Now here's a study Canadian lawmakers should read as they consider Bill C46, which seeks to amend the Criminal Code with respect to drug-impaired driving.

A study recently published in the American Journal of Public Health has found that states with legalized recreational cannabis do not have a greater rate of car accident deaths than states where cannabis is illegal.

Conducted by doctors and researchers from the University of Texas-Austin and Rice University, the study compared car accident fatalities in Colorado and Washington, where cannabis is legal, to similar data from eight control states where the drug is still prohibited.

Researchers used a federal fatality reporting system to determine the annual number of fatalities from motor vehicle accidents over six years, between 2009 and 2015, in the 10 states.

By comparing year-over-year changes, they found that the number of fatalities did not increase after recreational cannabis was legalized in Colorado and Washington, and was consistent with the number of deaths in the eight control states. 

"This is the first time researchers have actually looked at the real-life effects to see if there have been any major population changes in injuries on the road after marijuana was legalized in these states," lead study author Jayson Aydelotte told KTVU News in Austin last month.

In both Colorado and Washington, drivers with five nanograms of active tetrahydrocannabinol (THC) per millilitre in their bloodstream can be prosecuted for driving under the influence. Some marijuana lobbyists claim this is far too low, as frequent cannabis users can have residual amounts of THC in their bloodstream well above five nanograms—and for days after consumption.

Others argue the amount is too high, especially because the numbers determined are based on policy, instead of evidence of impairment.

Bill C46 currently outlines a limit of just two nanograms of THC per millilitre.

To put that in context, Canadian Olympian Ross Rebagliati won the gold medal for snowboarding in 1998 with more than eight times than what the federal government has suggested, at 17.8 nanograms per millilitre.