The objectives of the research described in this report were threefold: (1) to analyze in detail the theoretical solution that forms the basis of the data interpretation of the diametral compression test as recommended in the American Society for Testing and Materials (ASTM) D4123-82 (1987) and Strategic Highway Research Program (SHRP) Protocol P07 (1993) procedures, (2) to derive a solution that incorporates the viscosity of asphalt concrete which is present even at low temperatures, and (3) to anlayze improvements in both the measurements and in the interpretation of the test data.