Environmental impact assesment instruments serve the purpose of detecting the negative effects that a certain element or set of elements is producing in a particular area, measure the importance of such effects through the definition of indicators, establish the correct diagnosis based on optimacy criteria previously defined, and search for the introduction of corrective planning actions based on the identification of the causes that produce the impact. This impact is defined by the authors as the ways by which standards of living may be altered through an irrational decision of urban transportation. The research project, from which this article stems out, was aimed at establishing the functional structure and the components of a transportation system, imagining the theoretical effects that its inadequate operation, of is elements individually or as a whole in relation to others, could produce on a particular environment and, thus, set the basis for constructing the adequate hipothesys about the environmental problems generated by the system. The procedure is applied in downtown Santiago as a way of testing the theoretical development and setting a diagnosis of environmental problems of this area related with the transportation system.