I once had to evacuate from the 23rd. floor. It was a real fire alarm, not a drill, but at least there was no fire.

It took well over an hour. The staircase was narrow and the fire doors opened from each floor INTO the stairwell, effectively reducing its width by half. The crowding of people exiting into the stairwell meant I sometimes had to stop for five minutes or more because no one could move.

If there had been a serious fire on a lower level, the doors would have been forced open by people escaping, allowing the smoke and fumes into the stairwell. The pressure and congestion of those escaping would then prevent the doors from closing again, effectively trapping everyone above the fire level.

This was not a large-footprint building - I would guess an average of 50 persons per floor would need to exit. Still, if there HAD been a big fire on a lower level, the smoke and panic would have been awful and I probably couln't have gotten down at all.

RichM is right. Though I seldom go into (or even see) buildings like that any more, I would immediately exit stage left if there were any inkling of danger. I wouldn't wait for instructions, either.