This report focuses on the attempts to provide answers through the analysis of comprehensive time history smoothness data collected by high-speed
inertial profilers under the Long-Term Pavement Performance (LTPP) program. A key factor in the long-term performance of both asphalt and portland cement concrete pavements is initial pavement smoothness. In general, the smoother a pavement is built, the smoother it stays over time, resulting in lower maintenance costs and more comfort and safety for the traveling public. State highway agencies recognized in the 1960s the importance of controlling initial pavement smoothness, and began developing and implementing smoothness specifications. As the technology and equipment for measuring pavement smoothness advanced, two predominant methods emerged.