Assessing freight structure and its effect on transport CO₂ emissions: heterogeneous and mediating effect analysis

Understanding the effect of freight structure (FS) on carbon dioxide (CO₂) emissions is significant for policymakers in the transport sector. However, few studies have investigated fully how the freight structure influences transport CO₂ emissions. In the paper, a comprehensive freight structure index is proposed to assess the freight structure in 16 provinces of China during 2005-2019. Then, the heterogeneous effect of the freight structure and other factors on CO₂ emissions are quantified by the quantile regression model developed. The mediating effect between freight structure and CO₂ emissions is further examined. And the panel causality test is used to explore the directions of causality among variables. The main results indicate that (1) the level of freight structure shows a distribution of increase from western and central regions to eastern regions in 16 provinces. (2) Freight structure has a positive effect on transport CO₂ emissions, and its effect is significantly heterogeneous in all quantiles. (3) Freight structure has an indirect impact on CO₂ emissions through the scale effect. (4) The bidirectional causality is confirmed between economic growth, industrial structure, and information and communication technology with carbon emissions. A unidirectional causality from CO₂ emissions to freight structure is also found. Finally, some policy recommendations are proposed for sustainable transport development in China. The results facilitate the formulation of the regionally tailored carbon emission reduction strategy in the transport sector.