We have found that we can load 72 passengers onto a 40ft RTS bus safely. (That's how we got the radio code Signal 72 for full bus). Our only requirements are that there be no one in front of the yellow/white line, and no one in the rear stairwell.
For the low floors, no one can be in front of the yellow line, or sitting on top of the front wheel wells. Those busses have a rated capacity of 80... however, 72 is more feasible.
One thing I will mention... from personal experience... 75% of the time, it's the passengers themselves that will try and pack themselves in when a bus nears capacity. We as drivers will ask folks to move back, and fill any open space (often times there is quite a bit). However, when the bus is near full, and theres only room for 4... a group of 7 will try and cram on. If we tell them they can't fit, they'll argue and cram themselves somehow behind the line. Once they're all behind the line, the only thing I can do is drive.
On related topics of safety... I do want to add... in collisions with automobiles... the bus will always win. Typically, the large size of the bus will absorb most of the energy of the crash. With the RTS bus, the car will simply enter the bus, and travel below the floor of the passenger compartment.
With the new Low Floor busses, you'd be AMAZED at how structurally safe those things are. The following pictures are not of the type of bus that Disney uses, however, our busses would have simmilar results. The crash was done at about 30mph impact speed, I beleive.