Each neighborhood separates into two parts along the central boulevard, and the areas on either side of the boulevard can be treated as separate neighborhoods with populations of about 6,000. It is possible that certain buildings should exceed this limit, but they should never be buildings for human habitation. It would be considerably simpler to design a carfree city in the absence of this limitation, but I believe that this pattern is so important that I have adopted it in the reference design for carfree cities. In practice, many wonderful urban areas have buildings as high as six stories, but my experience is that areas with buildings taller than that are almost never agreeable places to live. Jane Jacobs was probably the first to make this argument, in The Death and Life of Great American Cities .
Mixed uses lead to interesting, vibrant, and healthy neighborhoods.
I think that this can be stretched to five minutes if the walk is pleasant, safe, and unimpeded by cars.