What will the great American cities of the 21st century look like? Just as the great American cities that were developed during the 20th century look vastly different to the great American cities that were developed during the 19th century, it seems completely reasonable to expect that the development pattern that dominates American cities during the 21st century will be just as dramatically different. We know this, because it is clear that the current pattern of development is financially unsustainable and will eventually break (because there will be no money to maintain it), and when it breaks, Americans will still need places to live, and so a new development pattern will emerge.
During the 21st century, we should expect to see urban renewal projects that see entire neighborhoods redeveloped and new neighborhoods emerge as we pull down urban freeways, abandoned big box stores, and other non-places, unlocking hundreds of acres within our cities for redevelopment.
What should this redevelopment look like during the 21st century? We've spent the 20th century building cities for cars, and that has led to a lot of negative consequences, so car dependency is something we want to avoid. This means redeveloping and retrofitting neighborhoods to be more human-scale. As a city is a collection of people, we can accommodate the car, but we shouldn't build for the car.
Street width is important to our perception of scale; street width affects how far away destinations feel (does the other side of the street feel as if it is divided by a highway, or mere steps away?), and the character of a place (is it cozy and intimate, wide and spacious, or bland and barren?) But, the street width also has practical implications on land usage.
Compare the examples in the image below. If the green represents a 300 by 300 foot city block, and the left block is surrounded by a 15 foot street, while the right block is surrounded by an 80 foot street, you can see how much extra space is taken up just by increasing the street width. The right example is 94% larger than the left example;