1. Research Associate, Dept. of Engineering and Public Policy, Carnegie Mellon Univ., 5000 Forbes Ave., Pittsburgh, PA 15213-3890.
2. Professor, Dept. of Civil and Environmental Engineering and Dept. of Engineering and Public Policy, Carnegie Mellon Univ., 5000 Forbes Ave., Pittsburgh, PA 15213-3890.
3. Chief Economist, U.S. General Accounting Office, 441 G St., N.W., Washington, DC 20548.
4. Professor, Dept. of Engineering and Public Policy, Carnegie Mellon Univ., 5000 Forbes Ave., Pittsburgh, PA 15213-3890.