Nor does alcohol mix well with heavy machinery, but just because someone may drink alcohol at some point in the future doesn't mean they will before they operate deadly machinery. The same would apply to marijuana. A stable considerate employee wouldn't use either substance while performing their duties, so maybe we should be screening for something like mental stability, aptitude for identifying/avoiding high-risk behavior etc. instead of trying to run a catch-all for potential drug users.

Marijuana can be detected in a urine test for 3 days after use for a non-user.

If you're a frequent smoker, it can last be detected over a month after your last use. Even longer in hair tests.

Obviously being high at work isn't a good thing, but neither is being drunk at work. But you don't get fired for having a few beers on the weekend.

I believe it's on the order of weeks. I don't buy that someone who smoked two weeks ago is liable to get high on the job. If someone drinks on the weekend, should we automatically worry they're liable to operate machinery drunk?

>I can't speak to how long you have to be clean to pass a test

Several days at least. Weekend smokers are out.