Weakening synergies in carbon and air pollution co-control necessitate robust structural transitions in China's transportation sector.

A fossil fuel-dominated country like China is promoting a policy shift toward synergistic governance integrating climate change mitigation and air quality improvement, especially in the on-road transportation sector. However, the efficacy of current emission control policies in mitigating greenhouse gas (GHG) emissions and air pollution remains poorly quantified through a synergy-focused lens. Here, we develop an integrated analytical framework incorporating a Synergy Index to evaluate policy performance across GHG abatement and air pollution-related health burden reduction. We find that China's on-road environmental policies reduced GHG emissions by 427 Mt CO2e and averted 104 000 premature mortalities during 2010-2015, but these gains declined to 278 Mt CO2e and 72 000 mortalities in 2015-2020, marking an 18.7% decline in the overall Synergy Index. Policy-specific evaluations reveal that traditional policies like tightening emission standards and fuel quality and removing high-emitting vehicles drove early synergies. Conversely, emerging structural transitions, including promoting electric vehicles and a modal shift from road to more efficient modes, grew in prominence but failed to offset declining efficacy. Strategic optimization of vehicle fleet and transportation structure to meet 2025 targets could reverse this declining trend. These findings underscore the urgency of accelerating structural transitions to sustain effective carbon and pollution co-control in the transportation sector, with global relevance for fossil fuel-dependent economies.