hot boxing your car is fine. driving it afterwards isn't.
just remember that the key part is prevention. this is about education, not enforcement; people don't make the crucial choice to not get in because they're worried about getting caught, they make it because they're worried about getting dead.
http://ottawacitizen.com/news/local-news/reevely-pot-legalization-brings-tougher-penalties-for-all-impaired-drivers-in-ontario