However, direct epidemiological evidence for this belief exists only for ethanol. The goal of this investigation was to determine the incidence of the psychoactive ingredient for marihuana, along with ethanol and other drugs in blood specimens from a carefully defined population of dead drivers. Although THC and other drugs were present in a small number of the blood specimens, the large number of specimens that had high blood ethanol concentrations indicated that alcohol is still the major drug affecting highway safety. (Author abstract)