The differences between the elastic and viscoelastic responses of a pavement structure, as measured by the FWD, may begin to explain some of the differences normally observed and reported between the laboratory and backcalculated moduli of a pavement material and/or subgrade soil. These observed differences should be studied in depth in future data analysis studies regarding the Long-Term Pavement Performance (LTPP) data base, especially when developing mechanistic-empirical pavement performance models.