A theory-driven intervention was carried out to initiate the try-out of a new travel mode behavior (try out taking public transport instead of car) in a sample of habitual car users (N = 297). A modified moral decision making model based on the theory of Schwartz and Howard (1981) served as background for the design of an intervention combining a habit-defrosting technique (temporary gift of a free ticket) with a norm-focused technique (plea for commitment). The sample consisted of 297 German citizens with good car access and living in areas with a convenient supply of public transportation. Participants were randomly assigned to four groups (commitment preceded by a free ticket, commitment only, free ticket only, and control) and had to report their travel mode choice for a particular, regular trip (e.g. trip to work) for a period of 8 weeks and a 2-week follow-up period. Additionally, model variables (personal norm to reduce car use, social norm, perceived behavioral costs, and habit) were recorded every 2 weeks. Although overall effects of the interventions were small, results indicate that a moral motivation is a relevant predictor for travel mode choices and can be stabilised by a commitment intervention preceded by a temporary change of the situation.