As Canada moves closer to ending pot prohibition, a new report is raising questions about what impact legalization has had on deadly traffic accidents south of the border.

After Washington State made recreational marijuana use legal, the number of drivers who were involved in fatal crashes and had pot in their systems doubled, according to the American Automobile Association’s report.

The number went up from eight per cent in 2013 to 17 per cent in 2014.

“I think it’s an indication that marijuana use and driving is going to be a growing contributing factor to fatal crashes,” said Jake Nelson, director of traffic safety for AAA.

The drivers all tested positive for THC, the active chemical in marijuana, but the research doesn’t prove they were impaired, only that they’d smoked within several hours of crashing.

It’s also unclear whether legalization actually contributed to an overall increase in deadly crashes. Statistics from the U.S. Department of Transportation show there were 429 fatal accidents in the state in 2014, which is 28 more than 2013 but still fewer the seven-year average of 430.

AAA’s research still has some politicians in B.C. concerned. Mike Morris, Minister of Public Safety, said he’s “extremely worried” about the potential impact of legalization on road safety.

The province is currently studying the issue, including whether roadside devices can detect impairment.

“There’s a number of devices that are being tested right now that show promise,” Morris said. “The question is: What is that [dangerous] level of THC? Is it two per cent, five per cent, half a per cent? We don’t know.”

According to AAA, the legal THC limits that have already been set in a handful of U.S. states are arbitrary, and not supported by science.

“It’s not that it’s not good enough, it’s that it’s entirely meaningless,” Nelson said.

“There isn’t a concentration of active THC in the human body that can allow us to reliably predict when that person is impaired.”

The organization is warning Canadian lawmakers not to make the same mistake, insisting the most reliable way to prove impairment may require a combination of observing physical symptoms and administering a THC test.

With a report from CTV Vancouver’s Scott Roberts