For trips of under a block, walking (or limited mobility equivalent) makes sense. For short local trips of a few kilometers bicycling makes sense. Then comes car, bus and mass transit for a few hundred kilometers trip. After that long distance trains, ship and air come into play.
It has been said that daily commutes have a constant at 20-30 minutes (each way).
http://en.wikipedia.org/wiki/Marchetti's_constant
These above fall into the class of how far can you get in ten to twenty minutes. Multi-mode trips (for a daily traverse from home to work) should have their aggregate be 30 minutes, and each mode thus have some fraction of the whole.
The issue is the elements and comfort. The long distance comutes have environmental control (heating/cooling, sounds, smooth ride).
The short travel options are generally spartan. Though there are interesting exceptions. The interior of a mall provides a controlled shopping trip environment. Pedestrian tunnels used in northern climates are another example. Even the shade provided by buildings and trees, and humidity from fountains in arid climates provide a similarly controlled environment.
But in the absense of ubiquitous infrastructure, there is wearing a snow suit or a raincoat, or using shade parasols, or sticking to tree lined venues.
The point is that it is NOT really about energy efficiency or exercise or other costs related to energy and dollars, as much as it is about "comfort".
If one takes this a starting point then the conclusions one can get to are unusual... A safe velomobile, can perhaps nicely fill the needs for walking and biking distances.
The Flintstone car is not so crazy - with a canopy, radio, communications, and safety. If the friction and weight could be small enough, yet all the above be in place, and with modest boost in basic "foot and hand powered" efficiency, then it is almost rational.
[Update 140714: Some getting close: But powered.
Elio motors
LIT motors
Tuk Tuks (aka auto rickshaw) ]
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