We estimate the effects of highways on the suburbanization of Spanish cities. First, we extend previous findings for the US and China by providing evidence for Europe: each additional highway ray built between 1991 and 2006 produced a 5 per cent decline in central city population between 1991 and 2011. Second, our main contribution is at the intrametropolitan level. We find that highway improvements influence the spatial pattern of suburbanization: suburban municipalities that were given improved access to the highway system between 1991 and 2006 grew 4.6% faster. The effect was most marked in suburbs located at 5–11 km from the central city (7.1%), and concentrated near the highways: population spreaded out along the (new) highway segments (4.7%) and ramps (2.7%). To estimate the causal relationship between population growth and highway improvements, we rely on an IV estimation. We use Spain’s historical road networks – Roman roads, 1760 main post roads, and 19th century main roads – to construct our candidates for use as instruments.