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This is about why cannabis impairment is hard to measure, not about what is legal where you live. Driving laws differ by country and by state or province, and a limit existing is not the same as it meaning something. Nothing here is advice about driving or about any test you may be asked to take.
With alcohol there is a number. Blow into a tube, get a figure, and that figure lines up reasonably well with how impaired someone is.
That relationship is why the whole system works.
Does the amount of THC in someone's blood tell you how impaired they are?
Peak impairment often arrives when THC in the blood has already started falling.
So a measurement taken later can point the wrong way entirely.
Alcohol dissolves in water and spreads through the body fairly evenly, so a blood level reflects what is reaching the brain.
THC dissolves in fat. It moves out of the blood into tissue quickly, on its own schedule.
A regular user can have a high blood level and drive competently. Someone unaccustomed can be genuinely unsafe at a low one.
Tolerance changes the whole relationship between number and person.
Motoring bodies reviewing the evidence have concluded that a threshold number for cannabis cannot be scientifically supported.
That is a strong statement, and it is about the science, not the policy.
Several jurisdictions have per se limits — a number above which you are impaired as a matter of law.
That is a legal decision. It does not create the scientific relationship.
What does a THC blood level tell you?
Tap an item, then a bucket
Assessing the person rather than the chemistry. Trained observation of attention, reaction, coordination and eye movement.
Slower and more contestable than a tube, and closer to the actual question.
Nobody can tell you a safe number of hours, because there is not one that holds across people.
The honest guidance is about how you are, not how long it has been.
Cannabis with alcohol is worse than either alone, and people underestimate that combination badly.
Two mild amounts do not make a mild total.
Because the alcohol model taught everyone that a test settles it. For cannabis, no test does — and both the person and the law can end up trusting a number that is not carrying the meaning they think.