Jokes / Breathalyzer test

One night, a police officer was staking out a particularly rowdy bar for possible violations of driving under the influence laws. At closing time he saw a fellow stumble out of the bar, trip on the curb, and try his keys on five different cars before he found his. 

Then he sat in the front seat fumbling around with his keys for several minutes. 

Finally everyone left the bar and drove off, and he started his engine and began to pull away.
The police officer was waiting for him. He stopped the driver, read him his rights and administered the Breathalyzer test. 

The results showed a reading of 0.0. The puzzled officer demanded to know how that could be. The driver replied, "Tonight, I'm the Designated Decoy."