Because cannabis can have effects up to a week after if I remember correctly. And it’s not very predictable. Alcohol is cleared out at a predictable rate and has no effect after a few hours, you’ll just be tired.
Marijuana’s effects last about one to three hours. A bit longer if ingested. There might technically be longer-term effects noticeable in a clinical setting, but nothing to a degree that would impact somebody’s ability to function at a typical level.
To my understanding "being tired" (which is absolutely part of being hungover the next day after a night of heavy drinking) is comparable and according to some sources pretty much equivalent to being "under the influence" in terms of how much it can impair you when operating heavy machinery.
I know a couple railroad engineers. They basically had to give up drinking because they could lose their job.
They do some kind of test that can pick up if you drank in the last couple days.
The way they assign shifts too causes the problems. If a shift pops up, they wanna take it, and that could happen at any time.
In any case I'd wager it's related to the hangover problem. You don't want a tired, distracted (by headaches or whatever), grumpy person moving 12000 tons.
You did NOT remember correctly. The only way you're going to have effects for a week is by using it for a week! The effects are VERY predictable, as studies have shown first-hand through trials for many decades. The only thing that isn't absolutely certain is an individual's rate of processing THC-COOH, which is only a leftover metabolite of what was once psychoactive THC (THC-A). You will absolutely test positive for THC-COOH a week after a single usage, sometimes up to a month; because it's stored in your fat. For this reason, it can even be detected up for multiple months in heavy users! An inactive metabolite is unable to cause any psychoactive effects.