In the accord with conception of sustainable development the analysis of negative externalities caused by traffic comes out from the analysis of social costs of personal and freight traffic and from comparison of countable costs caused by external impacts of traffic. Social traffic costs can be divided into two groups. Costs of the user of traffic infrastructure and costs of its acquirer form the group of internal costs. On the other hand costs from external effects of traffic, having an impact on the whole society, present external costs. This group includes not only direct financial costs caused by another subject of traffic, but also indirect costs, including damage to persons or natural ecosystems. They have not market price, so they have to be evaluated by different means. The article deals with the aims and methodology of this research in Slovakia.