A cop is staking out a bar for drunk drivers. At closing time, he sees a guy stumble out of the bar, trip on the curb, and fumble for his keys for five minutes.
When he finally gets in, it takes him another five minutes to get the key in the ignition. Meanwhile, everybody else leaves the bar and drives off.

When he finally pulls away, the cop is waiting for him, pulls him over, and gives him a Breathalyzer test. The test shows he has a blood alcohol level of 0.0.

The cop says, 'How is this possible?' The guy says,'Tonight I'm the designated decoy.