Roads can be categorized in many different ways. In relationship to Fig. 3.2, a classification may be based on road design (construction), use (traffic characteristics), or function. For planning purposes, a classification by function is most useful. This is elaborated in the next sections, for both industrialized and developing countries. Classifications by road design are commonly based on pavement width and type. The distinction "paved/unpaved" is the simplest classification. Road surfaces can be paved with seales or oil-based materials as well as asphalt or concrete. Unpaved roads are characterized as earth, sand, or gravel. Traffic characteristics embody features such as traffic volume and composition by mode. The narrow relationships between the three classifications are self-evident. The more important the function of the road, the higher the volumes, the bigger the construction of the road, and the wider the pavement. Table 3.1 gives an idea of such relationships.
Divisions into categories have been developed with the intention of forcing the adjustment of the driving behavior to conditions of road and traffic. To enable road users to do this, differences among the categories should be clear to them. The clearest road feature is the width of the pavement. Therefore, in each country, standard sizes are handled and recommended.
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