The concept of highway has been applied in Spain to a high capacity road that does not meet all the requirements of motorways. These routes were designed to increase the network capacity, reduce travel times and improve road safety. However, all the projects of highways were initially developed as a splitting of existing roads, in order to reduce costs, so their standards were quite away from those of toll motorways. The Spanish network of high capacity roads is therefore an heterogeneous one. The Strategic Plan of Infrastructure and Transport 2005-2020 has tackled as a priority the reform of these infrastructures, the proposed actions are held in the form of public works concessions, remunerating the concessionaire using a shadow toll scheme. As part of the project financing, concession companies were given access to equity loans to finance initially unforeseen works. Traditional methods of investment valuation do not quantify accurately the extent and effectiveness of such measures. The aim of this study is to provide a methodology based on the real options theory to quantify the real value provided by these loans to concessionaires, or, from another perspective, to quantify the true cost they represent to the public sector.