We estimate the effect of urban form on driving. We match the best available travel survey for the us to spatially disaggregated national maps that describe population density and demographics, sec- toral employment and land cover, among other things. To address infer- ence problems related to sorting and endogenous density, we develop an estimator that relies on assumption of imperfect mobility and exploit quasi-random variation in subterranean geology. The data suggest that increases in density cause small decreases in individual driving. Apply- ing our estimates to the observed distribution of density and driving in the us suggests that plausible densification policies cause decreases in aggregate driving that are small, both absolutely and relative to what might be expected from gas taxes or congestion charging.