Mobile map apps such as the Baidu Maps app have become a ubiquitous and essential tool for users to find optimal routes and get turn-by-turn navigation services while driving. However, interacting with such apps while driving through visual-manual interaction modality inevitably causes driver distraction, due to the highly conspicuous nature of the time-sharing, multi-tasking behavior of the driver. In this paper, we present our efforts and findings of a 4-year longitudinal study on designing and implementing DuIVA, which is an intelligent voice assistant (IVA) embedded in the Baidu Maps app for hands-free, eyes-free human-to-app interaction in a fully voice-controlled manner. Specifically, DuIVA is designed to enable users to control the functionalities of Baidu Maps (e.g., navigation and location search) through voice interaction, rather than visual-manual interaction, which minimizes driver distraction and promotes safe driving by allowing the driver to keep "eyes on the road and hands on the wheel'' while interacting with the Baidu Maps app. DuIVA has already been deployed in production at Baidu Maps since November 2017, which facilitates a better interaction modality with the Baidu Maps app and improves the accessibility and usability of the app by providing users with in-app voice activation, natural language queries, and multi-round dialogue. As of December 31, 2021, over 530 million users have used DuIVA, which demonstrates that DuIVA is an industrial-grade and production-proven solution for in-app intelligent voice assistants.