A road surface complying with the new International Standards Organization (ISO) specification was installed at an Arizona test facility (DPG site) in the winter of 1995/96. As part of the acoustic qualification of this site, comparative tests were conducted between this new surface, a Society of Automotive Engineers (SAE) sealed asphalt surface and an existing ISO surface in Michigan (MPG site). Initial testing with one vehicle and tire combination indicated that the new ISO surface produced ISO 362-1994 passby and coastby levels about 2 dB lower than sealed asphalt. Relative to the Michigan surface, the levels for the new Arizona ISO surface were 3 to 3½ dB lower. These differences were much greater than expected based on previously published studies of these two test surface types. Since the new surface was constructed to the ISO specification and meet the physical requirements for sound absorption coefficient, porosity, and surface texture, further investigation was conducted to determine if sound propagation or tire noise generation differences accounted for the differences. Experimental work to understand this difference included the use of on-board sound intensity measurements to isolate tire noise generation under both acceleration and coast and static sound propagation tests to isolate surface reflective properties. Analytically, a sound reflection model was developed to predict differences in attenuation based on measured surface impedance data. Taken together, the results of this investigation support the conclusion that a majority of the differences observed are due to tire noise generation. However, in comparing the new ISO surface to the SAE, a significant portion was also found to be attributable to sound propagation differences.