An empirical approach is one which is based on the results of experiments or experience. Generally, it requires a number of observations to be made in order to ascertain the relationships between input variables and outcomes. It is not necessary to firmly establish the scientific basis for the relationships between variables and outcomes as long as the limitations with such an approach are recognized. Specifically, it is not prudent to use empirically derived relationships to describe phenomena that occur outside the range of the original data used to develop the relationship. In some cases, it is much more expedient to rely on experience than to quantify the exact cause and effect of certain phenomena.
Many pavement design procedures use an empirical approach. This means that the relationship between design inputs (e.g., loads, materials, layer configurations and environment) and pavement failure were arrived at through experience, experimentation or a combination of both. Empirical design methods can range from extremely simple to quite complex. The simplest approaches specify pavement structural designs based on what has worked in the past. For example, local governments often specify city streets to be designed using a given cross section (e.g., 100 mm (4 inches) of HMA over 150 mm (6 inches) of crushed stone) because they have found that this cross section has produced adequate pavements in the past. More complex approaches are usually based on empirical equations derived from experimentation. Some of this experimentation can be quite elaborate. For example, the empirical equations used in the 1993 AASHTO Guide are largely a result of the original AASHO Road Test.