Long term explorations (15 to 40 years ahead) of the future development in travel demand are fraught with uncertainties. In particular, in long term policy development it is essential to use comprehensive and consistent scenarios and to incorporate the essential factors, constraints and feedback mechanisms in the analysis. It is on the long term that complexity arises because many of the usual assumptions on steady conditions on the short term are not valid. Moreover, long term developments in society which determine travel demand are hard to predict. It is no surprise that there is little consensus among the professionals on an adequate concept for the passenger transport system which is valid on the long term.