Both the rating and index determines the pavement condition in relation to the vehicle as it drives over it. These calculations determine how much fuel is used and how much maintenance is put into the road and vehicle.
The International Roughness Index (IRI) measurement is expressed in kilometers. This measurement determines the slope, suspension motion of the vehicle and distance traveled.
The Present Serviceability Index (PSI) determines the variances of slopes and cracks, and determines whether the surface is acceptable. The performance of the pavement is determined by a group of researchers and analysts who physically drive the road themselves.
