By considering that surface irregularities of building facades are sufficient to produce diffusion in streets, the diffuse sound field modeling by the mathematical theory of diffusion is used to predict sound propagation and reverberation in rectangular streets. This modeling was first developed to predict the sound field in rooms with diffusely reflecting boundaries [J. Picaut, J.-D. Polack, and L. Simon, Acust. Acta Acust. 83(4), 614–621 (1997)]. In this paper, the modeling is applied to urban acoustics without significant modification. A 3-D diffusion equation is derived for the sound energy density and solved for time-varying sources and in steady state. A diffusion coefficient is introduced to characterize the degree of diffusion of the street, and an exchange coefficient depending on the absorption coefficient is used to take the wall absorption into account. When the diffusion coefficient is chosen correctly, predicted reverberation times and sound attenuation are in accordance with experimental data obtained in a scale model of a street. With new developments, by introducing in the model, for example, intersections, moving sources, meteorological effects..., this original modeling should be in the future, a simple solution to predict the noise impact in large urban areas.