Driving while intoxicated is its own separate crime. Whether it's booze, weed, blow or prescription pills, you can't do that, and if you get popped, you get popped. And liquor and marijuana are apples and oranges. One is a powerful, addictive, CNS-depressant that kills many people each year. The other is marijuana.
Cops have a LOT of discretion, most places. And legally, tobacco and marijuana are heading in two different directions. Do pipe smokers want allies when it comes to smoking bans, or do they want to carve out exemptions only for themselves (and fail)?
Sorry. I know this isn't a political forum, but working in a police office, every day I see people justify what they do while condemning what others do, when morally, there isn't much difference. You know what the difference between someone who takes 8 hydrocodones a day and a heroin addict is? Health insurance and access to a doctor. Just sayin'