Marihuana and driving†

Abstract

Survey studies have found that marihuana use is increasing and that users frequently drive under its influence. But there is little direct epidemiological evidence to indicate if the presence of marihuana in drivers increases accident probability. However, there is a large body of experimental evidence indicating that marihuana impairs the performance of skills important for driving. Perceptual and attention functions show large decrements under marihuana with a less certain deficit for various tracking functions. Marihuana studies in driving simulators have found the greatest deficit in perceiving and responding to potential dangers from the environment. Simulator studies of risk taking have found no evidence for impairment. Several studies of performance in actual cars have also demonstrated performance decrements but the behavioral functions impaired have not been clarified. In summary the experimental evidence suggests strongly that marihuana use while driving produces a performance impairment.