Having driven drunk and driven stoned on multiple occasions, I can say with certainty that driving drunk is more difficult and more dangerous. On top of that, I'm pretty sure the mythbusters showed that talking on a cell phone while driving is as bad or worse than drunk driving. Which should be obvious to anyone on the road.

Anyway, everything is relative, and that is why writing laws is not necessarily easy. I would tend to say they should enforce after the fact, but most people will disagree with this because people may die in the process. Who is to say when I become too impaired to drive? BAC tests are unreliable because intoxication relative to BAC varies a lot from one person to another. Some seasoned alcoholics can be at 0.20 BAC and seem sober, while some teenage girl might seem shit-faced at 0.08 BAC. Some people might be able to drive and talk on their phone at the same time without killing people. Some people might be able to smoke a joint and still drive fine. I could possibly even take this further and say that some people might be able to speed and not injure others. When should speeding, drunk driving, cell-phone-talking, etc. be considered crimes? I would say they should be considered crimes once it can be shown that the person in question harmed another person by doing one of the above.

WTF am I talking about?