Data are presented showing that small thicknesses of foamed plastic insulation prevented frost penetration into a highway subgrade in an area where the freezing index is less then 1000 degree days. The data were obtained from a test road (flexible pavement) built in northern Indiana, which consisted of: a control (normal) section with no insulation; the normal section plus 1 inch of insulation; and the normal section with the 6-inch subbase eliminated and 1 1/2 inches of insulation added. Analysis of the five-variable subsurface temperature problem was conducted, holding three of the four independent variables, viz., three-dimensional subspace and time, constant, while examining the effect of the fourth on temperature. Additionally, limited data are presented with respect to differential surface icing of adjacent insulated and uninsulated sections. The information indicates that the insulated pavements are colder during a seasonal cooling, while the uninsulated ones are colder during a seasonal warming. The overall performance of the insulated sections is satisfactory after 3 winters of service.