Background Encouraging alternatives to the car such aswalking, cycling or public transport is a key cross-sector policy priority to promote population and planetary health. Individual travel choices are shaped by individual and environmental contexts, and changes in these contexts – triggered by a stimulus – can translate to changes in travel mode. Understanding how and why these changes happen can help uncover more generalisable findings to inform future intervention research. We aimed to identify common mechanisms underlying travel behaviour change, and identify the contexts in which these occur.

Methods We conducted a prospective longitudinal qualitative cohort study utilising semi-structured interviews, nested within a randomised controlled trial (RCT) of financial incentives to promote alternative travel modes to the car. Participants were residents in a new town in Cambridgeshire, UK, where design principles to promote walking, cycling and public transport were used at the planning stage. We purposively sampled RCT participants according to baseline travel patterns, car ownership and intervention group allocation and invited them to participate via email. At baseline (in 2021), three- and six-month follow up interviews, we followed a topic guide asking participants about previous and current travel patterns and future intentions. All interviews were audio recorded and transcribed. Data analysis used the framework approach based on realist evaluation principles identifying the context and mechanisms described by participants as leading to travel behaviour change. Data was analysed in NVivo 12.

Results We conducted 42 interviews with 16 participants. At baseline 63% of participants had lived in the new town for less than 12 months; 75% lived in privately owned accommodation and 50% travelled to work by car frequently. Participants reported six common mechanisms for changes in travel mode. These entailed increasing or reducing access, reliability and financial cost, improving convenience and experience and raising awareness. Participants described that these led to changes in travel mode in contexts where their existing travel mode had been disrupted, particularly in terms of reducing access or reliability or increasing cost, and where there were suitable alternative travel modes for their journey.

Conclusion Applying realist evaluation principles to identify common mechanisms underlying changes in travel mode has the potential to inform future intervention strategies. Future interventions using mechanisms that reduce access to, reduce reliability of, or increase the financial cost of car use may facilitate modal shift to walking, cycling and public transport when implemented in contexts where alternative travel modes are available and acceptable.