P.A. SYSTEM (MALE VOICE): The red zone is for the immediate loading and unloading of passengers only. There is no stopping in the white zone.
P.A. SYSTEM (FEMALE VOICE): No, the white zone is for loading and unloading, and there is no stopping in the red zone.
This banter, which only gets more ridiculous as it continues, is one of the first gags we get in the movie. It's possible that the audience might not even notice this conversation as it develops in the background, but in typical straight-faced Airplane! fashion, it doesn't draw attention to itself. It just is, and it is hilarious.