We investigate the determinants of driving speed in large us cities. We first estimate city level supply functions for travel in an econometric framework where both the supply and demand for travel are explicit. These estimations allow us to calculate a city level index of driving speed and to rank cities by driving speed. Our data suggest that a congestion tax of, on average, about 1.5 cents per kilometer yields welfare gains of about 30 billion dollars per year, that centralized cities are slower, that cities with ring roads are faster, and that the provision of automobile travel in cities is subject to decreasing returns to scale.