Yes, there actually are studies that have looked at that.
The problem is, because there are pharmaceutical medications that people are on, there's the issue of tolerance. So I don't know that we'll ever get to a point where you can say that a specific concentration of drug is going to cause impairment. In fact, other countries in Europe, places like Sweden and Germany, have gone with zero tolerance, meaning that if they find the presence of cocaine or heroin--a drug you're not supposed to be taking--in your body, then you are charged. It doesn't matter how much; it's the fact that it shouldn't be there in the first place.