How this exceptionally American land-use system came into being during the late 1800s and through to the 1930s is the primary focus of Hirt's book. In chronicling this period, many apparent paradoxes present themselves: for instance, although the United States of the late 19th century prided itself on being the most democratic nation in the world, its citizens had a low level of trust in their elected municipal officials. The progressive municipal reformers of the time might therefore have campaigned for planning to be guided by state or federal governments, but instead pushed for non-discretionary municipal-level zoning. As Hirt observes, zoning reformers such as Lawrence Veiller argued that "zoning rules should vary as little as possible in districts that were as large as possible and that zoning relief should be granted only under a very limited set of circumstances, if at all."
But if the planning powers were delegated from state to city, and the city was to have little power to alter the apparently infallible choices of the initial zoning commissions, who was left to actually engage in city planning? No one, as it turns out. Planning commissioners were seemingly intended to be little more than curators of the city zoning map, and Hirt finds, as I have also noted, that zoning maps have changed relatively little in their basic allocation of space since the 1920s. As I've written about before, the actual policy that zoning was intended to serve was almost an afterthought, and was primarily concerned with protecting the investments of wealthy homeowners. By default, and perhaps unintentionally, city planning (to the extent it existed at all) was turned over to the emerging highway engineering profession.
American zoning policy, in sum, was a negative and reactive vision -- through its implementation, it viewed cities as incapable of honest and effective self-government, and by its actual regulations, it viewed urbanization as a threat to not only investments but to civic spirit and even the American way of life itself. As Hirt writes, "[t]he single-family home had the right to the city: it was always seen as being there first. It was the gracious host, the delicate victim, and the original citizen that was always haunted, followed, invaded, and taken advantage of by other housing types." In this sense, Hirt's book echoes the conclusions of Steven Conn's recent Americans Against the City: Anti-Urbanism in the Twentieth Century.
The key takeaway of this is fairly simple: American zoning was already being used as a tool to prevent redevelopment as early as the 1920s. Zoning theory, while it grew out of nuisance law and European (especially German) practice, was already being adapted to exclude rather than include -- zoning, by design, is a hyperlocally-devolved tool. Indeed, the basic idea seems to have been to zone land such that intensification of existing development was made impossible, a practice which inevitably would concentrate redevelopment in the most intensively-used areas -- and on the greenbelt edges.
And because there was no systemic planning practice designed to rezone, it has been left to politicians' whims, which often result in downzonings once communities realize that existing zones allow more intensive land use than what currently exists, and state DOTs -- whose highway extensions (and not mass transit extensions) have been, since midcentury, what has opened new land up to development.
What is curious to note, then, is that American sprawl is driven by what is essentially a units problem. In the hard sciences, units problems rise when units are misapplied, or when they're inadvertently mixed. For example, while a velocity calculation might be mathematically simple under constant acceleration, the result has to be in the same units as the inputs (e.g. m/s, cm/s, mi/h, etc.). Mix up the units -- say, by measuring an object's weight when what you want is its mass -- and you throw your calculations off. Similarly, inappropriate aggregation can often hide more than it illuminates: this is one of Jane Jacobs' key critiques of mainstream macroeconomics, that the sovereign nation is fundamentally an inappropriate unit for economic measurements, as it almost always represents an aggregation of very much economically disparate regions.
By over-empowering land use planning at the hyperlocal scale, we inadvertently disempower reasonable land-use planning at the scale where it matters most, as far as optimizing spatial efficiency relative to amenitization is concerned: the greater conurbation. Static land use paradigms undercut the efficacy of mass transit (as has often been pointed out at Urban Kchose); because zoning law is not designed with spatial efficiency in mind -- and, indeed, almost every solution to any given land-use problem increases spatial inefficiency -- we find ourselves unable to respond save by sprawling more ... and more ... and more.
Sometimes small-scale intensification is not aesthetically pleasing: witness D.C.'s pop-up house, which, now that one has been built, has been banned. Indeed, this has generally been zoning's modus operandi -- regulate nonconformity into nonexistence. The problem with this is, however, that at a certain level, small-scale intensification requires aesthetic flexibility. Indeed, this may be old urbanists' strongest criticism of New Urbanism: that it overfocuses on form, while underfocusing on the land-use development process -- the true locus of the sprawl process. This is perhaps where Strong Towns has come into play: it has gained traction and has been growing, as a grassroots movement, precisely because it has correctly identified the core problem: a broken process: a process that enshrines a static, rather than dynamic, vision of the city.
But here is an interesting issue, and once where there might actually be natural friction between Strong Towns and old urbanists: short of total curtailment of land use regulation (an extremely libertarian solution), while the issues in transportation engineering are clearly defined by its excessively top-down paradigm, the problems with spatial planning are caused by it being excessively bottom-up. That is, resistance to change is driven not by new inhabitants but by the ones who already live there, and have the time and energy to mount opposition (which, due to the way the playing field is skewed, is almost always effective opposition). Even very simple changes -- for example, liquidation of excessive parking regulation, making small multiunit structures legal, moderate height increases, and other examples of incremental redevelopment -- are opposed so fiercely that, outside of large urban cores where an effective development counterpresence can be built, they are unlikely to get any traction.
The long and the short of this can be summed up by saying: The current land-use planning paradigm is fundamentally broken. Moreso, it is fundamentally broken because current American regulation creates a static vision of the city, rather than a dynamic one; this fundamentally empowers interests that favor their static vision (NIMBYs) while disempowering ones whose vision is dynamic (developers). And, despite many planners' attempts to make dynamic redevelopment easier, the truth is that the discipline's entire toolkit, over-oriented towards a static vision, greatly inhibits what can be done, and yields marginal results of even the most sweeping efforts.