[1] Univ Libre Bruxelles, Fac Sci, DGES IGEAT, Ave FD Roosevelt,50,CP 130-03, B-1050 Brussels, Belgium
[2] Tel Aviv Univ, Transport Res Unit, Dept Geog & Human Environm, POB 39040, IL-69978 Tel Aviv, Israel
[3] Univ Northern Colorado, Dept Geog & GIS, Candelaria 2200,Campus Box 115, Greeley, CO 80639 USA