Many people find public Wi-Fi networks convenient but these networks harbor security and privacy risks. As public knowledge of these risks becoming common, we investigated whether the risks were still at large and what factors influenced users to use the networks-being the first study to draw evidence from Japan. Adapting the methodology from a previous study in the UK, we first set up an experimental open public Wi-Fi network at 11 locations in downtown Nara and captured Internet traffic. From approximately 7.7 million packets captured from 196 unique mobile devices during a 150-hour experiment, we found private photos, emails, documents, and login credentials being transmitted in clear text without encryption-confirming that not only did many applications still fail to encrypt data-inmotion but also did many users continue to use unsecured public Wi-Fi networks. We then used a scenario-based survey to examine factors affecting the users' decision to use the networks. From 103 participants, we found that the desire to conserve mobile data-a form of resource preservation heuristic-instigated a risk-taking attitude and influenced participants, especially among those usually having a small monthly data plan, to use unsecured public Wi-Fi networks. Female and those having finish highschool only, were also more likely to use the networks.