First let me say I'm a little sceptical that you would drive better while stoned than sober, but I have heard other people say that, and there is even some limited testing that shows that is possible - but don't try to sell that to non-smokers, because it goes against everything they think they know. There is a swab test that will show recent use, which is far more fair than a blood test or hair test. On the other hand, the hair and blood test have to be done at a hospital rather than on the road. Unfortunately, I don't think the law distinquishes a safe limit. If it shows up in a test it is presumed you are DUI. Somebody correct me if you think I am wrong for CA. There has been some discussion I've read about what would really constitute being under the influence to the extent that you might be impaired, but I haven't seen any agreement on an amount. I'd say your best bet is to wait 3 hours or at least until your red eyes and "tired" look goes away. Also, if you have one, don't show your Med Card, and don't have anything in the car to suggest use. That way, it might not even become an issue. I remember being stopped by a CHP once when I was stoned. He never mentioned it and neither did I. He was just happy to give me a ticket for an incorrect lane change, and I was happy to take it.