TAG Unit A1.1: Cost–Benefit Analysis (2025) provides methodological guidance for evaluating transport interventions using social cost–benefit analysis. It explains how analysts should identify, quantify, and monetise impacts such as travel time savings, vehicle operating costs, safety improvements, environmental effects, and wider economic impacts. The guidance emphasises comparing a “do-minimum” baseline with alternative project options to determine net benefits to society. Future costs and benefits are discounted to present values using prescribed social discount rates, enabling calculation of indicators such as Net Present Value (NPV) and Benefit–Cost Ratio (BCR). The framework promotes consistent appraisal across transport schemes while incorporating uncertainty and risk analysis. It also highlights distributional impacts and the importance of transparency in assumptions, data, and modelling. The methodology supports evidence-based decision-making and prioritisation of transport investments that deliver the greatest economic and social value.
