Travel Time Budgets are known to be stable over decades. This conjecture is established on the Zahavi's studies and supposes that the mean daily travel time of a city is a constant amount of one hour. This stability suggests a paradoxical management of travel time savings at two levels. First, at urban level, increase in speed are not used to save time on travel but to extend travelled distances. Second, regarding about the individual behaviour of time allocation, the time savings are not spent on other activities. The progressive questioning of this conjecture leads us to a three steps approach. The first part redefines the sense of the Zahavi's proposition and specifies his innovative view of mobility behaviour. In the second part, the international comparison of travel time budgets explores the structuring of the urban space-times. It seems to indicate the reinvestment and a likely over-investment of travel time savings in supplementary travel. It leads to the limit of the urban and transport policy in terms of regulation of mobility and urban sprawl. The third part deals with the expected substitution between travel time and activity time and question their relationships and the way to represent the derived demand of travel. Our analysis of the temporal dimension of the individual mobility is based on the idea that the choice of travel time is both an access cost to activity and an activity in itself. A microeconomic model is proposed to integrate the double role of travel time. And a duration model is estimated for the travel time budgets of Lyon. It reveals the influential variables relative to the individual attributes, it indicates relationships with other activities duration and characterises the temporal dynamics of the travel time process.
