Decarbonizing public transit fleets: A Review of Emerging Technologies, Greenhouse Gas (GHG) Emissions Accounting, and Policy

Public transit agencies are decarbonizing bus fleets through efficiency upgrades, alternative fuels, and zero-emission buses, while also pursuing ridership gains that shift travel from private automobiles to transit. This scoping review synthesizes peer-reviewed studies and agency reports to compare propulsion pathways across advanced diesel and hybrids, compressed/renewable natural gas, battery-electric buses, and hydrogen fuel cell buses, through the lenses of life-cycle greenhouse gas emissions, total cost of ownership, and operational constraints. We also review emissions estimation approaches used in planning and reporting, from factor-based calculators to route-level simulation and machine-learning models, and summarize policy and implementation strategies that accelerate adoption while safeguarding equity. Across the literature, reported emissions and cost outcomes are highly sensitive to local inputs and assumptions, particularly electricity and hydrogen pathways, duty cycles and climate, charging strategy, utilization, and battery manufacturing and replacement.