Non-uniqueness of Single-number Indicators of Longitudinal Road Profiles

Currently used single-number indicators (SNIs) of longitudinal road unevenness do not provide information on the frequency content of the road profile. The frequency content can be defined in terms of a power spectral density (PSD) measure, waviness, and is important from the viewpoint of vehicle vibration response. In this paper, the problems associated with the non-uniqueness of single-number indicators are discussed, quantitative measures of waviness are proposed and corresponding values are estimated for some common SNIs. The methods for providing waviness information to complement particular SNIs are presented and verified with three real road profile records. Being respectful of the weakness of single-number indicators, only the standard unevenness index, C and a modified International Roughness Index (IRI120) for v = 120 km/h can conditionally be admitted, Generally speaking, to compensate for the non-uniqueness of single-number indicators, the addition of a waviness value is recommended for all indicators considered and suitable calculation methods are proposed.