As a condition of certification, aircraft manufacturers are required to demonstrate that their aircraft can be completely evacuated in 90 seconds in an emergency.
In order to prove this, the manufacturer will conscript a planeload of "passengers" of different ages, genders and sizes, block a couple of exits, introduce smoke into the cabin and set off the alarms – and set the stopwatch ticking.
There's plenty wrong with this scenario.
The aircraft has not just come to a screeching, terrifying, metal-shredding halt.
There's no flaming engines, no blood, no hysteria, no banged-up bodies to climb over, nobody is trying to wrench their carry-ons from an overhead compartment.
The passengers in this demonstration are prepped, they know they're going to have to get out of a "stricken" aircraft, with zero chance of dying.
Does this mean the 90-second rule is futile?
Not at all. In a real life emergency, your chances of survival are far better in an aircraft that can be totally evacuated in 90 seconds, even if the proof fell short of reality.