In order to reduce crash risk, transportation policies must address driver impairment, a significant factor associated with crash risk. Cannabis is one of the most commonly consumed drugs in America and has been suspected as a source of driver impairment. Policies have yet to specifically target cannabis use and driving mainly due to a lack of research on the topic. This article provides a summary of the research literature concerning how cannabis affects driving ability. Most of the research on the effects of cannabis have occurred in laboratory settings and have shown that cannabis has the most significant impairment effects on memory and attention. Prevalence rates for cannabis use and the psychopharmacology of cannabis are reviewed; estimates indicate that as many as 90 percent of individuals are willing to drive following cannabis consumption. Several studies have directly examined the effects of cannabis on driving performance. These studies indicated that cannabis users were more aware of their impaired state than alcohol users and, as such, exercised more caution while driving to compensate for the impairment. Crash risk was found to be elevated for drivers under the influence of cannabis, although the crash risk for drivers under the influence of alcohol was significantly higher. The limited amount of research on this topic does not lend itself to effective policy develop and, as such, more research on the effects of cannabis use on driving ability is necessary. Table, note, references