Background: The over representation of youth in road crash injury and fatality rates is a major public health problem globally. In New Zealand, youth drivers are most vulnerable in the restricted license period, when they can drive without the requirement for supervision by an experienced adult. Behavioral change interventions delivered using smartphone technology to young drivers could serve as a useful feedback mechanism for safe driving in this context, but this potential remains to be fully explored. Objective: We have applied behavioral change principles to design and develop a smartphone based intervention with the aim of helping youth drivers develop and hone safe driving skills. Methods: An iterative process was used to support development of the smartphone intervention. We reviewed behavioral change literature, identifying fundamental principles and exploring use of behavioral change techniques in other areas of public health. We engaged with key stakeholders, including young drivers, government agencies and relevant organizations. We also took into account technology adoption considerations when designing the application. Results: We developed BackPocketDriver, an Android smartphone app that uses inbuilt sensors to monitor and infer driver behavior. The application implements features that were identified during the design process and which are traceable to behavioral change techniques. Features identified include messaging to enhance motivation and driver education, journey feedback summaries, goal setting, social support and gamification elements (achievements and leaderboards). Conclusions: BackPocketDriver's design rests on a sound foundation -behavioral change theory. With explicit links between the theory and app features, the app aims to be an effective intervention to change and improve youth driver behavior. The next phase of this work is to run a small pilot study to assess BackPocketDriver's effectiveness.