The concept of high-occupancy vehicle (HOV) has long been an attraction for policymakers wanting to minimize greenhouse gas emissions (cf. the Intermodal Surface Transportation Efficiency Act, 1991 (Jaskevich 2001)). Since carpooling reduces the number of single-occupancy vehicles (SOV) on the road, research is needed to identify approaches that would be useful in increasing carpools (Burbank and Brinckerhoff (2009)).