Your interview is not over until you drive down the road.

In a few cases, I've known hiring managers to watch candidates from their office window as they exit the building and get into their car. People can do some pretty outrageous things like spitting, lighting up a cigarette, arranging themselves, yapping on their cell phones for 20 minutes while leaning on their car, chowing down on a sandwich in their car, and other things you would not believe.

So, stay in professional mode until your tail lights are out of sight. Also, you may also be observed arriving for your interview.