1. Department of Economics, University of North Carolina at Greensboro Bryan School of Business and Economics, PO Box 26165, Greensboro, NC 27402-6165, and National Bureau of Economic Research (NBER).
2. Institute of Transportation Studies, University of California, Davis, One Shields Avenue, Davis, CA 95616.
3. Department of Economics, University of California, Davis, One Shields Avenue, Davis, CA 95616 and Institute of Transportation Studies, University of California, Davis, University of California Energy Institute, and NBER.