Be careful when using the International Roughness Index as an indicator of road unevenness

Currently, from more road unevenness single-number indicators, the International Roughness Index (IRI) is the most popular. However, some roads with irregular forms of unevenness power spectral density (PSD) require analytical description with a number of independent parameters. In such cases the IRI cannot be used to distinguish between road profiles of distinctly different features. To indicate possible erroneous assessments of road unevenness using the IRI only, a simulation study was conducted in which seven different homogeneous road profiles were considered with waviness values in the range from 1.5 to 3.0, and 0.1-variance component of single harmonic undulation with most dangerous wavelengths, all having the same nominal value IRI=2.21mm/m. Values of the comparison single-number indicator, the standard deviation of elevation, were found in the range from 1.48 to 7.065 mm. The effect of selected road profiles on two vehicles, a passenger car and a truck, and on people sitting in the vehicles was also studied yielding even more dissimilar results.