1. Center of Urban and Regional Studies, IST, Technical University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal.
2. Institute of Transportation Studies, University of California, Irvine, 522 Social Science Tower, Irvine, CA 92697-3600.
3. Department of Geography, University of California at Santa Barbara, 3611 Ellison Hall, Santa Barbara, CA 93106.