Geography may not be conductive to biking everywhere. While I mostly bike around the city I live in, it's a very hilly place and even a short trip requires an amount of effort that cannot be expected of most people.

As for pedestrian traffic, my experience with medium US cities is that they are far too spread out for walking to be an option for most things. It's hard to see how this could change without Americans abandoning their beloved individual suburban homes with yards.

I think in most places public transportation is the only effective replacement for cars.