Multiple exits is a great way to protect people in the event of fire, but it's not the only one.

In the US and Canada where the 2 stairway rule is in use there has been a huge unintended downside of dictating the form of all low-ish and medium rise apartments for decades.

When you need 2 stairs the natural, efficient form is a wide building with a long central corridor with stairs at both ends. Building something that wide generally requires assembling multiple parcels of land and a long, drawn out planning process and large amount of capital investment. Small apartment buildings on a single parcel can't ever pencil out due to the space required to be dedicated to stairs.

Combine in the additional code requirement for bedrooms to have windows (for both quality of life and safety egress) and you get the vast majority of apartments having a limited number of bedrooms with windows only on one side of the living space. 3 and 4 bedroom units that could economically efficiently amortize the high per sq ft cost of kitchens and bathrooms waste an enormous amount of space, so they simply don't get built. Anyone who wants to raise a slightly larger family, live with multiple generations, or split costs with multiple roommates has no apartment options, they are forced to look at high cost, low density urban sprawl.

We currently have prescriptive requirements that dictate things like multiple stairs. We need to switch to outcome driven requirements that require fire suppression and safety over a given period based on occupancy and building size to give firefighters a big safety margin to respond. Then develop those outcomes into tested wall structures with a combination of fire suppression and prevention techniques that developers can adopt. Multiple stairwells might be part of the fire plan for a building, but it also can include things like sprinklers, concrete stairways, and automatic fire doors.

Redundancy matters when it protects from plausible issues.

> Hospitals don't stop carrying backup power supply for life-saving equipment just because "the power grid downtime has improved."

When power goes down, there is an immediate harm to patients. The grid downtime would need to be really implausible. Also, batteries would probably be okay now.

When fire happens - which is rarer now - it also rarer blocks the exit in a way that causes harm now.

Good example is ETOPS - https://en.wikipedia.org/wiki/ETOPS - some time ago, we required flights over ocean to have three or four engines. Now, reliability improved a lot, and the engines got good enough that single-engine operation is not so catastrophic as before.

Hospitals have all critical equipment on battery backup. The non-critical but important stuff can turn off for a minute while the backup generator starts.

Power outages tend to correlate strongly with situations where the hospital is most needed - bad weather injures a lot of people and takes the power out

Almost all apartment fires start inside an apartment. Improved standards for fireproofing between units, automatic doors, sprinklers etc means fires stay contained long enough for people to get out.

>Good example is ETOPS - https://en.wikipedia.org/wiki/ETOPS - some time ago, we required flights over ocean to have three or four engines. Now, reliability improved a lot, and the engines got good enough that single-engine operation is not so catastrophic as before.

And yet, single-engine jets are prohibited from operating under Part 121 and must use Part 135 (the analogy here might be to an apartment building where the developer takes on the responsibility of structural code-compliance and safety, vs. a single-family home where the homeowner does).

There's a big difference in going from 4 to 2 engines, vs going from 2 to 1. And to be certified single engine jets must have battery backups, rapid depressurization tech, and a host of other things twin engines don't have to. In one case (Vision Jet) they use a whole-plane parachute to compensate for the lack of a backup powerplant

Safety advocates (I.e. fire experts) are on board with changing the Ontario building code to no longer require multiple stair wells in new structures below a certain number of floors because of the lowered risk factors. Just need policies to catch up.

Not if the central stairwell is designed as, effectively, a bunker in the middle of the building. Build it out of concrete, with door closures and ample sprinklers, and the stairwell will never be the source of a fire mandating total evacuation. Today we even have "areas of refuge", normally at landings, where mobility-impaired people may wait for rescue rather than attempt stairs.

Fires almost never start in ‘common areas’ like corridors and lobbies. Tenant spaces and apartments inside multi-tenant buildings have their own four hour fire barrier so if a fire starts inside of a tenant space or apartment, it’ll be four hours before it blocks the stairs at a minimum.

a couple of decades ago, I worked in the top floor of this old building in glasgow https://www.google.com/maps/@55.8601694,-4.2542338,15a,24.1y... . Ground floor of the building was a TGI fridays. The evac route was up onto the roof, across steel walkways to another corner of the building, and down a back stairwell.

There was a regular alarm test, so we knew where to go. Then one day there was a REAL alarm, we all trotted off in our designated directions, but as we came down that back stairwell, it was full of smoke (not thick, but clearly something going on...) then at the bottom we saw why: TGIF had been stacking cardboard waste in there, plus their employees had been taking cigarette breaks in there, beside the no smoking sign... and you'll never guess what happened. Yep, fire in the fire escape.

The fire marshall was getting very stabby pointy fingering at some sheepish looking people as we went by to our muster points.

(it was just a stack of cardboard and there was nothing else flammable in the stairwell, it wasn't life threatening, just reckless)

Eh, this isn't really true. An apartment fire on a windy day with the doors/windows open to allow drafts will burn through in less than an hour, and if it's extreme wind faster than that.

Why not 3 exits? It's a matter of balancing costs and benefits.