Flexible pavement performance, as expressed by the present serviceability index (PSI) is related to local factors, such as surface course thickness, base course thickness, subbase course thickness, axle load and number of load applications. On the premise that functional pavement design should recognize the relationship between these factors and performance, data from 134 AASHO road test flexible pavement sections were examined for behavioral trends. This resulted in a mathematical expression on which it is theorized that performance, expressed in terms of PSI, can be incorporated into design procedures.