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 in transport planning and policymaking, and forms a foundation for the development of future transport systems. However, existing empirical approaches predominantly treat time as a single, undifferentiated construct and overlook the roles of its three coupled yet conceptually distinct dimensions, namely, age, period, and (birth) cohort (APC), in underpinning such changes. In this research, we present a conceptualisation of travel behaviour within the APC framework. Building upon our conceptualisation, we argue that attempts to investigate these changes must first scrutinise the necessity of adopting an analytical framework designed to simultaneously disentangle APC. We further formulate theory-driven insights for empirical analyses within the APC framework, addressing issues of operationalisation, interpretation of estimates, and methodological limitations. In a case study application, we examine changes in vehicle miles travelled by car in England, from 2002 to 2023, and find distinct patterns in car use across age, period, and cohort. For practice, 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. Grounded in this perspective, we discuss the potential of our research as a foundation for projecting and understanding future landscapes of travel behaviour and associated outcomes of more direct policy relevance.