To evaluate the road traffic noise, a simplified traffic model is usually used, that is, a model with vehicles of the same acoustic power and the equal headway distribution. But in an actual road, fast and slow vehicles and noisy and quiet ones are mixed randomly. The resulting increase inLeqdue to variance of PWL and velocity of vehicles has been investigated for the following three cases: (1) when variation coefficient δvof velocity of vehicles is rather small (i.e., δv≪ 1), (2) when the velocity distribution of the vehicles is the normal distribution, or (3) in the case of gamma distribution of the velocities. As the variance of PWL is 2.55–3.27 (according to the results by Y. Watanabeet al.), the estimated increase ofLeqis about 1 dB. And as variation coefficient of velocity of vehicles is 0.13–0.18 (according to the published data on Route 1, Route 16, and Meishin highway), the increase ofLeqis about 0.1–1.0 dB. Summing up these effects, the increment ofLeqis supposed to be 1–2 dB.