A key input to quantitative evaluations of transport infrastructure projects is their impact on transport costs. We propose a new method of estimating this impact relying on widely accessible customs data: by using the route choice of exporters. We combine our method with a spatial equilibrium model to study the effects of the massive expressway construction in China between 1999 and 2010. We find transport costs are 20% lower on expressways than on regular roads. The expressways construction increases aggregate exports by 10% and domestic trade by 14%. It generates 5.1% welfare gains, implying a 150% net return to investment.