Feasibility regimes and the limits of Voluntary Travel Behaviour Change: evidence from a MaaS-enabled university experiment

European metropolitan areas face increasing pressure to reconcile everyday mobility with decarbonisation, congestion mitigation and social equity. In this context, Mobility as a Service (MaaS) and Voluntary Travel Behaviour Change (VTBC) programs are widely promoted as demand-side instruments capable of fostering sustainable modal shifts through information, integration and personalised support. Yet their behavioural effectiveness remains empirically fragile and theoretically under-specified. In particular, existing evaluations largely rely on average treatment effects and attitudinal models, while neglecting the structural constraints and stabilisation mechanisms that govern everyday mobility. This thesis advances a regime- and threshold-based interpretation of voluntary travel behaviour change. It develops an integrated theoretical framework that combines process models of change, intention-formation theories and dual-process accounts of habit, and reconceptualises the intention–action gap as a feasibility problem generated by coordination costs, reliability risks and routine anchoring. Behavioural change is theorised as a discontinuous phenomenon: durable reconfiguration occurs only when interventions enable travellers to cross feasibility thresholds separating bounded internal rebalancing from genuine regime-level transitions. The framework is examined through a staged, longitudinal MaaS-enabled VTBC experiment implemented in a complex metropolitan university context. The empirical strategy integrates revealed and stated preference data, personalised travel planning, differentiated exposure conditions and multi-phase behavioural tracking. Rather than estimating population-average effects, the study adopts a mechanism-oriented evaluation design that distinguishes regime-level change from reactivity, bounded adaptation and engagement filters, and treats selective participation and attrition as analytically meaningful scope conditions. The results demonstrate a systematic dominance of bounded internal rebalancing within stable feasibility regimes. MaaS-enabled choice contexts consistently increase salience and reflective activation and induce limited portfolio reweighting, but fail to trigger durable regime reconfiguration in the absence of substantial feasibility gains. Behavioural stability emerges as an adaptive response to systemic fragility rather than as attitudinal rigidity or resistance. The thesis contributes a regime–threshold theory of VTBC, a feasibility-centred mechanism map linking habits, behavioural control and system architectures, and a governance-oriented reinterpretation of MaaS as a feasibility device rather than a mere informational interface. It concludes that soft behavioural interventions can support sustainable mobility transitions only when embedded in integrated governance arrangements capable of reducing coordination burdens and implementation risks, thereby transforming sustainable options from desirable alternatives into credible and stable everyday practices.