In the context of a 2-wave panel study, we used Ajzen's (1991) theory of planned behavior (TPB) as the theoretical framework for deriving and systematically testing hypotheses as to how an intervention (a “free” ticket for public transportation) influences the travelmode choice of students. The empirical results show that this intervention caused a drastic decrease in students' car use. The effect of the intervention on behavior is mediated by the causal chain postulated by the TPB. In the second step, we analyzed whether there were subgroup-specific reactions to the intervention. Surprisingly, the subgroup analysis shows that students with more negative attitudes toward policy measures restricting car use reacted more strongly to the intervention than did students with a more positive attitude.