Built Environment and Taxi Commuting Carbon in Shanghai: Volume, Not Intensity, and a Definitional-Coupling Trap for Low-Carbon Urban Planning

Decarbonising urban passenger transport is central to China’s carbon-peaking commitments and to sustainable cities, yet through which channel the built environment shapes travel carbon remains contested. This study applies a geocomputational framework—from trajectory-based emission accounting to counterfactual optimisation—to morning- and evening-peak taxi commuting carbon across 1494 grid cells and 29 built-environment indicators in central Shanghai. Because the taxi fleet was predominantly new-energy (electric), the outcomes are internal-combustion-equivalent carbon; taxi travel is a self-selected minority of trips, so the associations show where taxi commuting occurs, not a mode-independent effect. Higher density is associated with higher total commuting emissions predominantly through more trips, not higher per-trip carbon. Distance to the second-nearest centre and public-service point-of-interest density are the leading predictors, and empty cruising—about half of fleet carbon—co-locates with commuting demand. The framework diagnoses a definitional-coupling trap in accessibility-based optimisation, where an objective built from the decision variables mechanically registers a trade-off, and supplies a construction audit. Associations are predictive, not causal. For low-carbon urban planning, the results point to trip generation and deadheading, rather than per-trip intensity, as the channels to act on; we recommend treating essential-service accessibility as a minimum requirement rather than a tradable objective.