Natural gas for automotive purposes is an appealing alternative: curbing local and global pollution and dependence on foreign oil are among the most remarkable advantages. The other side of the coin implies building and maintaining an on purpose network entailing financial requirements. The final aim of this work is to compare its advantages with economic rationale. A system dynamics model is built and taken as reference for all quantitative assertions. It contains data referring to two scenarios: business as usual versus expansion. The model treats separately global and local emissions and infrastructure needs. Quantitative results are the basis for the final assessment, that is grounded on the externalitiesí theory. By analyzing the scenariosí gap numerous remarks follow. Regarding global emissions, beneficial effects seem modest. Local emissions would either decrease or not vary depending on the pollutant. Avoided externalities estimates exceed infrastructure financial requirements. Natural gas is a suitable answer in tackling some issues related to the road transport industry.