Environmental and welfare gains via urban transport policy portfolios across 120 cities

City-level policies are increasingly recognized as key components of strategies to reduce transport greenhouse gas emissions. However, at a global scale, their total efficiencies, costs and practical feasibility remain unclear. Here we use a spatially explicit monocentric urban economic model, systematically calibrated on 120 cities worldwide, to analyse the impact of four representative policies aimed at mitigating transportation greenhouse gas emissions, also accounting for their economic welfare impacts and health co-benefits. Applying these policies in all cities, we find that total transportation greenhouse gas emissions can be reduced by 31% in 15 years, compared with the baseline scenario. However, the consequences of the same policies vary widely between cities, with specific effects depending on the policy considered, income level, population growth rate, spatial organization and existing public transport supply. Impacts on transport emissions span from high to almost zero, and consequences in terms of welfare can either be positive or negative. Applying welfare-increasing policy portfolios captures most of the emission reductions: overall, they reduce emissions by 22% in 15 years. Our results highlight that there is no one-size-fits-all policy. However, with context-specific strategies, large emission reductions can globally be achieved while improving welfare.