| Summary: | In this paper, we examine transportation sustainability in American metropolitan areas using transportation-related CO₂ emissions, public transit accessibility, and commuting times as indicators. Though variations in these indicators may stem from historic contexts, policies, institutional arrangements, social and cultural origins, the spatial structure of metropolitan areas—in particular their formal characteristics—may also be a contributing factor. To test this relationship, we identify metropolitan form metrics from prior literature that are expected to impact transportation outcomes, and choose five metrics to which we introduce significant improvements. We apply the metrics to all 166 Combined Statistical Areas in the US, using an open-source GIS toolbox released along with the paper. Our findings demonstrate that form-based metrics provide a better explanation to CO₂ emissions, public transit accessibility, and commuting times in US metro areas than the simpler population size or density metrics typically used in practice. We also show that counter to prior literature on urban scaling laws and economies of scale, which have argued that larger cities and metro areas are more sustainable per capita, transport-related CO₂ emissions and transit accessibility are actually less favorable in larger CSAs when controlling for formal characteristics of metropolitan areas. Instead of scale, compactness has the highest elasticity with respect to transportation sustainability of metro areas.
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