Marv, you are right for all the wrong reasons! Behold:
. It moves forward. When the bus moves forward, the heavier air moves to the back of the bus. The helium balloon is lighter than air, so it is pushed forward. Give it try yourself sometime!
Interesting. I'd have thought that it would move with the air before being pushed forward. I'll have to try it, but only after I try to replicate the WTC demolition with chicken wire and lighter fluid.