The vertical subgrade strain criterion is probably one of the best known and internationally accepted mechanistic-empirical design models in current use. This model relates the imposed vertical strain at the top of the subgrade to the number of load cycles that can be sustained before a certain terminal condition will be reached. Recent research in South Africa has shown that the vertical strain at the top of the subgrade is in fact a poor predictor of subgrade permanent deformation and therefore even worse in terms of predicting the permanent deformation of the full permanent system. A sample of 35 HVS tests for which reasonable material. MDD deflection and MDD permanent deformation data were available, were used in the investigation of subgrade permanent deformation to calibrate permanent deformation subgrade permanent deformation than the vertical subgrade strain or the vertical stress at the top of the subgrade as long as the subgrade is protected well enough to prevent high shear stress from occuring. A set of design models was calibrated for different levels of subgrade permanent deformation using the subgrade elastic deflection (SED) as the critical parameter