Drugs and Driving: Different Studies, Different Conclusions?

With the recreational use of cannabis becoming legal in Canada, many unknowns about the potential impact on road safety remain.  First are the absolute unknowns: whether more people will become recreational users, resulting in more drivers, cyclists, and pedestrians being drug-impaired; whether, as designed, it will reduce teenage usage and, if not, whether this already most vulnerable age category of road users will be further at risk.

Perhaps ‘unknowable’ is a more accurate label for this category.  Second are the practical unknowns: what tools law enforcement agencies will use, and what procedures they will follow to legally and safely obtain blood samples from suspected drug-impaired drivers; and how our already overburdened court system will cope with the expected increase in drug-impaired cases.

Ideally, we would already have answers for this category of unknowns; practically, we can hope for iterative development of workable approaches.  The most fundamental unknown, however, given it underpins most of the new rules and procedures, is the effect of usage on ability to drive safely.  On this we are inching towards some understanding, furthered now by published research from McGill University funded by the Canadian Automobile Association [CAA].

Polling by the CAA found that one in five millennials (18 to 34-year-olds) thinks being drug intoxicated has no effect on their driving ability or actually improves it.  This study clearly shows otherwise.   It involved 45 recreational users aged 18 to 24 who used cannabis up to four times per week in the three months prior to their participating in the study’s set of four test sessions.

In the CAA study, the sessions involved driving simulator tests, and computerized tests measuring divided attention, speed at detecting objects in the periphery, and distractibility.  Cannabis usage varied from none—setting a baseline for the individual participant—to, after inhaling a standard, 100 mg dose of cannabis performance at one, three, and five hours later. Each session also asked participants about their perceived confidence in their driving ability—a probing of the above-noted belief in the non-effect of being stoned on driving ability.

The results were as follows:

  • Performance of simple driving tasks was ‘largely unaffected’ …but
  • Performance of more complex driving tasks involving normal distractions like responding to a car ahead suddenly braking or a child crossing the street “was affected at all time points after cannabis use.” These effects were most pronounced at three and five hours after usage.
  • Up to five hours after usage participants said they did not feel “safe to drive.”

The authors of this study offered the following explanation of these results: During the acute “post-cannabis” phase, people “are indeed able to effectively focus on tasks, …but three and five hours later, a different kind of impairment sets in as [they] come down from the “high” and become more tired and more easily distracted.”  Some media reports have concluded these findings fit within Canadian guidelines for “lower risk cannabis use,” which recommend waiting six hours after using pot before driving.

Other studies have shown that the effects of cannabis usage on driving include increased breaking, more weaving within lanes, and distorted time perception.