I must admit, as much a fan as I was of the show Taxi, I had forgotten about this bit. Here's how it went:

Jim had to take a driving test to get his cab license:

JIM (WHISPERING TO BOBBY): What does a yellow light mean?
BOBBY: Slow down.
JIM (SLOWING SPEECH) What...does...a...yellow...light...mean?
BOBBY (HISSING): Slow down!
JIM (EVEN SLOWER): Whaaaaat...dooooooes...a...yellllllllow...llllllllight...meeeeeean?