My mom, who tells me all my best jokes, told me this one:

[Ed: Reportedly appears in July 1991 Playboy, but is also probably older than
that.]

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 doesnt 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: cant touch nose, cant walk
straight, cant stand on one foot, cant 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 guys blood-alcohol level is legal.

All right, says the cop, how can you pass a breath test when youre
so obviously falling-down drunk?

Well, its like this, replies the guy. Im the designated decoy.