Until fairly recently, the main driver of growth of travel demand in developed economies was growth of personal incomes. This is no longer the case, a key change that is an important contributor to the Peak Car phenomenon. For the future, demographic change is the main driver of demand growth. There are four aspects: population growth, population ageing, the young deferring maturity, and the spatial consequences of these developments.
I have a chapter summarising changing demographics in a new Handbook on Transport and Urban Planning in the Developed World. A copy of the manuscript is available Changing Demographics chap 23-1-15