There is optimism that government use of »big data» will improve the outcomes of public policies. Despite such optimism, however, little evidence exits on the role big data can play in improving government service. In this study, we undertake one of the first quantitative studies showing the potential utility and limitations of »big data-based policymaking.» We explore the recent use of big data in the policymaking of the Seoul Metropolitan Government. In 2013, the government introduced the »Owl Bus» - a late night bus system - whose routes were selected based on government analyses of big data relating to citizens' late night taxi and mobile phone use. The findings suggest that the average number of passengers on the Owl Bus routes was significantly greater than on other daytime bus routes with comparable characteristics. That said, we also present the potential limitations of evidence-based policymaking in general, especially when politics and equity considerations are factored into the policymaking.