Abstract
This paper covers, among various possible definitions of volume and density, those which relate these characteristics to average speeds of vehicles. Also discussed are ways of obtaining expressions which are differentiable functions of time and space. Such an approach leads, on a rigorous theoretical basis, to results of the kind represented in the model by PROGOGINI and, on a practical level, to approximations satisfying the different properties of volume and density. The choice of these approximations presents several problems.