There’s a test and a legal limit for alcohol. At the moment for drugs, even prescribed ones, any amount is enough for a DUI. In my state, I can legally have 3 beers in an hour and drive but couldn’t have one hit of a joint and legally drive.
I didn’t say what you alluded to in your last paragraph. Legally people can and do drink and drive, it’s illegal to consume drugs and drive in any amount (in my state).
My personal experiences in high school and college lead me to believe a lot of people think it’s ok to smoke and drive and to some extent do other things. While they will avoid drunk driving just because it’s much easier to detect alcohol.