Assessing IRI vs. PI as a Measurement of Pavement Smoothness

A pavement specification that allows either the International Roughness Index (IRI) or Profile Index (PI) to measure pavement would be advantageous to both industry and the Missouri Department of Transportation (MoDOT). This brief study provides for MoDOT an understanding of the relationship between IRI and PI, an analysis comparing the two systems, and a review of potential specifications, and offers a smoothness adjustment table that would fit into Section 502.15.3 of the Missouri specifications. Furthermore safeguards against the potential manipulation of the data within both IRI and PI systems are discussed. With proper certification procedures for equipment and operators and a random verification program on the measurement of smoothness, it is unlikely that the state DOT will encounter a problem with systematic alterations of inertial profiling data. Sample specifications from the Connecticut DOT, the Ohio DOT, and the Minnesota DOT are included in the report as they are good examples of specifications that MoDOT may wish to use as a pattern for its specification.