The results of a study conducted to determine the relationship between changes in the surface distress of flexible pavements and incremental changes in the international roughness index (IRI) or ride quality by using Long-Term Pavement Performance (LTPP) program data are presented. The results of the regression analyses completed to identify those distresses found to be important and related to incremental changes in the IRI were obtained under the sponsorship of NCHRP Project 1-37A (Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures). The results from the study have shown that selected distresses do have a significant effect on incremental changes in IRI with time and traffic. The results summarized can be used for the management, design, or evaluation of pavement structures.

Most state transportation agencies in the United States use the 1986 or 1993 version of the AASHTO design guide. The AASHTO guide for the design of flexible pavements uses the resilient (elastic) modulus as the property for characterizing all pavement materials and soils. However, state agencies do not routinely measure the resilient modulus in the laboratory but instead estimate the value from strength tests or physical properties. Physical property and repeated load resilient modulus tests are being performed on all unbound materials and soils within the Long-Term Pavement Performance (LTPP) program. Other correlation studies have been performed relating physical properties to the k coefficients of the resilient modulus constitutive equation but have been confined to specific soils. The LTPP program includes a diverse range of soils and unbound pavement materials for which the physical properties and resilient modulus are being measured. Thus, statistical analyses were initiated to confirm these relationships and define the accuracy of predicting the elastic response parameters for use in design. Results from these analyses suggest that the correspondence between the physical properties and the elastic parameters of the resilient modulus constitutive equation was fair to poor. It was also found that sampling technique had an effect on the elastic parameters of some unbound materials and soils. On the basis of the findings, it was recommended that resilient modulus tests be performed to characterize unbound materials and soils accurately for use in design.