New infrastructure projects may affect CO2 emissions and, thus, cost benefit analyses for these projects require a value to apply for CO2. This may be based on the marginal social cost of emissions or on the carbon price resulting from present and future policies. This paper argues that both approaches are necessary, but for cost benefit analysis of infrastructure projects the latter should be the primary tool. A series of complications arise when applying this principle in practice. These are discussed in the paper. Even if the complications make the implementation of the approach difficult, we argue that it is still preferable to a social cost approach.