[wludavid]

Highly inaccurate. Dueller corrected himself. Personally, I don't trust them. They're too hard to use, and not accurate. You might blow a 0.05 at the bar, and then your last beer enters your bloodstream and you go to 0.09. You get pulled over and you go to jail and get blood-tested. By that time all the beer has entered your bloodstream and the [accurate] tests read 0.10. And that's after getting in the car at an honest 0.05!

Unfortunately, Dueller's suggestion that even asking means you shouldn't drive isn't very useful. One of the first things to be affected by alcohol is the prefrontal cortex, where we (attempt to) make rational decisions. You might be feel fine, but your decisions have been impaired. Those third and forth pints become a good idea, and you're FINE to drive. Maybe, maybe not.

My advice, FWIW, is to make a plan if you're going to drive home. Something like: cocktail, burger, beer, water. If you deviate from that plan, don't reassess your level of intoxication, as your decisions are already being affected. Either stop drinking or make other plans to get home.