Perspectives on marijuana in motorists

It is legal to drink, it is legal to drive, and it is legal to drink and drive in Canada within the regulatory limitations imposed by law. Similar provisions are contemplated regarding the use of marijuana, for which precedents exist in the states of Washington and Colorado. Reports from these areas reveal a significant increase in the proportion of THC-positive drivers involved in motor vehicle fatalities.[1,2] An increase from 8.3% in 2013 to 17.0% in 2014 occurred following marijuana legalization in Washington. In Colorado it increased by 3.4% between 2005 and 2013, and climbed to a 12.1% increase with legalization in 2014.

At first glance the proportional increases are indeed a cause for sobering thought, as termed in an article in the October 2016 issue of the BCMJ.[3] A meta-analysis, however, based on the actual number of fatal accidents, presents an alternative perspective. The increase in the proportion of marijuana-related traffic deaths could merely mean that more people were using the drug, not necessarily that more people were under the influence of marijuana. The doubling in the proportion of THC-positive drivers in motor vehicle fatalities in Washington was associated with a net increase of 5.2% in fatalities. In view of the random annual variation of ±4.7% in the 4 years preceding marijuana legalization, the association between the incidence of fatalities and THC-positives in 2014 is not significant statistically. The tripling in the proportion of THC-positives in Colorado occurred with a 21.9% decrease in the overall number of traffic fatalities. A significant negative correlation is apparent. The limited data from the two states suggest that controlled integration of marijuana usage did not have a detrimental effect on traffic safety.

The existing evidence that marijuana poses a potential health hazard to adolescents is unequivocal. In the matter of accident causation, however, the Traffic Injury Research Foundation of Canada in a comprehensive review found that in subacute doses THC does not significantly alter muscular coordination, attention, general alertness, or reaction time.[4] A US National Highway Traffic Safety Administration study reported that there was no indication that marijuana without other drugs was a cause of fatal accidents, and that those who used marijuana without alcohol were found to be less culpable than nondrug users.[5] It is of some interest to note that no THC-positives were found in a study of 774 drivers admitted to hospital in BC during the rampant marijuana-usage years of the beat generation in the 1970s.[6] The hypothesis that THC decreases driver aggressiveness, thus reducing the risk factor, bears merit.

The drugged-driving issue is complex, and drug testing and reporting is not uniform. Current data are insufficient to allow standardized comparisons of drug use across years or across jurisdictions, and confounding reports are numerous. Marijuana’s spectrum of behavioral effects on motor vehicle operation are the subject of a recent comprehensive review.[7]
—R.A. Rockerbie, PhD
Medico-Legal Pharmacologist, Victoria


1.    Tefft BC, Arnold LS, Grabowski JG. Prevalence of marijuana use among drivers in fatal crashes: Washington, 2010–2014. Accessed 19 October 2016.
2.    Colorado Department of Transportation. Drugged drivers involved in fatal crashes (2003–2014). Accessed 19 October 2016.
3.    Rumball C. Driving stoned: Marijuana legalization and drug-impaired driving. BCMJ 2016;58:477-478.
4.    Warren RA, Simpson HM. Cannabis and driving: Implications of moderation in penalties, issues in road safety. Ottawa, ON: Traffic Injury Research Foundation of Canada; 1980.
5.    Parkin HE, Rockerbie RA, Martin GR. Blood alcohol in hospitalized traffic crash victims. BCMJ 1980;2:58-60.
6.    Terhune KW, Ippolito CA, Hendricks DL, et al. The incidence and role of drugs in fatally injured drivers (DOT-HS-808-065). Springfield, VA: National Technical Information Service; 1992.
7.    Rockerbie RA. Alcohol and drug intoxication. 4th edition. Victoria, BC: AlcoTrace Publications; 2014. p. 355-363. Accessed 24 October 2016.

R.A. Rockerbie, PhD. Perspectives on marijuana in motorists. BCMJ, Vol. 59, No. 1, January, February, 2017, Page(s) 10-11 - Letters.

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