The shift to a low-carbon society is required to deal with global warming. CO2 emissions in urban areas are increasing rapidly as Asian developing cities become increasingly motorized to keep pace with economic growth. However, reducing CO2 emissions in urban areas cannot be achieved simply by switching from automobiles to public transportation, so it is necessary to estimate how much CO2 emissions will be decreased by implementing multimodal transport policies for a low-carbon society. Therefore, this study assessed the long-term CO2 emission reductions based on the dynamic simulation results of the performance of the multimodal transport policies using the “Metropolitan Activity Relocation Simulator (MARS),” one of the land use and transport models. For this case study, the developing city of Ahmedabad in India was selected because public transport will be introduced to establish a low-carbon society. Consequently, the reduction of CO2 emissions of up to approximately 18% in the whole urban areas compared to the baseline scenario was indicated by implementing those policies because people will shift from private vehicles to public transport. This study revealed that the impacts of multimodal transport policies are effective for CO2 emission reductions and contribute to the design of a low-carbon society in developing cities.