Pré-publication, Document de travail: Most seats in private cars are empty when drivers hit the road. Carpooling could thus represent a low-cost strategy to reduce carbon emissions in the transportation sector. Using revealed preference data from actual long-distance carpooling trips in France, we estimate passengers' preferences for the different characteristics of a ride. We find that passengers are highly price-elastic and value significantly the convenience of pick-up and drop-off locations. In contrast, their value of time once in the car is significantly lower than typical reference values. Finally, we discuss the effectiveness of a number of counterfactual policies aimed at promoting carpooling.