Abstract
A widely held belief in transport planning is that there is an automatically expanding mobility, based particularly on the increasing use of private motor car use. But the "law" of constant travel time rates states that the average number of daily trips per person and the time budget allocated to transport show stability. This law has raised a number of objections, notably that there has been no satisfactory explanantion of it offered- it is merely an incomprehensible black box. This paper aims to rectify that.