I haven't seen any definitive "data" on exactly how long it stays in your system, not like I looked but I suspect it can depend on several factors, individual metabolism and so on. Like if you smoke once can you be guaranteed you will test clean in a week? I would stress over it for sure.
When weed was starting to get legalized I wondered how they would handle the DUI issue. I mean if you have smoked heavily for a period and then stop you might test positive for a month or three, depending on who you ask. Would this mean you would be driving under the influence for all that clean time? Basically no one who smoked even semi regularly would be able to drive. I saw some docu and they talked about THC having a weird mtabolism, it was hard to prove when a person had actually smoked relative to testing.
What was also interesting is that blood concentration had little to do with if you were high or not. Inebriated. So from what I understand is that determining if someone is inebriated or not is up to the arresting officer to decide. Also used those roadside balance tests. So I guess you are "legal" if you look sober and can act normally?
A question: does weed affect driving ability negatively? A few say no, there's no proof, most say yes potentially. You might feel you have full control but it might also creep up on you sometimes even if you are very experienced, suddenly you are way too high. Are high people a problem from a public safety pov?
I worked in oil and gas for many years. We all used to buy testing kits for urine, as that is what they used for random tests. I smoked a few joints and took the test 24 hours later and passed. A lot of use would only smoke weed on Friday's because the earliest we would get tested was Monday. Nobody ever got popped.
The only time a guy got popped was the day after new year's, he tested positive for coke and weed, but have been blasted on it for 3 days.
It does depend on your metabolism and bodyfat percentage, as THC metabolites are stored in that way. There was a guy i worked with years ago who posted on this marijuna-addicts forum, he said there was a group of people who studied these detection times and if you only smoked once it literally was out of your system in 24 hours.
Hair follicle tests are harder to pass, but i heard it takes a lot of weed smoking to get it in your hair. I honestly don't know how true that is, but blood and piss tests it's gone in a day or two unless you smoke it regularly. I used to get tested 3-4 times a year and never had a problem, but i only smoked once or twice a month.
Cannabis is legal in New Mexico, but i can still lose my job if i fail a piss test. Since it does stay in your system longer than alcohol, i don't think we will have a solution on how to test for impairment.
When i get stoned i do everything more carefully, including driving, but it does impair your reaction time a lot.