Walkable neighbourhoods with excellent public transport are perfectly (and I would bet more cost-effectively) possible with low and mid rises as you can with high rises. Good examples are Barcelona and Paris.

America’s weird zoning laws are a problem unto themselves. Here in the Old World it’s perfectly normal to have a row of shops and cafes and whatnot in the middle of the suburbs or on the bottom floors of a block of flats. That is the biggest factor in creating walkable neighbourhoods.

Edit: I accidentally a word.