UCalgary researchers find consuming both alcohol and cannabis detrimental to driving performance

A new study by University of Calgary and Dalhousie University researchers has revealed how much cannabis affects driving performance relative to another common drug involved in crashes: alcohol. They also report that while driving under the influence of either substance on its own can be dangerous, combining both substances and getting behind the wheel is worse than either drug alone.

Led by Dr. Sarah Simmons, PhD, when she was a doctoral student at UCalgary, the meta-analysis is the first of its kind to focus on driving performance and behaviour within the context of combined cannabis and alcohol consumption. The analysis took four years to complete and merged the findings of 57 studies representing about 1,725 participants.

The research shows that when consumed simultaneously, both cannabis and alcohol generally impair vehicle handling, such as the ability for a driver to stay in their lane, more so than either substance used separately.

"With cannabis and alcohol detected a lot together in fatal crashes, it is important to understand how the two affect our driving abilities when combined," Simmons explains.

Especially because the two drugs are generally thought of as having opposite effects on speed, we were interested in what happens when the two are combined. Although some may think these two drugs could cancel each other out on driving speed, we did not find evidence for that. Additionally, the research shows when people consume both, it leads to more driving performance decrements.

Gaining a better understanding of these drugs and how they affect driving is paramount to public safety.

When used alone, cannabis has similar effects on driving as that of low blood alcohol concentrations. However, the two drugs had opposite effects on speed — cannabis was associated with decreased speed, whereas alcohol was associated with increased speed.

Although drivers under the influence of cannabis slowed their driving speed, which may reflect attempts to compensate somewhat for the impaired state, those drivers still demonstrated reduced lateral control of the vehicle. So, while there is a common notion that cannabis can make someone a better driver, Simmons’ research clearly contradicts that. 

Even though this research has made huge strides in gaining a better understanding of cannabis-impaired driving, and the combined effects of cannabis and alcohol on driving, there is still much to learn.

“The number of studies on alcohol and cannabis impaired driving is limited and leaves significant gaps for future research, especially around chronic users of cannabis and whether different forms of cannabis consumption, such as edibles, would affect driving differently,” says Simmons