In this paper, we explore using large digital displays in combination with a personal mobile application to publicly and privately encourage people to make healthy choices. We designed, built, and deployed an experimental system called Lunch Line that promoted healthy eating. Lunch Line includes a public display that enables passersby to view the reported eating behavior of a group of people and take on daily "food challenges," and a mobile web application that allows users to record personal food choices, report challenge achievement, and compare their choices with other users and with USDA recommendations. Results from a 3-week field evaluation at a company cafeteria showed that our integrated system was effective in drawing public attention, delivering challenges, enabling self-tracking and self-reflection, and providing feedback on personal and group choices. We share lessons on how to design future systems that integrate situated public displays and personal mobile devices to encourage healthy choices.