A normal collision would be an unintentional one. Like a gas leak exploding. That's as opposed to a fully fueled 767 meant to be used as a bomb.

A typical modern building, let's say an apartment building, would have a burn time of approximately 2 hours between apartments and 4 hours on opposite sides of the firewalls. This is plenty of time to get out, and actually, the evacuation procedure in the situation of a "normal" fire wouldn't call for an immediate evacuation of people 2 floors below the fire.

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