Pressures to cut public expenditure and to reach high value for money of projects that use scarce public money are evident across the globe. At the same time there seems to be a lack of decision support tools for pin-pointing whether public services are yielding net benefits. Accountability is called for but the ‘accounting systems’ that validate the right choices in service delivery are not yet thoroughly established. As a response, an impact evaluation via a real-world case study of a multi-modal transport safety agency is presented. The main contribution of this article is methodological, including a summary of study cordoning; description of methods to map impact mechanisms; quantification of socio-economic impacts of services; the benefit to cost (B/C) appraisal of services and service bundles, and evaluation of an agency's overall B/C ratio by applying the findings to systems level. The described analytical process is repeatable elsewhere with modifications or as it stands.