False Friends of Sustainable Mobility: A Fork-to-Wheel Method for Human Metabolic Energy in the Life-Cycle Carbon Footprint of Passenger Transport in Italy

Transport is among the hardest sectors to decarbonize, yet life-cycle carbon-footprint (CF) assessments of mobility systematically omit the human metabolic energy expended while traveling. This study introduces a Fork-to-Wheel system boundary that incorporates metabolic energy expenditure into the unit CF (gCO2eq/p-km) of passenger transport modes, alongside vehicle production–maintenance and Well-to-Wheel energy, computed from mode-specific Metabolic Equivalent of Task values and diet-weighted carbon intensities for Italy. A Monte Carlo sensitivity analysis tests the robustness of the modal ranking. The results show that the metabolic component contributes only 1.1–2.3% of the total unit CF for private cars (89.0–188.4 gCO2eq/p-km), but 20.5–78.8% for micromobility and active modes, reaching 78.8% for walking and 77.1% for cycling. Consequently, walking (43.6 gCO2eq/p-km) exceeds both e-bikes (31.8) and e-scooters (26.4), while cycling (21.9) is of the same order as e-scooters, despite both being regarded as zero-emission. The ranking of walking against micromobility modes remains robust across all of the tested scenarios, and a break-even dietary intensity is derived for each pairwise comparison to support transfer to other contexts. Framed as the “false friends of sustainable mobility”, these results show that neglecting biological energy flows can distort carbon-based rankings. Carbon intensity alone, however, does not determine the sustainability of modal shifts: once mode-specific trip lengths are applied, the per-trip ordering differs from the per-kilometer one, and health, safety, and congestion effects lie outside the impact category assessed here.