A cop is waiting across the street from a bar parking lot late on a Saturday night, watching for drunks trying to drive home. After a short wait, one particularly sad case stumbles out the door, bleary-eyed, confused, wandering the parking lot looking for his car. He locates his car, fumbles for his keys, gets in and drives off, bumping the curb on the way.
Of course he doesn't get more than a half-dozen blocks before the cop is on him, and he immediately pulls over. The cop has him step out of the car, sizes him up, and administers several field sobriety tests, with much effort (the driver has trouble understanding some of the tests). The driver fails all the tests miserably: can't touch his nose, can't walk straight, can't stand on one foot, etc.
The final legal step, of course, is the Breathalyzer, so the cop asks his subject to blow into the tube. Green light. In disbelief, the cop checks the Breathalyzer and has the suspect try again. Another green light -- the guy's blood-alcohol level is legal.
"Alright," says the cop, "how can you pass a breath test when you're so obviously falling-down drunk?" "Well, it's like this," replies the guy. "You've heard of being the Designated Driver? I'm the Designated Decoy."