The International Roughness Index (IRI) is a worldwide standard for measuring road smoothness longitudinally (i.e. in direction of driving). The index measures pavement roughness in wheel path in terms of the number of metres per kilometre that a laser, mounted in a specialised van, jumps as it is driven along a road. The lower the IRI number at given speed, the smoother the ride felt by road user.
A mathematical model of the dynamic response of a real vehicle traveling along a single wheelpath of longitudinal road profile (referred o as the quarter-car or World Bank model). It is expressed n terms of accumulated displacement of the simulated suspension in metres per kilometre. (m/km) - Glossary of Austroads Terms.