Recent surveys of the slugging population in the Washington DC area and the casual carpool population in the San Francisco Bay area suggest that drivers and passengers choose to share rides for very different reasons. For drivers, the largest benefit from picking up passengers is the travel time savings from the use of the HOV lanes. For passengers, it appears that the motivations to share rides are more diverse, with cost savings and travel time savings remaining the most important factors, but flexibility and environmental benefits ranking strongly as well. This finding is important as groups seek to recruit additional drivers and passengers in rideshare arrangements.