1 Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ 08544, USA. tsearchi@princeton.edu robert.edwards@jrc.it.
2 Joint Research Centre, European Commission, Ispra 21027, Italy. tsearchi@princeton.edu robert.edwards@jrc.it.
3 Joint Research Centre, European Commission, Ispra 21027, Italy.
4 Agricultural Conservation Economics, Laurel, MD 20723, USA.
5 Institute of Transportation Studies, University of California at Davis, Davis, CA 95616, USA.