You're right. It is a difficult thing to do because, when the police are coming into contact with a person who could have been intoxicated by both alcohol and drugs, they will be reporting by the substance they think is impacting the impairedness of the driving of the individual the most. There is a likelihood that many of these cases will be reported as alcohol-impaired. That's a police practice that they are using. It might be related because it's easier to prove than drug-related impaired driving.
On September 19th, 2017. See this statement in context.