What’s the economic value of greening transport infrastructures? The case of the underground passages in Lisbon

Green infrastructure have been widely spread to counter the negative effects of urbanization. However, their economic feasibility still generates discussion between researchers, investors and suppliers, leading to the need for more accurate and case-specific cost-benefit analysis. This study pretends to analyze the economic feasibility of greening transport infrastructures, in particular road tunnels. The proposed methodology includes the life-cycle costs and benefits of green systems. Its incremental approach considers economic appraisal components (financial, economic and socioenvironmental) and transport infrastructures dimensions (infrastructure, user and surroundings). This study simulates five green case studies in underground passages of a roadway with high traffic intensity in Lisbon, Portugal. Four out of five scenarios are economically feasible using a life-cycle of 40–50 years and a discount rate of 4,8 %. The final net present values range from 33.961 to 5.079.356 €. Discount and inflation rates, financial costs, reduction of the noise inside the tunnels, creation of new functional areas and aesthetical improvements have great significant impact on projects’ economic return.