Yes, in an ideal world we would do that. But by the time we come up with a perfect test that can be done roadside by police to determine driving impairment, that has very few false positives and holds up in court, we'll most likely have self driving cars and this won't be an issue anyway.

I understand that everyone reacts differently to different drugs. Dale Earnhardt can probably be a little tipsy and still drive better than me. But even if you are an above average driver, it's still immoral (and I think should be illegal) to use substances that will increase your risk of killing someone else in a car accident, even if you are having a small amount of those substances that just brings you down to average driver levels.

Car accidents are one of the leading causes of death. It's an inherently risky activity to be doing every day. But we've structured our society in a way that makes it unavoidable. Given that it's dangerous even for sober people, I don't think we should be giving people the right to drive under circumstances that make it even more dangerous.