The issue with level of impairment is interesting; I don't think anyone would defend habitual drinkers should be able to drink more before being prosecuted for driving under the influence. I don't see how it should be any different for THC unless it's significantly more subjective.
>> I don't think anyone would defend habitual drinkers should be able to drink more before being prosecuted for driving under the influence. I would defend exactly that (in a sense). In an ideal world, the traffic-stop test would measure impairment alone - from any cause (i.e. sleepiness or injury as well as substance use or abuse, of all kinds). Not impaired? No prob. The old physical walk-the-line tests are the closest thing I know of, to a real impairment-measuring test.
It's too subjective with booze or pot. There is no doubt that there are people that are far better equipped to handle either, but proving that is rather difficult. My wife's friend Niki is drunk after one glass of wine. I'd be fine with b_b driving me around after a full glass of bourbon, but I don't think Niki could even stand after that.