What pavement performance indicator directly quantifies ride quality?

The International Roughness Index (IRI) is the pavement performance indicator that directly quantifies ride quality. IRI is a standardized measure of the longitudinal profile of a pavement surface, calculated from measurements of the vertical deviations of a simulated vehicle axle as it travels along the pavement. A higher IRI value indicates a rougher pavement surface and a lower ride quality, while a lower IRI value indicates a smoother pavement surface and a higher ride quality. IRI is typically expressed in units of meters per kilometer (m/km) or inches per mile (in/mi). It is widely used by transportation agencies to assess pavement condition, prioritize maintenance and rehabilitation projects, and monitor the effectiveness of pavement treatments. Other measures, like Present Serviceability Rating (PSR), were used in the past, but IRI offers a more objective, repeatable, and automated method to assess ride quality. The measurements for IRI are taken using specialized equipment, such as inertial profilers, which use accelerometers and distance sensors to measure the pavement profile. The data collected is then processed using standardized algorithms to calculate the IRI value. Because IRI is a direct measure of the pavement's longitudinal profile, it is highly correlated with subjective measures of ride quality, such as driver comfort and vehicle operating costs.