Having been in numerous accidents (as a passenger) where the people driving were under the legal drunk limit. I can tell you that the amount is actually a lot lower before it can start to impact and detract from manual skills such as driving. I personally do not drink, I have no qualms about others drinking, but even 1-2 beers is too much if you plan on driving in the next 20-30 minutes.
When I did drink a little (about 7 years ago) I would always make sure that I had no more than 1 drink an hour and at least 1/2 hour with just soda or water before I drove. I have also seen Marijuana and Mushrooms used while driving, and I can tell you they mess you up even faster.
Most of the accidents were as a child with my mother being the impaired one...She owns a Hell of a lot of telephone poles.