The aim of this article is to conduct an analysis of the consequences of public modal shift policies on interurban journeys in France (people who live and work in two separate functional urban areas). It measures and analyses three facets of these policies: the potential for a modal shift from the car to public transport, the environmental consequences (CO2 emissions) and the social consequences (transport costs) of such a shift. The increase in travel costs brought about by higher fuel prices or from charges for access to urban centres, together with a reduction in the costs of travelling by public transport, are the three elements which - according to the models - have the biggest influence on modal shift and the reduction in CO2 emissions. However, our findings also show that these policies are socially regressive in that they financially advantage the higher socio-economic categories.