In most urban areas, mobility is predominantly reliant on automobiles, leading to significant negative environmental impacts, such as noise pollution, air pollution, and greenhouse gas emissions. To meet the objectives of the Paris Agreement, urgent action is required to decarbonize the mobility sector. This necessitates the development of assessment and planning tools to create effective decarbonization scenarios. Urban mobility must evolve to reduce dependency on fossil fuels by increasing public transport options and promoting active modes of transportation. This research presents a methodology to estimate the modal share required to shift car users to active modes and public transport, thereby achieving future CO2 emission reduction targets in the road transport sector. A case study in Braga, Portugal, demonstrates that to meet the 2040 target of 59,150 tons of CO2, 63% of trips must be made using active modes (e.g., walking and cycling) and 32% by public transport.