A new study of Canadian drivers who were injured in car crashes shows once again how difficult it is to measure marijuana’s impact on road safety and how misguided it is to use THC blood levels as a legal standard for impairment. The study, reported in the journal Addiction, found no statistically significant relationship between testing positive for THC and driving that contributes to accidents.
The researchers, led by Vancouver emergency physician Jeff Brubacher, looked at crashes in British Columbia that injured about 3,000 drivers whose blood was tested for marijuana, alcohol, and other drugs. They used police reports, which were available in about 2,300 cases, to determine which drivers bore responsibility for the accidents.
Among drivers with THC blood concentrations of less than five nanograms per milliliter, they report, “there was no increased risk of crash responsibility.” Drivers above that threshold were 74 percent more likely to be responsible for crashes than drivers who did not test positive for potentially impairing substances, but the difference was not statistically significant.
By contrast, drivers with blood alcohol concentrations of 0.08 percent or more—the current cutoff for driving under the influence (DUI) in almost every U.S. state—were six times as likely (i.e., 500 percent more likely) to be deemed responsible. The additional risk was 82 percent and 45 percent, respectively, for recreational drugs other than marijuana and for sedating medications such as Benadryl or Xanax. All those differences were statistically significant.
These findings, Brubacher et al., conclude, “suggest that the impact of cannabis on road safety is relatively small at present time.” They caution that the results might have been different if they had looked at fatal crashes and that the picture could change if marijuana legalization in Canada leads to a substantial increase in stoned driving.
Two important factors help explain the failure