Driving Stoned

Cannabis is now the most frequently detected illicit drug in drivers involved in motor vehicle crashes. Experimental research and epidemiological data have demonstrated psychomotor impairment associated with THC intoxication and quantified impairment with heightened risk of motor vehicle fatalities. Experimental laboratory studies have repeatedly demonstrated that the primary component of cannabis, Δ9-tetrahydrocannabinol (THC), impairs psychomotor skills including reaction time, focus, executive function, decision making, impulse control, and short-term memory, all important assets needed for safe driving.

Research reveals that the high from marijuana peaks during the first hour after smoking and slowly declines over the following 2 to 5 hours (Hartman and Huestis 2013), sometimes longer. Studies of aircraft pilots who failed to safely execute an emergency landing 24-hours after smoking marijuana should have been the wake-up call we needed regarding the prolonged impairment, in some case 1-2 days after the subjective “high” wears off. But well-financed proponents of marijuana legalization were successful at circumventing the conventions of scientific inquiry to assess safety and efficacy, thus allowing a largely uneducated public to vote the use of this addictive drug into law. As a result, the prevalence of cannabis use is expected to increase as ongoing legalization of both medical and recreational use proceed, despite any scientific evidence regarding the efficacy as a medicine or individual and public safety concerns.

In 2014, Urfer et al, published research in the J. Annals of Toxicology showing that the percentage of a positive finding for THC in motor vehicle crashes in Colorado, confirmed at or above 2 ng/mL (n = 1,598) had increased significantly from 28% in 2011 to 65% in 2013. The mean and median THC concentrations were 8.1 and 6.3 ng/mL, respectively. These data illustrate a statistically significant increase in positive THC confirmations in arrests for Driving Under the Influence of Drugs (DUID) in Colorado during the legalization efforts. The level of driving impairment was generally less among heavy users but still comparable with a BAC of 0.05 g/dL–clearly impaired and potentially lethal.

The best available scientific evidence germane to cannabis use and driving impairment sharply contrasts with public attitudes regarding driving under the influence of marijuana. Regular users often admit to driving under the influence of marijuana and erroneously believe that their driving performance is not affected, or that they have learned to compensate for the impairment.

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Why Does This Matter?

Consuming marijuana causes psychomotor impairment and substantial risk to drivers and others on the roads. A policy brief by the World Health Organization stated that the influence of cannabis was estimated to be responsible for approximately 8,700 traffic deaths worldwide in 2013. This is still far less than the number of deaths due to alcohol-impaired driving in the same year. Yet, it is the combination of alcohol and THC that produces the greatest risk—which is commonly practiced among marijuana users. Moreover, how best to test for THC at the scene of an accident is still a developing science. But in all, these data underscore the importance of developing policy and legislation based upon solid scientific evidence and not popular opinion.


Ramaekers JG1. Driving Under the Influence of Cannabis: An Increasing Public Health Concern. JAMA. 2018 Mar 26. doi: 10.1001/jama.2018.1334. [Epub ahead of print]