Under What Conditions Can Urban Rail Transit Induce Higher Density? Evidence from Four Metropolitan Areas in the United States, 1990-2010

Ample empirical evidence shows that dense urban forms can promote rail transit use and reduce car dependence. However, evidence of the reverse causal link---the impact of urban rail transit investments on neighborhood land use forms---is less clear. Previous studies that evaluate the land use impacts of the urban rail transit systems yield mixed results on whether and how these systems could affect land use forms. Those mixed results suggest that the existence and magnitude of land use changes due to rail projects are likely influenced by certain contextual conditions, pertaining both to the project location and to its regional setting. It is the focus of this study to explore what those conditions are and to examine how they could affect the land use impacts of urban rail transit projects on the nearby neighborhoods.;Out of many dimensions that describe land use impacts, this research chooses to examine the changes in population and housing densities---the densification effects, in particular. After theorizing the mechanisms of the densification effects of urban rail transit, this study hypothesizes on the key factors that may interfere with such land use effects and tests those hypotheses in empirical studies. It takes into account both the internal and external factors that could interfere with the land use impacts of urban rail transit. The internal factors include the type of rail transit and the station features; the external factors include the pre-existing conditions of the neighborhoods where the rail stations are located. To provide the most recent evidence on this topic, this research selects four metropolitan areas in the United States as the study cases---Chicago, Denver, Los Angeles, and Washington, D.C., each of which constructed new urban rail lines in the 1990s. Applying a difference-in-differences design and a mixed methodology of spatial and regression analyses, this study quantifies the effects of new rail stations on neighborhood population/housing density changes and investigates the conditions that may promote or mitigate such effects.;The findings on individual cases show that a new rail transit station is more likely to help increase population and housing densities when it is introduced in a moderate-income neighborhood with a pre-existing condition of compactness and relatively few single-family houses. A cross-comparison of the results from the four difference cases reveals that heavy rail lines are more likely to trigger increase in population and housing density than light rail lines. In addition, the network effect also matters---a new urban rail line that is an extension to an existing rail transit network is more likely to promote density increase than a brand new urban rail system built from scratch.;This study contributes to the long-lasting debate on the costs and benefits of urban rail transit investments. It provides the most recent empirical evidence on the densification effects of urban rail transit in the United States. Furthermore, it is the first of its kind to systematically study the interference of both the internal and external factors of a new urban rail transit project on its potential land use impacts. The findings of this study can be used to help transit planners make informed planning decisions on the site selection of a new rail station in the future, if densification is one of their planning goals.