"Driving dangerously" should be the metric. Not whether your body contains chocolate milk, weed, alcohol, oxycotin, aspirin, or whatever.
Too many people under the influence of alcohol or drugs are under the impression that because they can drive in a straight line and stop at red lights they are not dangerous and are being 'careful' and 'responsible', yet their judgement and reflexes are nonetheless impaired and this makes them dangerous.
Besides, you can't reasonably equate aspirin and chocolate to the dramatic effects that even a moderate amount of weed and alcohol have on your attention and brain response.
I'm all for the decriminalisation of drugs but I'm also in favour of zero tolerance if you choose to get behind the wheel.