Cannabis — marijuana — is the most widely consumed illicit substance in the world, and the number of Canadians confessing to driving within an hour of using pot is growing, researchers from Dalhousie University write in this week's issue of the British Medical Journal.
In addition, "surveys of young drivers have also shown that rates of driving under the influence of cannabis have surpassed rates of drinking and driving in some jurisdictions," the Halifax team reports.
Not only is cannabis relatively easy to get, "many young people really don't believe that cannabis impairs," said lead author Mark Asbridge, an associate professor in the department of community health and epidemiology at Dalhousie.
"They know that alcohol impairs their driving, so if they're the designated driver at a party they'll switch alcohol for cannabis. I've heard stories: 'I'm the designated driver so I'm just going to smoke up.' "
"We just simply don't have the same messaging around drugs and driving" that exists around drinking and driving, he said.
Past studies into cannabis and crash risk have been mixed. Some have found an increased risk of being involved in a collision after using marijuana, while others have found either no association whatsoever, or even a lower risk — suggesting people were actually safer driving while intoxicated by pot than not.
The Halifax team set out to disentangle the evidence. They performed a "systematic" review, scouring the literature for the best-designed studies they could find. In the end, they pooled data from nine studies that combined involved 49,411 drivers from Australia, New Zealand, the United States, France and the Netherlands.
All the crashes involved in the analysis took place on public roads and involved one or more moving vehicles such as cars, vans, trucks, buses and motorcycles.
The researchers found a 92 per cent increased risk — a near doubling — of a driver being involved in a collision resulting in serious injury or death, to themselves or others, if they used marijuana within two to three hours of getting behind the wheel.
The strongest association was with fatal crashes.
The study wasn't designed to answer the question: How much pot does it take before the crash risk increases?
Most studies in the analysis used any amount greater than zero as the cutoff for a positive test result. But, "for cannabis, there's not necessarily a cut off that we can identify where risk was most heightened," Asbridge said.
Still, studies have shown that cannabis impairs the psychomotor skills needed for safe driving, he said. Marijuana affects perception and spatial awareness. Drivers have difficulty staying in their lanes, Asbridge said. "There's actually a psychological process where people often believe that they're driving safer than they really are and they don't recognize that they're following too closely, or making these lane violations."
Earlier studies that suggested it might be safer driving under the influence of cannabis often relied on urine samples. The problem there, Asbridge says, is that markers for cannabis in urine "can stay in your body for weeks or even over a month so that's not a measure of recent use at all."
His team only included studies that measured active THC metabolites from blood samples, which is a more accurate way of measuring whet