> Cannibis can be in your system for weeks or months
No.
You are confusing the tests for cannabis metabolites (that can tell if you smoked weeks ago) to tests for THC, the substance that gets you stoned and leaves the bloodstream as you sober up.
A driving impairment test will test for THC, not metabolites. Nobody cares if you smoked last week, only if you are under the influence when driving.
In the UK, having 2ug / litre of THC in the blood is a drug-driving offence. At a minimum you will loose your licence for 1 year.