Serviceability is a concept derived during the AASHO Road Test. This concept is related to the primary function of a pavement structure: to provide the traveling public with a smooth, comfortable, and safe ride. A scale ranging from 0 to 5 is used to evaluate a pavement’s present serviceability index (PSI); pavement with a rating of zero is impassible and a rating of 5.0 would be perfectly smooth. Figure 2-12 illustrates the concept of serviceability index.

All pavements, when newly constructed or rehabilitated, are expected to begin at a high level of serviceability with a decrease in serviceability over time and traffic loading as the pavement becomes more distressed and rough.

Typically, the initial serviceability of a pavement will be about 4.0 or higher (5.0 is a lofty, but inaccessible goal, and levels below 2.0 are not considered acceptable). Pavement managers strive to keep the minimum “terminal” serviceability index at a level that will not be indicative of a public safety hazard or discomfort or be excessively costly in rehabilitative effort.

Smooth highways minimize dynamic axle loading; typically, the smoother a pavement begins its life, the longer the time to initiate rehabilitation. The change in serviceability over time is also known as “performance.” General pavement performance relationships are illustrated in 'Pavement Performance Relationships.'