Net Carbon Reduction of Free-Floating Bike Sharing for Sustainable Urban Mobility: A Distance-Based Modal Substitution and Rebalancing Framework

Bike-sharing systems are widely promoted as a low-carbon transport option for sustainable urban mobility, but their environmental benefits depend on the travel modes they replace and the emissions generated by system operations. This study develops a net operational carbon-reduction framework for free-floating bike sharing (FFBS) that jointly considers distance-based modal substitution and bicycle rebalancing. Distance tolerance and the fuzzy maximum-membership principle are used to identify plausible modal substitutions, while traveller characteristics and surrounding points of interest (POIs) are incorporated to refine substitution shares. The framework is applied to large-scale bike-sharing trip data from Nanjing, China, for October 2019, February–March 2020, and June 2020. Under the baseline accounting assumptions, the system was estimated to achieve positive net operational greenhouse-gas (GHG) reductions in all three periods, with most riding-stage benefits retained after accounting for rebalancing emissions. Carbon-reduction density was higher in June 2020 than in October 2019 in five of the six analysed districts, while changes in demand density varied across districts. The period differences are interpreted alongside seasonal and pandemic-related changes in travel conditions. These findings highlight the importance of jointly considering modal substitution, operational emissions, and spatial heterogeneity when evaluating the environmental performance of bike-sharing systems. The proposed framework can support district-level assessment of operational carbon performance, inform transport decarbonisation and sustainable urban mobility planning, and identify the data that would be most beneficial to such an assessment.