A smooth riding concrete pavement increases driver comfort but there are other benefits as well. Pavements built to exceed minimum quality standards for surface smoothness also have a longer life. Present Serviceability Index (PSI), a measure of pavement ride quality, decreases with the number of wheel loads applied. When the PSI reaches a lower limit, called the terminal serviceability index, the pavement no longer provides an acceptable ride. Repair or rehabilitation is needed. Pavements with a higher initial PSI last longer. Because pavement smoothness strongly affects the initial PSI, increasing numbers of state highway departments are measuring smoothness and specifying desired levels.
The California profilograph is the most commonly used quality control tool for acceptance testing. It is a rolling straightedge that produces a pavement profile trace on a sheet of continuous graph paper. Trace reduction consists of measuring deviations from a reference plane, then summing them to calculate a Profile Index reported in inches per mile. The higher the Profile Index, the rougher the pavement.