This paper presents a critical, theme-based methodological review of (i) the estimation of the wider economic impacts (WEIs) of transport investments arising from agglomeration economies, and (ii) their use as an additive element in cost–benefit analysis (CBA). We examine fundamental methodological refinements that can enhance the robustness of agglomeration impact appraisal. We identify priority areas, including eliminating overlap between WEIs and direct user benefits (DUBs), incorporating amenity externalities, and refining agglomeration measures, often represented via access to economic mass (ATEM). Our analysis highlights a persistent gap between transport appraisal practices and recent advancements in the field of urban and spatial economics, where theoretical insights have yet to be fully operationalised in the mainstream CBA framework. To bridge this gap, we outline key empirical research priorities and propose refinements to WEI estimation methodologies to improve both their empirical validity and practical applicability in transport appraisal.
Anupriya et al. (Wed,) studied this question.