1. Wuppertaler Stadtwerke Mobilität homepage , http://www.wsw-online.de/mobilitaet/
2. Passenger information services : A Guidebook for Transit Systems, Transit Cooperative Research Program (TCRP) Report 45, Transportation Research Board, National Research Council, National Academy Press, Washington, D. C. 1999.
3. Note on two problems in connection with graphs (spanning tree, shortest path);Dijkstra E. W.;Numerical Mathematics,1959
4. Ortúzar J., Willumsen L. Modeling transport , John Wiley, 1990.