VDOT uses the 1993 AASHTO pavement design guide for the design of its new or rehabilitated pavement structures. The AASHTO guide is based on empirical relationships that were developed during the 1950s as a result of the AASHO road test. As VDOT moves to implement the Mechanistic-Empirical (M-E) design guide, characterizing existing pavement conditions, including the resilient modulus of the subgrade, is an enhancement of the input level compared to using the M-E default values.