To me, this sentence would mean that someone has smoked so much marijuana that they can't think straight. "Stoned" can mean any drug, but it's typically used for marijuana. Drug users have different languages for different drugs. The slang that exists for drugs is complex and vast. Unless you're really interested in drugs, I wouldn't waste time learning the vocabulary. I can't say I know all the terms that are used. "High" and "stoned" are the two most common.

The person has been smoking too much marijuana, and cannot mentally function. Why is the same person behind the wheel?

It means 'I've taken so many drugs that I can't think properly'.