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Abstract: I quantify the effect of U.S. Interstate Highway System on suburbanization and urban core decline via trade costs reductions beyond just commute costs reductions. I leverage variation in highway construction dates and driving time reductions to show that new highways affect both trade costs and commute costs. I find that, on average, a rise of one standard deviation in market access raises population and employment by about 10 percent after 40 years at the county level. However, a rise of one standard deviation in commuting access raises population and employment by about 1 percent after controlling for market access changes. I develop a quantitative model of trade, commuting, migration, and scale externalities. I map the model to the average effect of market and commuting access through indirect inference. Using the calibrated model, simulations show highways account for 15 percent of suburban growth and 33 percent of the decline in urban cores. I find that suburbanization and urban core decline were driven not only by reductions in commute times but also by trade cost reductions, which shifted economic activity away from urban cores.