Matthies et al 2006: Commiting to use public transportation increased willingness to try it among German citizens over five months
German citizens with good car access living in areas with convenient public transit were randomized (a) to receive a free public transit ticket (or not) and (b) to receive information about the negative effects of car use on the world’s climate, a list of ten personal commitments to alternatives to driving (e.g., “I commit to change the way I drive to safe fuel during the next two weeks”), only two of which referenced public transit (“I commit to using public transportation at least one/two times during the next two weeks for my regular trip”), from which they selected one or more to endorse (or did not). In the two weeks following the free ticket manipulation, people who received the free ticket were more likely to try public transit. Then the commitment manipulation was delivered. While there was no main effect, and no interaction in the first two weeks following its delivery, in the subsequent two weeks there was an interaction between commitment condition and the degree to which people personally endorsed reducing car use. Among endorsers, those in the commitment condition were more likely to try public transit (36.3% did) than those in the no-commitment condition (7.1%). There was no effect for people without this strong personal norm (commitment: 0%; no-commitment: 7.4%). The same pattern was observed, albeit marginally, in an assessment 20-21 weeks following the commitment manipulation.