It's clear and provable that impaired driving is dangerous and increases the risk of accident. It's reasonable to infer from the data mentioned in this piece and others that legalization has increased the incidence of cannabis usage and as well as the incidence of impaired driving. 

From a policy perspective though, the harms brought by increased usage and impaired driving, tragic as they may be, are outweighed in the aggregate by the benefits of regulated legalization and the reduced harms of prohibition. Stoned driving should be addressed in much the same way as drunk driving, through education and enforcement efforts.