What is Pavement Smoothness?

Pavement profiles can be visualized as imaginary lines “drawn” along the surfaces of the pavement. Normally, pavement profiles are measured either longitudinally (down the road) or transversely (across the road). The former is more common, and is called a longitudinal profile. The latter, a transverse profile, is used for more specialized purposes.

When measuring pavement profiles, we take “samples” of the elevation at evenly spaced points. Measuring techniques have evolved over time. In earlier years, we used manual equipment such as a Profilograph. Today, we commonly use highway-speed laser-based systems termed Inertial Profilers. However, even with modern equipment, pavement profile measurement isn’t without its challenges. For example, certain surface textures, including longitudinal tining for concrete pavement, have sometimes resulted in unintended errors.

To quantify smoothness, we often use indexes that reduce the pavement profiles to a single number (a degree of roughness). Many states today still use a Profilograph Index, based on either a measured or simulated Profilograph trace. Recent trends have us moving towards the International Roughness Index (IRI), which was developed from a World Bank study in the 1980s. IRI has been demonstrated to not only better simulate a vehicle’s response to the road, but also correlates well to the driver’s perception of ride quality.

There are currently several standards for pavement smoothness under consideration by the American Highway Association of State Highway and Transportation Officials (AASHTO). These include specifications for an inertial profiler, certification processes for inertial profilers, profiler operation and results evaluation, and ride quality specifications. Initiatives such as this offer proof that this field continues to evolve, and as a result will continue to take center stage in many of our decisions.