It is neccessary to calculate excess attenuation due to buildings correctly for the prediction of traffic noise at the back ground of buildings. In previous studies, building coverage ratio (Ba) is used for a parameter in prediction of the excess attenuation. However, there are some problems in Ba model : when a building stands in front of the measuring point, the attenuation depends on the distance between the building and the measuring point, even if the value of Ba is constant. Therefore, Ba cannot reflect the built-up situation and is not sufficient for noise prediction. In this paper, a new parameter is proposed based on the ratio of the length of sound path obstructed by a building, normalized by equivalent effect of diffraction (Bph). Numerical results of the predicted noise levels by Ba model and Bph model are discussed through comparisons with measured results, and it is confirmed that Bph model can offer a better prediction of the excess attenuation due to buildings than the Ba model. The present Bph model is farther refined by adding two independent parameters, and we confirmed that we can predict noise levels accurately at the back ground of overcrowded buildings using the refined Bph model.