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This research revisits the debate on transportation impacts of spatially decentralized development by measuring the formats and extents of polycentric spatial development in USA metropolitan areas and then examining the associated commuting outcome.The treats polycentrality as the third dimension (in addition to population size and density) of the metropolitan built-environment examined from the regional scale, and reflects the spatial organization of a metropolitan region in terms of the degree and the format of its internal density variation.The research uses a spatial statistical approach to measuring polycentrality for every USA metropolitan areas, using tract level census data of 1970 and 2000.It then examines the variation of polycentrality among metropolitan areas, its changes from 1970 to 2000, and how degree and formats of polycentrality are related to commuting time in the top 50 metropolitan areas.Cross-sectional analysis and longitudinal analysis not only reveal that polycentric development has significant impacts on commuting time, but also reveal suggests that suburban centers has to reach to a certain density level in order to create transportation saving in a decentralized region.