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Addressing Marijuana-Impaired Driving

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Marijuana use can impair attentiveness, reaction time, motor coordination, and judgement, raising the risk of automotive accidents. These risks are exacerbated when the driver also consumes alcohol. Marijuana use is associated with impairments in psychomotor and cognitive function, which poses a threat on the road to both the users and to other drivers around them. 


  • Some legalizing states have followed the alcohol impairment model and established “per se” limits (e.g., 5 nanograms of THC/ml blood) above which drivers are presumed impaired. Other states have no per se standard and rely instead on generic approaches to judge impairment, such as evidence of erratic driving or poor performance on roadside coordination tests.
  • Alcohol testing methods such as breathalyzation are generally effective at measuring recency of alcohol use and level of impairment. However, comparable technology does not exist for cannabis, which complicates assessment of driver impairment.


Many researchers are attempting to develop new technologies to measure cannabis impairment, but currently available technologies have numerous shortcomings for addressing this complex problem. Until technology improves, a combination detection approach is probably most prudent, i.e., relying on both biological tests and generic performance impairment tests used for a range of substances.

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