"Drinking and driving" joke

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, front of his shirt soaked,
bleary-eyed, confused, wandering the parking lot looking for his car.
He locates his car, fumbles for his keys, gets in (bumping his head
in the process) and drives off, bumping the curb on the way.
Of course he doesn't get very far at all 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 nose, can't walk
straight, can't stand on one foot, can't recite a speedy alphabet.
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.
"All right," 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. "I'm the designated decoy."