Pavement Concepts > International Roughness Index

Riding quality as measured in terms of the International Roughness Index (IRI), was developed at the International Road Roughness Experiment held in Brazil in 1982 under sponsorship of the World Bank (Ref.2). The IRI summarizes the longitudinal surface profile in the wheel path and is computed from surface elevation data collected either by a topographic survey or a mechanical profilometer. It is defined by the average rectified slope (ARS), which is a ratio of the accumulated suspension motion to the distance traveled, obtained from a mathematical model of a standard quarter car traversing a measured profile at a speed of 80 km/h. It is expressed in units of meter per kilometer (m/km). Generally, the initial IRI directly after construction, ranges from 1.2 m/km to 1.9 m/km and with structural damage, such as faulting, shattered and cracked areas, IRI may increase to an unacceptable 3.0 m/km.

IRI depends on % shattered concrete and % faulting. As such, it has an influence on road safety and should be taken into consideration in the final design.