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α: calibrated so average coauthorship-adjusted count equals average raw count
We estimate the marginal external cost and welfare losses due to congestion for a large number of roads in Rome, Italy. We show that the marginal external cost is substantial, equaling about two thirds of the private time cost of travel. About one third of this cost is borne by public transport users. Our results imply that policies designed to reduce congestion can result in important welfare gains.