Samenvatting
In the past several studies were devoted to behavioural constants such as the number of trips, constancy of travel time and travel costs and stable relation between travel budgets, travel speed, car ownership and spatial structure. In this paper another kind of behavioural constant is discussed. This concerns the constant ratio between travel time and activity duration by activity type, which has hardly received any attention till so far. One of the most important conclusions is that constant ratio's between travel time and activity duration exist. Some possible policy consequences of this empirical finding are discussed.