There's a lovely, if similar joke, set for no particular reason in a small county in Texas where the local hobby is drink driving.
A cop pulls up one Friday evening outside a bar notorious for it's drink drivers, and spies someone absolutely plastered stumbling aroung the parking lot. This is the one, thinks he, and lies in wait for the chap to drive onto the public road. Eventually the drunk finds his car and spends a good 5 minutes trying to get his key in the door. A few others start to leave but the cop sticks with his man, now curious as to the level of blood alchol that must be in him. He eventually gets into his car and there's another few minutes trying to get the key into the ignition. Others leave the bar, and the cop is about to go after one or two of them, when the lights come on full beam in the drunks car, the wipers go on full pelt and there's a beeeep as he nods onto the steering wheel. With the lot almost empty the drunk drives very slowly out onto the main road and the cop immediately pulls him over as the last few patrons leave the bar.
The cop asks for drivers licence and insurance which are forthcoming instantly. The drunk is very pleasant and doesn't seem in the least inebriated. He is asked to perform the ritual tests, which are all done flawlessly, and breathes into a breathalyser. Zero alcohol in him.
'But!' exclaims the cop, 'The stumbling about, the lights and the beeping, the slow driving! How on earth are you sober?'
'Ah' responds the driver, 'I'm the designated decoy'.