The present study tested an extended theory of planned behaviour (TPB) model within the domain of transport mode choice and identified the most important factors impacting on whether participants drove or used public transport to commute to work. Structural equation modelling of data from 827 participants showed that car use was determined by intention and habit but not perceived behavioural control (PBC), whereas public transport use was influenced solely by intention. The analysis also revealed that TPB variables (attitude, subjective norm and PBC) influenced use of both transport modes indirectly through their effects on intention and habit. In contrast, the incremental validity of variables not contained in the model (moral norm, descriptive norm and environmental concern) was mixed and varied according to transport mode. Theoretical and applied implications of the findings are discussed.