Ha ha.

A couple of policemen were sat in their car in the late evening keeping the carpark of a pub under surveillance on the lookout for potential drink-drivers. As the pub began to empty, their attention was drawn to one man who stumbled out of the door, tripped over a kerb, tried his keys on five different cars before he found his and then sat in the driving seat fumbling around for several minutes. By this time everybody else had driven off and the carpark was virtually empty. So they went over to this man, asked him to step out of his car, and breathalysed him. To their surprise, he registered zero on the machine. The man smiled and said "It's all right Officer - you see, I'm tonight's Designated Decoy!"