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Present serviceability index rating measures the longitudinal roughness, patch work, rutting and cracking within a road segment in the secondary direction.
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The PSI is based on data on the roads longitudinal roughness, patch work, rutting and cracking. Later studies have shown that PSI is mainly a fruit of unevenness, with a correlation of more than 90% between the two. Uneveness was measured with a mechanical profilograph, reporting a parameter called slope variance (SV). SV is the second spatial derivative of height. For a vehicle travelling at speed, SV is the exciting source to vertical acceleration; the second derivative in time domain of height. This makes very good sense, since 1 – 80 Hz acceleration is the parameter used when relating human exposure from vibration to perceived discomfort in the current ISO 2631-1 (1997) standard. Thus, SV is physically linked to ride quality.