There is an overreliance on personal vehicle travel for short trips in the United States. This paper describes a mobile application, called EcoTrips, that promotes walking and biking for short trips by tracking users' travel behavior and providing a variety of feedback. EcoTrips conveys environmental impacts of travel behavior, and also leverages a variety of co-benefits of green travel by providing feedback related to fitness, finances, and time management. Feedback is conveyed in conventional as well as metaphorical metrics to make the data more comprehensible and meaningful. EcoTrips is unique among similar apps in that it provides a very high degree of flexibility for users to tailor the metrics that are displayed. It is also unique in that it is both the subject of HCI research and publicly available in English and in the United States. In a small pilot field study, participants reported some increase in awareness of the impacts of their travel behavior, and some small changes in their travel mode choices. Participants also highlighted the importance of maximizing the personalization of data. Given the expanding landscape and prevalence of pervasive and personal mobile and wearable technologies, HCI researchers should continue to develop eco-feedback and trip planning technologies to promote green travel modes for short trips.