How the "neighborhood unit" transformed into sprawl
What’s most striking though is the difference in scale. The Perry diagram indicates an internal radius of 1/4-mile, and while the Boise plan does not provide any type of scaling map element, its implied internal radius is a full 1/2-mile (since the section line roads, indicated by the bounding thoroughfares, are one mile apart). So, while the majority of residents in the Perry diagram would live within a quarter-mile of retail shops (easily reachable with a five minute walk), the majority of residents in a new Boise neighborhood would live over a half mile from the proposed commercial land use. It’s easy to see that the framers of the Boise plan did not intend their residents to walk to retail but to drive, graphically evinced by the large areas of dedicated parking surrounding the new shopping area.
The story is common all across America, and indicative of the first wave of post-war suburbia. With the arrival of the car culture, professionals began immediately changing the DNA of our cities to make them easier for driving. But, they still knew enough of the older planning to plan for mostly continuous streets, some element of public space and some neighborhood-serving commercial areas. Quite a bit of this was also influenced by ULI's noteworthy book The Community Builder's Handbook, which was authored by J.C. Nichols among others. The Boise example is a later example, being from 1963, but some of the elements are obvious.
Today, interestingly enough, it's those early suburbs from the 1940's-1960's that have the most potential to transform themselves into more walkable places. Later versions of how to plan development along section lines became so auto-dependent that they have very little hope of growing up into anything else. For example, this diagram below is indicative of the 2nd wave of post-war suburban development. Just look at how few streets now actually connect:
This is a classic "access management" approach to planning, where it's a given that people will drive everywhere for everything. It focuses all through traffic on section-line arterials (spaced every mile apart) and precludes any efforts at walkability. Even biking is difficult in this pattern, unless some natural features are present (streams, low areas, etc) that over-ride the grid.
The challenge today is to look for design & livability solutions that work for each template. The underlying infrastructure is so expensive that it's likely not to change. And while it's tempting to try and figure out ways to transform later suburbia to allow for more biking and walking, we can't escape the reality that it's very, very difficult. Many of my New Urbanist colleagues would disagree with me, but in my opinion the wise path to pursue is to let the later phases of suburbia just be what they are, and focus on urbanity where it can have enough critical mass to succeed.