1. Graduate Student Research Assistant, Center for Sustainable Systems, School of Natural Resources and Environment, Univ. of Michigan, 440 Church St., Ann Arbor, MI 48109. E-mail: waicc@umich.edu
2. Codirector, Center for Sustainable Systems, and Associate Professor, School of Natural Resources and Environment, Univ. of Michigan, 440 Church St., Ann Arbor, MI 48109 (corresponding author). E-mail: gregak@umich.edu
3. Economist, Office of Asset Management, Federal Highway Administration, Washington, D.C 20590.