Non-motorized is probably a bit too purist and not practical for the broadest possible implementation.

Electric-assisted or hybrid human-electric transport should prove to be a lot more convenient and practical.

I'm not sure I know how to respond to this.

As you know most people do not use bikes to travel around the city and practicality is a major issue both in design of the city and the vehicles. For the most bike-friendly cities 40% currently seems to be the maximum number of users.

A number of months ago there were about 40 million ebikes in China (as posted on this blog in a NYTimes article) often banned from major cities because of various problems.

Suitable urban design (safety is of the utmost importance) and industrial design of the vehicles (likely including derivations of tricycle recumbents) would most likely make this type of transportation suitable for the broadest range of the population with ambitious goals substantially greater than 40% including the elderly, handicapped, mothers with small children, etc.

In a lot of situations people welcome powering either on an ad hoc basis or on a steady basis.

The key is to create vehicles that are much easier, safe, more fun and practical to use than cars and the current transit systems. Hybrid human-electric vehicles have this potential.

The Segway was a step in the right direction but, this is a fully powered vehicle and people have to stand up and it is not trivial to use among other issues. Ebikes like Optibike are another, but still lack the requisite level of practicality.

There will be a tipping point when suitable urban environments and vehicles have been designed.

Angus Grieve-Smith, My apologies for not understanding what you were talking about.

It is unfortunate that walking and cycling are often lumped together causing the confusion herein as each mode is different with different characteristics, applications, and needs.

In any case it would take the people you mention about one-fourth the amount of time to cross large distances using bicycles rather than walking.