Not really a joke but very funny true story.
At a local bar, a cop parks his car waiting to catch a drunk driver.
After a little while, a man stumbles out of the bar and gets into his car.
He then drives off and the cop pulls him over. The cop asks him, have you been drinkin tonite? The man replies, no I am the DD. The cop laughs and says he saw him stumbiling to his car. The man laughs, and replies, I really am the DD. You see, my buddies were drinkin and they left just after me. I havnt had a drink all nite, and that is why I am the Designated Distraction.(DD)!