1. Faculty of Physics, Warsaw University of Technology, Koszykowa 75, PL-00-662 Warsaw, Poland
2. Institute for Economics and Traffic, Dresden University of Technology, Andreas-Schubert-Straβe 23, D-01062 Dresden, Germany
3. Department of Sociology, Columbia University, 413 Fayerweather Hall, 1180 Amsterdam Ave., Mail Code 2551, New York, NY 10027, USA