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Marijuana Impaired Driving

For alcohol we have a driving per se limit: 0.08 g/dl. This means that if a driver has a BAC at or above 0.08 g/dl, he/she is presumed impaired and can be charged with DUI. But for marijuana, we have no such limit. So while high school seniors are more likely to report driving after smoking marijuana than driving after heavy drinking (O'Malley & Johnston, 2007), enforcement is hampered by vague guidelines for impairment.

Indeed there are perceptions among some in our target population that marijuana does not impair driving at all. Some claim that the tendency of marijuana users to drive slowly makes them better drivers. But recent research contradicts this belief, and there is general consensus that driving under the influence of cannabis is a serious public health concern.

A few years ago, a panel of experts came together to study the issue of THC limits for driving. Their report (see Grotenhermen et al., 2007) offers a nice overview of the research that has documented the crash risks associated with various THC blood levels. It’s clear that marijuana researchers have the work cut out for them; while we have roadside alcohol breath tests since the late 1930’s, there is still no good roadside equivalent for THC. However, the panel was able to make some very specific recommendations based upon laboratory research as well as sound epidemiological data. They recommend a per se limit of between 7 and 10 ng/ml THC blood level.

So this means we should be informing our students about the risks of marijuana impaired driving. To avoid driving over the safe limit, a marijuana user must allow for the passage of time. Using marijuana is different than drinking alcohol. Almost any use of cannabis will cause a spike well above 7 ng/ml. But that spike will dissipate rather quickly. By waiting 3 hours from the last use, the driver will likely be well below the recommended limit and also likely not still be experiencing any substantial impairments.

Finally, the limits recommended for THC change dramatically with even low levels of concurrent alcohol use. Mixing alcohol with marijuana substantially increases crash risk. Students should be explicitly warned about this, and encouraged to use designated drivers who will remain completely sober to assure everyone get’s home safely.


Grotenhermen, F., Leson, G., Berghaus, G., Drummer, O. H., Krüger, H., Longo, M., Moskowitz, H., et al. (2007). Developing limits for driving under cannabis. Addiction, 102(12), 1910-1917.

O'Malley, P. M., & Johnston, L. D. (2007). Drugs and driving by American high school seniors, 2001-2006. Journal of Studies on Alcohol and Drugs, 68(6), 834-842.