This paper discusses the practice of cost-benefit analyses of transportation infrastructure investment projects from the meta-theoretical perspective of critical realism. Such analyses are based on a number of untenable ontological assumptions about social value, human nature and the natural environment. In addition, main input data are based on transport modelling analyses based on a misleading `local ontology' among the model makers. The ontological misconceptions translate into erroneous epistemological assumptions about the possibility of precise predictions and the validity of willingness-to-pay investigations. (...) Accepting the ontological and epistemological assumptions of cost-benefit analysis involves an implicit acceptance of the ethical and political values favoured by these assumptions. Cost-benefit analyses of transportation investment projects tend to neglect long-term environmental consequences and needs among population groups with a low ability to pay. Instead of cost-benefit analyses, impact analyses evaluating the likely effects of project alternatives against a wide range of societal goals is recommended, with quantification and economic valorisation only for impact categories where this can be done in an ontologically and epistemologically defensible way. (shrink)