It's still illegal via US Federal Law, so what happens if you're caught depends on who catches you.
As far as Canada goes, Law Enforcement here is concentrating on effective means to identify and charge non-alcohol impaired drivers. There is also considerable new research going on to identify medical procedures that can be used to identify impaired drivers. I expect there will soon be "breathalyzer" type machines to aid that initiative, versus having to send the suspect to a hospital for tests. It's what happens when you can openly throw money at the problem ... marijuana research is not very well established in academia. That needs to change.
There is a lot of uncertainty about employee policy, but it's mostly industries that aren't involved in machinery operation. Any company involved in that already has policy in place, and it's enforceable. Most larger firms have mandatory detox or similar programs that employees would have to take with a violation, sometimes second and third chances there, after those options are exercised you're fired and the union (if it exists) can't and won't do anything about it ... you've lost your job.
Reality is people have been smoking and driving, or working, for decades.
Legalization helps to bring policy and procedure in place that should have been there all along. Municipalities and businesses in Canada are now putting in place "no-smoke" areas that again should have been there all along. In that respect it's not really an issue, in fact from a society perspective it's probably a net benefit.
I know that no-one wants their kids exposed to drugs but the reality is they all are well aware by the early teens if not sooner. Being able to talk about it, being able to have TV ads with more specific advise, and whatever help that can be put together for parents, since they are the ultimate tool to deal with these issues, it's a different way of looking at it but I think it will prove to be effective.
I think that as far as the US goes, there will be a lot of scrutiny and interest in what is happening in Canada, and without legalisation at the Federal level (which seems doubtful) there will be plenty of tools and information in general that will trickle down to the US. Law Enforcement will benefit.
"Smoking while driving" carries the same penalty, if deemed impaired, as Drunk Driving in Canada. Even in the last few months there has been an increase in charges laid.