July 2017 NHTSA report – The report describes the absorption, distribution and elimination of delta-9-tetrahydrocannabinal (THC) the primary psychoactive substance in marijuana, in the body. It contrasts this process with the absorption, distribution and elimination of alcohol in the body, as they are very different processes. The poor correlation of THC concentrations in the blood with impairment is discussed, along with the implication that setting per se levels is not meaningful. Some of the challenges of measuring driving impairment resulting from marijuana use are reviewed. State laws relating to marijuana and driving are presented. What is known about the prevalence of marijuana-impaired driving and the crash risk associated with marijuana-impaired driving is reviewed.
Finally, the report presents information on training for law enforcement to detect marijuana impairment in drivers, the feasibility of developing an impairment standard for driving under the influence of marijuana and recommendations for increasing data collection regarding the prevalence and effects of marijuana-impaired driving.