Feds will make it very tough for it to become actually legal. They have raided marijuana dispensaries in California. They will arrest people who smoke it in Colorado.
And it will be a huge sticking point with many jobs. A lot of businesses rely on OSHA to obtain a certain level of certification, so they can win bids for larger jobs via the government, larger corporations, etc. Drug testing is a huge part of that. You think a negative drug test will be "ok" after this? No, they will fire the person who tests positive, every time. OSHA is not about to reduce their standards, and so businesses will not either. And it makes sense - if you have 100 construction workers on a site, do you really feel safe if 20% of them test positive? You have no way of knowing if they smoked it last night, or during lunch. There is a lot of personal responsibility involved, but unfortunately, people are not very responsible.