Auto accidents are higher in 4 states where pot is legal, this comparison study says. However, the percentages of fatal crashes has not increased. Most other similar studies, like the July 2017 piece in The American Journal of Public Health, have found no significant difference in crash fatality rates after legalization, compared with similar states.
While there are certainly risks, it has to be many times safer than some of the psych meds people are driving around on. And of course compared to alcohol, there is really no comparison in impairment levels. However, marijuana intoxication does make alcohol impairment even more acute.
Back in July, Canada passed some laws related to cannabis impaired driving, allowing police to use oral-fluid screening devices to detect THC. Drivers with two or more nanograms of THC/ mL blood can face fines and worse. But these levels don't necessarily translate to impairment - THC is not metabolized consistently as alcohol is, and tolerance levels, body-mass index and method of consumption all play a role.