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Ever wonder if there should be a breathalyzer for substances other than alcohol? Well, a new study may make the case.

Researchers from Dalhousie University have found that people who smoke marijuana up to three hours before driving are twice as likely to cause a car, bus or motorcycle crash as those with no drugs or alcohol in their system.

Indeed, the researchers suggest marijuana "impairs brain and motor functions needed for safe driving," the BBC reports.

The study, which appears in the British Medical Journal, is a meta-analysis of nine studies of close to 50,000 people worldwide who had been in serious or fatal crashes in cars, sport utility vehicles, vans, trucks, buses and motorcycles featured in the studies.

All studies tested for tetrahydrocannabinol, or THC, the active chemical in cannabis, by analyzing blood samples or using direct reports of cannabis use from those involved.

Most studies used one nanogram per millilitre of cannabis or any amount greater than zero as the cut-off for a positive test result, with one study using a 2ng/ml cut-off, the BBC reports.

There was an almost double risk of a driver being involved in a collision resulting in serious injury or death if cannabis had been consumed less than three hours before.

Mark Asbridge, study author and associate professor at the department of community health and epidemiology at Dalhousie University, told the BBC the research was important.

"Our findings provide clarity to the large body of research on cannabis and collision risk.

"They also offer support to existing policies, in many jurisdictions, that restrict driving under the influence of cannabis, and direct public health officials to devote greater attention to this issue."

Still, researchers point out that alcohol remains the substance most often present in crashes.

Should efforts to stop impaired driving include more of a focus of drugs such as marijuana?