Transport analyst and writer Tom Vanderbilt has a four-part series in Slate about the crisis in American walking:
The United States walks the least of any industrialized nation. ... Why do we walk so comparatively little? The first answer is one that applies virtually everywhere in the modern world: As with many forms of physical activity, walking has been engineered out of existence. With an eye toward the proverbial grandfather who regales us with tales of walking five miles to school in the snow, this makes instinctive sense. But how do we know how much people used to walk? There were no 18th-century pedometer studies.
[S]ince our uncommon commitment to the car is at least in part to blame for the new American inability to put one foot in front of the other, the transportation engineering profession’s historical disdain for the pedestrian is all that much more pernicious. In modern traffic engineering the word has become institutionalized, by engineers who shorten pedestrian to the somehow even more condescending “peds”; who for years have peppered their literature with phrases like “pedestrian impedance” (meaning people getting in the way of vehicle flow).
As Vanderbilt says, traffic engineers and our obsession with the car have driven most of the problems. Even though engineer Charles Mahron and people like him crusade against the worst urban designs (see, e.g., Brainerd, Minn.), I don't think anything will change without a disruptive and permanent external shift. I don't really want $10 gas, but wow would that focus people's attention on driving.