3.5.1 Serviceability
The AASHTO design equations are developed around the concept of serviceability,
which serves as the pavement performance parameter by which a pavement’s condition
is valued. Present serviceability is defined as the momentary ability of a pavement to
serve traffic. The present serviceability rating (PSR) was developed to measure serviceability.
PSR is a rating of pavement ride based on a scale of 0, for impassible, to 5, for
perfect. For the development of the original AASHO equation, individuals (the raters)
236 CHAPTER THREE
would ride the pavements and assign a PSR value. To avoid riding and rating every
pavement to determine serviceability, a relationship is usually developed between PSR
and measurable pavement attributes. The value determined by this relationship is called
the present serviceability index (PSI). At the AASHO Road Test, the PSI was derived to
be related to slope variance, cracking, and patching for concrete pavements, and to slope
variance, rutting, cracking, and patching for asphalt pavements. The relationship
between pavement thickness and serviceability index is defined by the AASHTO pavement
design equations