There is a broad consensus about the great importance that transport and mobility have on the citizens’ welfare. Infrastructures and transport services contribute to this objective in a great variety of forms: for example improving on the one side population’s access to services and labour and on the other increasing our countries productivity. Must be said that not all the different projects and initiatives have to be executed, only those which can resolve accessibility problems and contribute with benefits must be financed, because opportunity costs in the field of infrastructures are particularly relevant when the public economic resources are shrinked; that’s why in the actual situation to priorize investments has to have much more importance. Moreover, the need to include private capital in the infrastructure’s financing through PPP’s or concessions requires strict evaluation of the projects involved, even though it isn’t an easy task The growing concern at all levels of society about environmental issues urgently asks for better evaluating techniques of the external costs and investments, so that efficient transport strategies can be implanted to internalise externalities equitatively and also to save resources. Moreover, the improvement of evaluation techniques of transport initiatives will allow the political responsible to give the stakeholders better, objective and systematic explanations. This master thesis has dealt with the social costs and benefits evaluation that an strategy can provide, to this end a great number of methodologies have been selected with the intention of evaluating not only the operating costs of the different transport modes, but also other aspects that sometimes are forgotten or remain little known. In addition, different methodologies have been applied to know the potential of reduction of the external costs that accumulate four different practical cases: the flexible carpooling implementation with the aid of a HOV lane, the bus lane creation by reducing the general purpose lanes width, the dynamic speed fixation and the optimal social speed of travel on highways obtaining. It has been verified that the great complexity of these methodologies requires of strict technical practice not to fall in doublecountings (normally by counting yet internalised external costs) and of great computation capacity, as the road system has lots of actors and variables that are intimately related.