> This distance must be fairly short.

Not really a factor in dense urban environments. Everything within 5km is easily reachable within ~15 mins. Especially so once you factor in ebikes.

> You must be relatively young/fit/healthy/able-bodied.

Relatively is doing a lot of work there. This is not true for the most part and again, ebikes.

> The weather must OK (not frequently too cold/hot/wet).

The weather is mostly ok in most places most of the time. Frequency barely matters, you can always leave your bike home and walk/use public transportation instead if the weather that day isn't suitable.

> The terrain must be fairly flat.

Gears exist. Ebikes nullify this regardless.

> The traffic conditions must be not-suicidal.

This is the only real problem - dedicated biking infrastructure, physically separated from cars is required. However, it's usually an excuse more than a real issue. Biking infrastructure is astoundingly cheap compared to anything else.

> There must secure bike storage at both ends.

Depends on the trip type. It's usually not a problem due to extremely low cost - 5 U-shaped pipes attached to the ground gives you 10 bike parking spaces in a tiny area. If popping into a shop for 5 mins it's usually enough to just lock the bike to itself. It's unlikely a thief would carry off a bike in that time. Otherwise, keep the bike is naturally secure at home/workplace.

Plus, if it does get stolen, bikes are cheap and a dime a dozen.

> Ideally, shower/changing facilities at both ends for bad-weather days.

Unnecessary for most people in most conditions, but if it is an issue, usually cycling slower or getting an ebike works fine.