Impact assessment methods for environmental policies in freight transport: an integrated process model

Freight transport is a major source of greenhouse gas emissions and other externalities, creating a growing need for robust Policy Impact Assessments (PIAs) to evaluate environmental measures in this sector. Yet existing PIAs are highly fragmented: most analyze specific instruments, modes, or isolated tools and rarely offer a process-oriented perspective that consistently links impact indicators, methodological categories, and assessed impacts. This paper addresses this gap through a systematic review of 60 peer-reviewed PIAs in freight transport (2000–2024), identified through a multi-database search and structured screening protocol. The analysis distinguishes 34 impact indicators (inputs) and 21 assessed impacts (outputs) and classifies PIA approaches into nine methodological families: cost–benefit analysis, operations research, computable general equilibrium models, systemic simulation, freight demand and network models, decision-support and multi-criteria methods, discrete choice models, agent-based models, and non-modelling approaches. Building on these findings, the paper develops an integrated PIA process model comprising four iterative phases: scoping and design, indicator and method selection, assessment execution, and impact interpretation and synthesis. The review reveals a dominance of ex-ante freight demand forecasting studies, a strong policy focus on modal shift in Europe and China, and persistent blind spots concerning social impacts, infrastructure capacity and resilience, and longitudinal evaluation. The integrated process model offers a coherent process logic that connects PIA elements systematically, thereby improving consistency and comparability across studies and providing researchers and policymakers with a practical structure for conducting transparent evaluations of environmental freight transport policies.