More than half the interviews I went to, the PC asked at the beginning of the day if anyone was going straight to the airport and if they needed a cab, those who said yes had one waiting for them at the end of the interview day. If that doesn't happen, just ask when you get there.
Also, if you're trying to make an early (before 6p) flight, be sure the PC knows that now so they adjust your schedule so you have time to make your flight. If you're in NYC, Chicago, Boston, SF, you need to be leaving your interview ~3 hours before your flight if you have any hope of making it on time (exception maybe for leaving UChicago and going to Midway). Don't expect to leave an interview @ 4:30 and make a 6p flight.