The age-period-cohort trinity and a demographic framing of travel behaviour change

Understanding population-level long-term changes in travel behaviour is vital for assessments of transport planning and policies and for informing the development of future transport systems. In this research, we present a conceptualisation of travel behaviour within the age–period–(birth) cohort (APC) framework. Building upon this conceptualisation, we argue that any attempts to investigate these changes must first scrutinise the necessity of adopting such an analytical framework designed to simultaneously disentangle these three intertwined yet distinct time-related dimensions. We further formulate theory-driven insights for travel behaviour analysis within such a framework, addressing issues of operationalisation, interpretation, and reflection on its limitations. Using a case study application, we examine changes in vehicle miles travelled (VMT) by car in England, from 2002 to 2023. Our theoretical insights and empirical evidence call for a reorientation of transport planning philosophies that recognise and address demographic dynamics (i.e. shifts in age-cohort composition), cohort succession in particular, as an independent, active driver of change. We discuss the capacity of our research, situated within this recognition, to provide a foundation for projecting and understanding future landscapes of travel behaviour and related outcomes of more direct policy relevance.