Research Institute (UMTRI), and the Insurance Institute for Highway Safety;
- Specialized computerized information services such as the Transportation Research Information System (TRIS) and its highway transportation subfile Highway Research Information System (HRIS), MEDLARS, MEDLINE, and EMBASE;
- Specialized information clearinghouses and abstracting services such as National Institute on Drug Abuse; Johns Hopkins University's Alcohol, Drugs, and Driving: Abstracts and Reviews; the Addiction Research Foundation (Canada); Alcohol, Drugs and Traffic Safety: Current Research Literature;
- General libraries having collections in related disciplines such as medicine, law, and the social sciences; and
- General repositories and information services maintained by governmental agencies such as the National Technical Information Service and the Library of Congress.
The UMTRI library was the central focus and coordinating element of the literature search and collection activities. The research library at Purdue University was also accessed in the literature search.
LITERATURE SEARCH PROCEDURES
The starting point in the search was recent bibliographies and reviews of directly related materials. Relevant bibliographies and reviews were identified through a search of the UMTRI library, and through discussions with subject-matter experts (including a project advisory group) and others.
The next step in the search was to examine specific journals and conference proceedings known by the principal investigators to contain pertinent materials. These documents were not necessarily concerned directly with highway safety, but tended to focus on other related disciplines such as human factors, toxicology, and drug studies in general.
Another source considered for this review is material generated by the various Drug Evaluation and Classification (DEC) programs throughout the country. This material appears in different forms, including newsletters and pseudo-journals that report claims on the accomplishments of the programs, some of which are supported by data. NHTSA staff in Washington identified a number of contacts for gaining access to such materials.
Each document acquired through the literature search was screened for inclusion in this review. Two levels of screening were performed: (1) an initial screening to determine whether a document should undergo further substantive examination by the Principal Investigator (Mr. Jones), and (2) a final screening by the Principal Investigator of documents surviving the initial screening. Criteria contained in each these levels are indicated below:
- Level 1 - Initial Screening
- Criterion 1.1 - The document must address pertinent topics as it appeared to when it was identified initially.
- Criterion 1.2 - The document must at least purport to have scientific validity. Documents merely reflecting the unsupported opinions of their author were not retained for review.
- Level 2 - Final Screening
- Criterion 2.1 - The study documented must actually have scientific validity, that is:
- The study's objectives and the hypotheses being examined must be clearly stated. The study design and the research method must be appropriate for accomplishing the study's objectives, and must be thoroughly described in the document. It is essential that the characteristics of the study's subjects and how they were selected be described, and that the selection criteria were consistent with study objectives.
- The quality of the data must adhere to generally acceptable standards for scientific research.
- Sample size (N), and the probability of a result occurring by chance alone (p) are reported.
- The actual amount of differences among groups that may be compared in a study (that is, effect size), and the level of significance for rejecting the null hypothesis of no differences (a) are reported.
- The analysis techniques used are appropriate and properly used.
- Criterion 2.2 - The treatment of the results of the study must be complete, objective, and balanced, and the findings and conclusions must be sound.
Failure of a document to meet either of the Level-1 criteria resulted in its elimination from the review. However, documents that met Level-1 criteria but were flawed with respect to one or more Level-2 criteria were not necessarily rejected. For example, a study that was well-designed and executed, but made conclusions that did not flow from its findings might have been kept in the update. The update's commentary on that study noted the inconsistencies between the research results and the conclusions, and offered a more consistent interpretation of the results. Some other flawed studies that were being widely quoted in the non-scientific literature were retained simply to document their flaws.
Finally, the last step in conducting the update was the preparation of this document, the update report. Bibliographic information on each article was entered into a computerized bibliographic database.