If you have kids on the way, I highly recommend knobs over levers for the exact reason Vancouver is specifying the opposite: they're harder to open.
Between 1 and 2, it's invaluable.
Between 1 and 2, it's invaluable.
I also installed this magnetic door stop on the same door. http://www.amazon.com/Stanley-Hardware-Magnetic-Doorstop-Nic...
These two combined have given me godlike powers of grocery transportation.
It seems to me that having the first group all buy $10 adapters per door will almost certainly be more expensive than having the few people who cannot operate door knobs buy lever handles.
E.g. it's about 1 in 5 Americans who are impacted by arthritis (of some form), plus you have people with repetitive strain injury and other non-arthritic conditions who will also benefit from lever handles. Compare that to the relatively small fraction of people in wheelchairs and this seems like a no-brainer compared to ramps.
As others have pointed out, the entire building code is basically a balance between cost and safety. Anything having to do with egress tends to get extra attention, as well. If there was a zero-cost way to prevent slips and falls, it might well end up in the building code, yes.
You can usually look on the box and find the coefficient of friction (COF) for any floor tile you buy. OSHA has requirements for what COF is appropriate for various uses (dry and flat or wet and sloped, for example). It wouldn't surprise me if that sort of thing eventually makes it into the residential building code.