Instability rutting generally occurs within the top 2 in. of the asphalt layer when the structural properties of the asphalt concrete are inadequate to resist the stresses imposed on it. Several researchers have presented observations in attempts to explain instability rutting, but a clear identification of the mechanism does not exist. Stresses in the asphalt layer caused by measured tire interface stresses were analyzed in three dimensions by using finite elements to identify possible mechanisms for instability rutting. The analysis showed that radial tires produce high near-surface shear stresses at low confinements, which are not predicted with traditional uniform vertical loading conditions, in the region where instability rutting is known to occur. The resulting shear stresses tend to be shallower than for the uniformly loaded case, and they are focused in areas where instability rutting has been observed. The observed stress states imply that the characterization of instability rutting requires testing at these low confinement (and sometimes tensile) stress states, rather than at the higher stress states typically used in the strength characterization of mixtures.