Road transportation infrastructure projects in Spain have been promoted, raising the quality of Spain’s road transportation network to European standards in a short period of time. The objective of this chapter is to measure the output effect of road transportation infrastructure in Spain in the period between 1997 and 2006. In particular we estimate a production function using a panel dataset of Spanish provinces (NUTS 3) in order to account for marginal productivity effects within a province and to document the existence of spillover effects outside the provincial boundaries through the use of spatial econometric methodologies. Road infrastructure endowment indicators are measured using three different indicators: an accessibility measure, the traditional road stock indicators and a variable to accommodate the stock indicator to the degree of utilization. Estimation results show that road transportation infrastructure positively impacts upon the performance of the Spanish economy. The spillover effects of road infrastructure stock and accessibility on neighbouring provinces are larger than direct impacts caused by the infrastructure where it is located.