Urban spatial structure, commuting, and growth in United States metropolitan areas
There have been "qualitative changes" in metropolitan spatial structure in recent decades. While these changes have been widely recognized, much less is known about the specifics---the forms, causes, and consequences of the spatial changes. In that regard, this research aims to address several questions: What are the prominent features of emerging urban forms? Are cities becoming more edgy or more edgeless? What are primary forces driving the spatial changes? What are the consequences of the spatial changes in daily commuting and urban economic growth?
To answer these questions, this research attempts to uncover the current stage and directions of spatial evolution, investigate the driving forces of spatial changes, and probe the links between urban spatial structure, commuting economies, and economic growth in the contemporary US metropolitan areas.
In Chapter 2, I defined and estimated sets of spatial structure variables by identifying employment centers consistently within 79 metropolitan areas with population over one-half million. These spatial descriptors show that one of the most important features of the modern metropolis is predominant dispersion. Average dispersed employment share was 82 percent by the GWR method and 73 percent by the minimum density method. Surprisingly, the majority of jobs were diffused outside any type of employment centers in all 79 metropolitan areas without exception. Findings from this Chapter parallel the results of Gordon and Richardson (1996) and Lang (2003)---spatial evolution "beyond polycentricity".
Chapter 3 presented a series of statistical analyses testing the determinants of manifold dimensions of urban spatial structure. The results provided valuable findings that generally conform to the predictions from urban economic theories and path dependence perspectives.
Larger metropolitan areas tend to have smaller CBD employment shares and more decentralized and polycentric structures. However, population size was not significant in explaining another spatial dimension, employment dispersion. Congestion was a significant contributor to subcenter formation, but was less significant in employment share models. Industrial composition was also found to be an important spatial determinant, confirming that different industries are subject to different agglomeration economies with varying geographical ambits.
The path dependence in urban spatial structure was indirectly identified in two ways. Recently developed metropolitan areas have smaller CBD and are more decentralized than pre-war metros; while metros that reached the half of current population 35 to 60 years ago had more polycentric structure. Second, metros with a strong agglomeration in the urban core tended to have fewer subcenters and smaller subcenter employment shares.
Chapter 4 explored spatial changes in six metropolitan areas for the last two decades to address the question whether they are increasingly edgy or edgeless. Findings paralleled the results of Gordon and Richardson (1996): Jobs continued to decentralize from the metropolitan core to the suburbs during the 1980s and 1990s and job dispersion was a more common phenomenon than subcentering.
Nevertheless, the results showed significant variation in spatial decentralization trends rather than a uniform linear process from monocentric through polycentric, and to dispersed structure. New York and Boston, with big and long established CBDs, were less subject to decentralization. The polycentricity of Los Angeles and San Francisco was reinforced in the last decade, while job dispersion was predominant in Portland and Philadelphia. Each metro seems to have developed a unique pattern of decentralization, in light of their histories and circumstances to limit the growth of commuting times.
These findings suggest two important theoretical implications. First, the geographical and historical contexts of an individual metropolis strongly affect the path that it takes in response to global trends such as ever decreasing transportation costs and information technology (IT) development. Also, it seems that there is a "self reinforcing" pattern in spatial development as firstly observed in technology adoption and industrial development. Second, industrial composition and restructuring is an important part of the path dependent spatial evolution processes.
Chapter 5 presented a study on the commuting impacts of metropolitan level spatial structure. Descriptive analysis identified large potential for commute time saving by spatial restructuring towards more polycentric and dispersed form, particularly in large metropolitan areas. Notwithstanding the potential, however, I found only partial commute time saving with the spatial adjustment in regression analyses. While employment dispersion helped reduce commute time, polycentricity was not significant.
The insignificance of polycentric spatial dimension can be interpreted in two ways. First, CBD/main center employment share may be already too small to affect metro wide average commute time. Second, polycentrization may have system wide effects such as increased cross commuting, offsetting potential commute time savings.
Chapter 6 examined how the links between metropolitan spatial structure and economic growth depend on the size of the metropolis. Consistent with theories of urban system and evolution, growth effects of employment dispersion were found to be dependent on metropolitan size. A metropolitan area with more clustered spatial form grows faster when it is small; whereas more dispersion leads to higher growth rate as it grows large. Just as a city needs to successfully take on higher order functions and economic activities to move upward within the national urban system, it also needs to restructure its spatial form in a way to mitigate congestion or other diseconomies of size for continued growth.
Therefore, attempts to find one particular efficient urban form may not be promising, just as the efforts to find the optimal city size have not been fruitful. Efficient spatial structure may depend not only on the city size but also on other urban attributes such as industrial structure and the shape of transportation networks, which are products of the historical path of urban development. Insignificant growth effects of polycentric versus monocentric structure imply that there may exist many plausibly competitive urban forms and different paths of spatial evolution.
Area planning & development
0999: Area planning & development