We investigate the determinants of driving speed in large us cities. We first estimate city level supply functions for speed in an econometric framework where both the supply and demand for driving are explicit. These estimations allow us to calculate a city level index of driving speed and to rank cities by driving speed. Our investigation of the determinants of speed provide the foundations for welfare analysis. Our results suggest that large gains in speed may be possible if slow cities can emulate fast cities and that the deadweight losses from congestion are large.