This paper investigates the causal impact of city shape on car use and emissions within French metropolitan areas. We in particular analyze the influence of a novel indicator of urban geometry that captures differences in the built environment likely to nurture or alleviate car dependence. Individual data allow us to separate the effects of urban geometry and households' spatial sorting, while historical and geological instruments help tackling ‘endogeneous density’ issues. Cities with a more compact, fractal and diverse spatial layout have lower car emissions per household. Urban geometry drives a bell-shaped relationship between city size and the ‘carprint’ of households: small cities compensate for their lack of Density or Diversity by an environmentally-friendly geometric Design, whereas medium cities exhibit heavier driving footprints.