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In urban areas, medium and high-rise buildings are usually constructed on both sides of an arterial road and a viaduct road often exists above it. There are many reflections by building facades, the pavement and the underside of the viaduct road in such places. Therefore the roadside noise levels may exceed the recommended limits in city street canyons. In this paper, a simple calculation model for estimating the mean increase in noise level owing to multiple reflections has been proposed in order to predict road traffic noise in city street canyons more accurately. This model is based on a diffusion method. We have also investigated the effects of multiple reflections on noise propagation using a 1:40 scale model of a city street canyon with a viaduct road. As a result of this study, it has been found that the mean increase in noise level owing to multiple reflections by building facades and the underside of the viaduct road in built-up urban areas is up to about 8 dB. Comparisons between the results of experiments and calculations show that the simple calculation model gives results consistent with experimental ones.