Inducing modal shifts away from car-oriented travel patterns is crucial to the development of a sustainable transport system. Multimodal travel behaviour, which is also termed as multimodality, refers to the behavioural phenomenon of using more than one mode of transport during a given period. Evidence suggests that encouraging individual multimodality may potentially constitute a first step to sustainable mode use change. Understanding correlates of multimodality provides more insights into how multimodality is distributed, how modifiable factors may influence multimodality, and based on these insights, how to support policies to promote multimodality. Recently, increasing scientific attention has been paid to correlates of multimodality. Nevertheless, little is known about the extent to which multimodality, especially the level of multimodality, is correlated with variables beyond socioeconomic, demographic, and residential dimensions. Indicated by studies on the use of single modes, it could be hypothesised that variables in several other dimensions may be linked with multimodality.
This thesis aims at extending the conceptual framework used to analyse correlates of multimodality by exploring variables in the temporal, situational, and attitudinal dimensions. Three elements around each of these dimensions, namely, age-period-cohort, trip purposes, and attitudes, are specifically focused.
This thesis used nationwide multiday travel diary surveys from England and the Netherlands. The results showed that age-period-cohort and trip purposes were significantly associated with multimodality. By contrast, mode-specific attitudes may not necessarily be influential for corresponding mode use decisions when multimodality is involved.
Chapter 2 showed that individuals tended to be less multimodal as they got older. The results do not support the view held by most existing studies, namely, that multimodality has increased in recent decades. Instead, multimodality presented a downward trend for recent (birth) cohorts. The existence of significant cohort-specific variations in multimodality also indicates the important role of early life conditions and formative experience in shaping multimodality.
Chapter 3 found that individuals presented higher levels of multimodality when they made trips that were more variable in departure time and travel distance, but only when sufficient trip stages (at least 3 stages) were made. Moreover, there were cross-purpose disparities in correlates of multimodality in terms of significance and variance explained.
Chapter 4 suggested that for mode use decisions of multimodal travel behaviour, attitudes may not be as influential as they affect decisions of single mode use. Moreover, the results support the view that multimodal travellers may have a high potential for a long-term modal shift due to high-level cognitive dissonance. This may also provide explanations of the psychological mechanism by which multimodal travellers tend to change their mode use over time.
As a whole, this thesis reveals the multifaceted nature of correlates of multimodality. The novel approaches, namely the hierarchical age-period-cohort model and Heckman selection model used in this thesis provided new tools for understanding multimodality. The main implication of this thesis is that policymakers need to take into account the complexity of correlates of multimodality to develop policies aimed at encouraging multimodality and to develop effective mode-shift interventions.
