its not that simple cosworthguy. Surely the point is road safety. People can perfectly legally go get stoned in parts of europe then come here and still have weed in thier system.

That doesnt therefore mean they are a risk behind the wheel.

Its exactly the same principle with alcohol needing a limit. You cant practically enforce a zero limit nor would it be respected from a road safety perspective.

however yes, in the vast majority of cases, detection of drugs in such cases is an indication of illegal drug use. Im pretty certain it could easily be used as justification for an on the spot drug search of person and car, and could be recorded for police intelligence reasons.

There are very very few prescription drugs that you cant legally drive on. However, there are thousands of drugs that potentially can make you drowsy. It is almost impossible to assess with those kinds of cases when someone is unfit except through the standard of thier driving. There are just too many drugs/parameters to assess. Its much simpler with alcohol and illegal drugs as the effects are much better documented.