In order to predict road traffic noise for environmental impact assessments, future sound power levels of the vehicles must be estimated. In this paper, the formulas of future sound power levels that were estimated based on the past regulation of accelerated running noise are compared with the ones based on sound power level data measured in 1996. As a result, the validity of the formulas has been partly verified. Further, we have estimated the future sound power levels with our proposed estimating method based on the regulation of steady running noise and with the usual one based on the regulation of accelerated running noise, reflecting the latest regulation for motor vehicles noise. Consequently, the future sound power levels calculated by both the methods are smaller than those obtained from the data measured above by 0.8-1.5dB.