90 seconds for evacuation
I have long held a theory on this often quoted figure.
I do not believe anyone in power, including the authorities (CAA/FAA) or manufacturers actually expect to be able to evacuate an aeroplane within 90 seconds in the real world. I have always believed the 90 secs is a nominal figure which each manufacturer must demonstrate in strictly controlled conditions, as alluded to in that quote from the cabin crew forum as posted by crewmeal.
I am sure we are all aware that the test tries its very best to simulate potential real life conditions such as making 50% of exits inoperative and night type visuals. They even try to offer inducements (including cash) to some of the "passengers" to encourage them to exit as quickly as possible, making it a competition in effect, to generate disorder and a "rush type" mentality.
How the 90 secs was ever decided upon is probably lost in the history of avaiation (someone out there may know) but it can only ever be a nominal figure because you simply cannot replicate real life scenarios to the degree required in order to "guarantee" a 90 secs evacuation. But the 90 secs is the only figure ever quoted so of course the media, even aviation media, leap upon it at every opportunity, as do many of our peers within the industry.
Trouble is I have never seen this written down anywhere so can't prove it but to my simple brain it makes absolute sense.