1. Taede Tillema is in the Department of Planning, Faculty of Spatial Sciences, University of Groningen, P. O. Box 800, Groningen, 9700 AV, The Netherlands,
2. Bert van Wee is in the Transport Policy and Logistics Section, Faculty of Technology, Policy and Management, Delft University of Technology, Delft, The Netherlands, .
3. Dick Ettema is in the Urban and Regional Research Centre, Faculty of Geosciences, Utrecht University, The Netherlands,