closing time at the bar

One night a police officer was staking out this particularly rowdy bar
for possible violations of the "Driving-Under-The-Influence
Laws". At closing time, he sees a fellow stumble out of the bar, trip on
the curb and try his keys on six different cars before
finding his. Then sits in the front seat, fumbling around with his keys
for several minutes. Everyone else left the bar and drove off.
Finally he starts the engine and begins to pull away.

The police officer waiting for him, stops the driver, reads him his
rights and administers the breathalyzer test. The results showed a
reading of 0.0. The puzzled officer demands to know how this could be
possible. The driver replies, "well, tonight I'm the
DESIGNATED DECOY!