A number of laboratory studies of information processing have been performed with both alcohol and marihuana treatments. Data from these experiments are reviewed in terms of implications for driving. Peripheral processes appear not to be significantly degraded by either substance, but central processes are affected by both. There is evidence that alcohol causes a slowing of central processes whereas marihuana effects appear to be related to attention lapses. Personality variables, as measured by the MMPI, are compared for alcohol and marihuana users. It is concluded that driving skills are likely to be impaired by marihuana, but there is no evidence that marihuana use creates a greater traffic hazard than alcohol use. The deficits in information processing differ for the two drugs, and related accidents may be expected to reflect those differences.