Americans spend trillions of minutes in cars annually, tens of billions of minutes traveling to health care providers, and hundreds of millions of minutes ridesharing (e.g., with Uber or Lyft). From July to October 2017, we recruited rideshare users (e.g., Uber or Lyft) to participate in a survey about health messaging during rides. Responses were collected anonymously on tablet devices. We interviewed 170 ridesharers and assessed their interest in health messaging delivered during rides. Participants ranged from 19 to 79 years of age, and most (87%) reported using their smartphones to search for health information. More than 70% expressed interest in health messaging during rides, and 55% of current smokers expressed interest in quit-smoking messaging. The most popular of suggested health topics included healthy eating (61.8%), exercise (60.6%), and weight loss (40.0%), and the preferred message formats were video (33.5%), smartphone apps (33.5%), and online social network (25.3%). Free time spent riding in cars represents an enormous untapped resource, and our findings suggest that riders are receptive to the idea of health messaging delivered during rideshare trips.