Drugs and traffic crash responsibility: a study of injured motorists in Colorado.

BACKGROUND: Alcohol is a contributing factor in a large proportion of traffic crashes. However, the role of other drugs is unknown. The objectives of this study are to determine the prevalence of recent drug use among drivers injured in traffic crashes, and to determine the extent to which drugs are responsible for crashes.
METHODS: We studied 414 injured drivers who presented to an urban emergency department within 1 hour of their crash. Demographic and injury data were collected from medical records. Urine toxicologic assays were conducted for legal and illegal drugs. Traffic crash reports were analyzed for crash responsibility by a trained crash reconstructionist. The causal role of drugs in traffic crashes was measured by comparing drug assay results in drivers judged responsible for their crashes (cases) and those not responsible (controls). Odds ratios and 95% confidence intervals (CIs) were calculated.
RESULTS: Thirty-two percent (95% CI = 27-37) of the urine samples were positive for at least one potentially impairing drug. Marijuana was detected most frequently (17%), surpassing alcohol (14%). Compared with drug- and alcohol-free drivers, the odds of crash responsibility were higher in drivers testing positive for alcohol alone (odds radio [OR] = 3.2, 95% CI = 1.1-9.4) and in drivers testing positive for alcohol in combination with other drugs (OR = 3.5, 95% CI = 1.2-11.4). Marijuana alone was not associated with crash responsibility (OR = 1.1, 95% CI = 0.5-2.4). In a multivariate analysis, controlling for age, gender, seat belt use, and other confounding variables, only alcohol predicted crash responsibility.
CONCLUSION: Alcohol remains the dominant drug associated with injury-producing traffic crashes. Marijuana is often detected, but in the absence of alcohol, it is not associated with crash responsibility.