I propose that before decriminalization of various substances could ever happen, the issue of what constitutes intoxication would need to be clarified. This goes for public, driving, etc. Perhaps less subjective measurements and standards could be instated; and there would be different standards for different situations. With present saliva/breath/skin/sweat/whatever non-intrusive testing technology we have and will have in the near future, I think more quantitative standards for intoxication will be easier.
The fact of the matter is that all substances are different, and effects vary by dose. I honestly think there is an amount of many different presently illicit substances that wouldn't effectively impair one's driving ability any more than a <.08 BAC. There are also substances which could seriously impair driving ability at any amount. It's easy to say alcohol is okay at <.08 BAC, but any amount of anything else is bad, when anything else is illegal. It's not easy to say that when those other things are legal. Same goes for public consumption. We have bars. We have alcohol at ball games, and in churches and, in samples at grocery stores. There are certain amounts of alcohol that are okay, certain behavioral changes that are not considered extreme enough to be legally considered intoxication, and this would have to be applied for other substances as well for any decriminalization to occur.