  Experimental studies on vehicular traffic provide data on quantities like
density, flux, and mean speed of the vehicles. However, the diagrams relating
these variables (the fundamental and speed diagrams) show some peculiarities
not yet fully reproduced nor explained by mathematical models. In this paper,
resting on the methods of kinetic theory, we introduce a new traffic model
which takes into account the heterogeneous nature of the flow of vehicles along
a road. In more detail, the model considers traffic as a mixture of two or more
populations of vehicles (e.g., cars and trucks) with different microscopic
characteristics, in particular different lengths and/or maximum speeds. With
this approach we gain some insights into the scattering of the data in the
regime of congested traffic clearly shown by actual measurements.
