ON THE CALCULATION OF INTERNATIONAL ROUGHNESS INDEX FROM LONGITUDINAL ROAD PROFILE

The international roughness index (IRI) was established in 1986 by the World Bank and based on earlier work performed for NCHRP. IRI is calculated from a measured longitudinal road profile by accumulating the output from a quarter-car model and dividing by the profile length to yield a summary roughness index with units of slope. Although IRI is used widely, there is no single, short reference document that describes what it is and how it is calculated. Instead, the critical information is spread over several large reports. A short, self-contained reference that defines IRI is provided, along with all the information needed to compute it from longitudinal road profile measurements. The development of the IRI is reviewed, the mathematical definition is presented, an algorithm for calculating IRI is derived, the performance of the algorithm is analyzed, tested Fortran source code for computing IRI is presented, and problems with IRI (and profile measurement in general) that have emerged since 1986 are identified.