In design, there are two primary considerations. The first is to assess the volume of traffic and the second is to determine the design speed of the road.
The volume of traffic will determine the classification of the road, e.g. motorway, dual carriageway, and the overall pavement construction thickness. For design purposes, it is generally considered that the loads imposed by commercial vehicles contribute far more significantly to the structural damage to the pavement than the loads imposed by private ears; and because of this, the design criteria are based upon the number of commercial vehicles which will use the road. There are many different types of commercial vehicle e.g. articulated lorry, refuse lorry, rigid lorry
which each provide different axle loads. In assessing the volume of traffic the different types of commercial vehicles are considered and their overall axle loadings are expressed as an equivalent number of standard axles.