An airbag on a bus would probably do more harm than good.
When you look down and see the headlight bucket above your toes, you know that anything the bumper doesn't catch is going to hit your feet next. Knowing that, you tend to drive a lot more responsibly.
An air bag would give a lot of people a false sense of security and, while they may "help" if you are in an accident, the total number of accidents would probably shoot through the roof!
