Humans can be deeply influenced by affective behaviors during social interaction. Specifically, emotional cues from others can be a powerful way to persuade people to modify their behaviors. With this motivation in mind, we explore how a social robot called AIDA (Affective Intelligent Driving Agent) can better persuade drivers to adhere to road safety guidelines as compared to existing technologies, and AIDA's persuasiveness as compared to a human passenger. An Android smartphone, which mounts in the robot's head, serves as AIDA's main computational unit. Because the smartphone contains personal information about the driver (e.g. contacts, calendar and music preferences), leveraging this device can create a more deeply personalized experience. We conducted a user study in which participants completed tasks in a driving simulator with the help of: 1) a smartphone alone, 2) AIDA as a static-mounted agent, 3) AIDA as an expressive robot, or 4) a smartphone plus a human passenger. AIDA was able to promote safe driving behaviors and reduce cognitive load better than a smartphone alone. Overall, the AIDA robot paralleled in performance as compared to the human passenger. The AIDA robot also facilitated more sociability with the driver than the smartphone or static agent. Further, AIDA's proactive launching of the driver's favorite music better promoted overall enjoyment.