We have a personal relationship with mobile phones, since they are closer to us than any other technological device. They are ubiquitous (60% of the world population owns one), individual, and pervasive in our lifestyle (we have them with us all the time and everywhere). These modern devices are nearly as powerful as personal computers, always connected to the Internet, and loaded with sensors like GPS and accelerometers. These mobile devices offer opportunities for persuading users to change attitudes and behaviors towards personal and societal issues. For this paper we focus on the design of a mobile application for reducing in carbon dioxide emissions in personal transportation choices that can change attitudes and behaviors. The paper presents design practices that have resulted in a prototype social mobile application.