Analyzing the city-level effects of land use on travel time and CO2 emissions: a global mediation study of travel time

This global study simultaneously analyzes how land use differences between cities affect travel time and CO2 emissions. The land use-travel time-carbon emission relationship in which the time is specified as a mediator is analyzed at the city level on an international scale based on individually reported data by moving away from the tendency of conducting country-level, regional-scale analysis based on estimates from travel survey data or satellite images. A major finding of the structural equation model is that because of the high population density in their high-density built-up areas, compact developments with urban cores reduce travel, and indirectly lessen CO2 emissions (complete mediation by travel time). In contrast, despite no effect on travel, gross population density in the entire city is found to have a direct effect in reducing per-capita emissions based partially on the economies of scale. The proportion of the low-density built-up area in the city also directly reduces the emissions, which might reinforce the concept of the paradox of intensification. Among the control variables, household size, fuel price, and congestion level may ultimately lead to less CO2 emissions because their direct effects, which are positive on travel time, would be overwhelmed by the negative effects on CO2 emissions.