Public roads are the principle source of excitation to ground vehicles. Objective roughness/severity classification of the public roads is essential to predict vehicle response/fatigue damage at an early stage of vehicle development program. It is observedthat IRI (International Roughness Index) and PSD (Power Spectral Density) classifications as per ISO 8608 are the most widely used roughness indicators worldwide.This paper describes the work carried out to study the usefulness of IRI (International Roughness Index) for a vehicle designer and test engineer. The intention is to demonstrate the link between IRI and vehicle response. It is shown that IRI numbers (m/km) can be easily converted to suspension velocity and displacement. The paper discusses various methods to interpret IRI and PSD (Power Spectral Density) of measured road profiles. © 2017 Elsevier Ltd.