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ABSTRACTThe purpose of this research is to develop a methodology for calculating the index of the spatial equity for the public transportation services for the city of Seoul using the Lorenz curve and Gini coefficient based on accessibility to the services. The index was estimated for 100 m × 100 m cells first and then aggregated for 424 traffic analysis zones (TAZs) in the city. The indices for Seoul’s subway and bus were represented with and without the weighting factor. On the basis of this methodology, this study assessed the effect of an additional subway line on the spatial equity before and after its operation. Furthermore, an index of relative difference indicating the change in population and employment in the station areas was applied to investigate the changed effects of spatial equity between population and employment from the new subway line. The core findings from this research are summarized as follows. First, the subway network of Seoul provides more unequal public transportation services for the TAZs than for the bus network. Second, an improvement in the operation frequency of both transit modes would deepen the spatial inequity of Seoul. Third, a subway extension plan for Seoul could noticeably reduce the spatial inequity. The plan would improve the spatial equity for the population more than that for employment. The most important meaning of this study is to be found in combining spatial equity on the supply side with transit-oriented development (TOD) on the demand side. So, by establishing a TOD policy for the additional subway station areas, an appropriate land-use pattern can be established for maximizing potential ridership in consideration of the combination.