But without the blood or urine test, how does anyone know when you smoked it?
How does the govt monitor alcohol intake?
As I understand it, there's an important difference, which makes marijuana more troublesome.
Testing for alcohol directly, or nearly-directly, tests the amount of actual alcohol in one's blood, at the time the sample is taken.
Pull someone over for suspected drink driving, take a sample, and if it tests above a certain level of alcohol, you can reliably establish that yes, he was drunk.
As I understand it, testing for most other drugs doesn't directly test the amount of drug active in the subject's system at the time; it tests for metabolites that are produced as the body processes the drug, and which can be present long after the effects of the drug itself have worn off. I understand that it's even possible to be directly under the influence of many drugs, probably including marijuana, and to test negative for the drug because the body hasn't yet processed enough of it into the metabolites that the test detects.
For many drugs, we just don't have a test that determines to what degree, if any, a subject is under the influence of an impairing drug at the time the sample is taken.
It seems to me that for now, the only viable solution is for it to be clearly understood by all involved that there are just certain professions, that you cannot be in, if you are going to use certain drugs. Truck drivers, forklift operators, heavy equipment operators, pilots, construction worker, etc. Period. If you want to be a druggie, then you need to choose a profession that doesn't involve putting other people in danger.