Transport infrastructure contributes to social welfare when a basic condition is fulfilled: social benefits should exceed social costs. It has been common to see infrastructure with a biased perspective leading to the belief that the more, the bigger and the more advance technologically the better. The concept of opportunity cost of public money does not seem to be understood by many who take crucial decisions on the construction of new large projects. In this paper, the economic evaluation of projects is defended as a way to avoid the construction of socially unprofitable projects.