We would like to acknowledge the contributions of K. Axhausen, D. Cavens, N. Cetin, C. Gloor, L. Mauron, and M. Vrtic. Our computational system is excellently maintained by the ``Informatics support group'' of the Department of Computer Science, in particular by M. Schmitt, for which we are particularly grateful. We also thank the Swiss federal administration which provided the real-world input data.