Rethinking green transport infrastructure: indirect emissions, mediated impacts, and decarbonization pathways in South Korea

Abstract

This study examines the relationship between green transport infrastructure and indirect greenhouse gas emissions, focusing on emissions that arise not from vehicle operations themselves but from energy use, thermal demand, and waste generation associated with infrastructure maintenance and service provision. In doing so, it revisits the conventional assumption that electric rail systems are environmentally neutral and highlights the need for a broader accounting framework for transport-related emissions. The study distinguishes between unmediated infrastructure-related associations and mediated associations operating through socio-structural change. To examine these relationships, the study combines a spatio-temporal Durbin model with mediation analysis using 12 years of panel data from 227 Korean cities and counties. The results suggest that expressways are associated with higher indirect GHG emissions mainly through mediated pathways, particularly those related to population concentration and traffic volume, while their unmediated association is not statistically significant. By contrast, urban and metropolitan railways are associated with lower indirect emissions, with the clearest lower-emission association observed for urban railway. Intercity railway does not show a broad-based significant association, suggesting that its environmental relevance may be more selective and context-dependent. These findings indicate that “green” transport infrastructure should not be evaluated through a single universal pathway. Rather, transport-related carbon evaluation should consider both infrastructure type and the socio-economic pathways associated with it. From a policy perspective, road-related investment may need to be assessed together with land-use management, travel-demand management, and energy-efficiency considerations, while rail-based systems may be more consistently linked to lower-emission outcomes when combined with station-area planning and commercial structures that support concentrated and less carbon-intensive urban activity.