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IRI correlates somewhat with human exposure to whole-body vibration in vehicles and thus to perceived ride quality as defined in the ISO 2631-1 standard. However, in many circumstances IRI can be a very misleading indicator of pavement rideability. One example is when driving over road roughness with long wavelengths (over 20 - 30 m) at speeds much lower than the IRI reference speed 80 km/h. I e on roads with speed limits 30 - 70 km/h. Then the calculated IRI can be extremely high (indicating very rough road / poor ride), whereas the real ride quality is very good. The relationship between IRI and the ride vibration affecting ride quality has been analyzed thoroughly[2]. The conversion from IRI to vertical human whole-body vibration in a passenger car can often be done by a factor 0.16. Depending on actual roughness wavelength and vehicle speed, the conversion factor ranges from 0.04 to 1.4. This shows that there can be a difference of a factor 30 between the worst and the best ride, given the very same IRI-value. Thus IRI can be a very misleading indicator of ride quality.
Lower number is smother