This article presents an interdisciplinary methodology that provides decision makers with key figures on the reliability of ageing structures in transport infrastructures. The methodology is closely connected to ongoing discussions about proactive and sustainable maintenance strategies under increasing economic and ecological pressures. Since the infrastructures of the main modes of transport in Germany, that is, roads, railways and waterways, pose similar challenges to the responsible governmental authorities and administrations, the development of the methodology presented in this article is embedded in the research programme 'BMVI Network of Experts'. Under the premises of the research programme, the methodology provides a step-by-step approach to creating a profound knowledge base which is required for the reliability assessment of ageing infrastructures. The methodology aims at generating meaningful key figures about the structural condition of an assessed asset for a risk-informed maintenance strategy. The examples of a condition assessment procedure based on a fuzzy criticality recommendation and a screening tool for ranking a large number of structures using operative reliability indices illustrate how the interdisciplinary methodology allows the development of user-friendly tools to support and facilitate the management of infrastructure maintenance in the responsible governmental entities.