Instability rutting generally occurs within the top 5 cm (2 in.) of the asphalt layer when the structural properties of the asphalt concrete are inadequate to resist the stresses imposed on it. It is generally believed that near-surface transverse shear stresses perpetuate instability rutting. Field observations of heavy-vehicle simulator testing noted greater rutting in one-way directional loading than with two-way directional loading, even at lower temperatures and with longer rest periods between load applications. An analysis of stress states in the asphalt pavement layer using the three-dimensional finite element commercial code ADINA showed that longitudinal stress path patterns varied between the different directional loadings. A hypothesis was developed that the differences in longitudinal plane stress path patterns between one-way and two-way directional loading could be attributed to the different levels of rutting. A viscoelastic model with load applications simulating the different directional loadings was constructed and used to test this hypothesis. The viscoelastic model results indicated qualitatively that even with greater relaxation times, one-way directional loading produces greater strains.