Alcohol-impaired driving has been a subject of intense interest and research for well over 60 years. When we compare the effects of consuming alcohol on behavior, attention, and cognition, we find that impairment in these functions strongly relates with alcohol concentration in blood or breath. Impairment increases with rising alcohol concentration and declines with dropping alcohol concentration.
While fewer studies have examined the relationship between THC blood levels (the psychoactive ingredient in marijuana) and degree of impairment, in those studies that have been conducted, the consistent finding is that the level of THC in the blood and the degree of impairment do not appear to be closely related. Unlike alcohol, there is no correlation between rising THC level and driver impairment. Some research studies have found that peak performance deficits are observed long after the peak THC level occurs.