Channel characterization is an essential step to the development of outdoor or indoor wireless networks. Indeed, for multimedia applications, new radio mobile systems must accurately take into account channel behavior. In this paper, we propose an efficient 2D ray-tracing method to characterize the narrow and wide-band radio channels for a very large number of receivers in micro-cellular configurations. It is based on a quick pre-calculation of an exact 2D visibility graph. The proposed method follows an ITU recommendation, which advocates, for wide-band characterization, only considering the paths included in a 18 dB dynamic range of power impulse response. Contrary to the classical approach, which consists in thresholding the complete impulse response in a post-treatment, our method only computes the significant paths. The interest of the proposed method resides in its significant computation time reduction factor, in comparison with the classical approach and this without any significant loss in accuracy. Received power and wide-band parameter maps are computed for about 40,000 receivers, in a dense urban environment, and are provided with an approximate reduction factor of 4 and 80% of null estimation error in comparison to a classical approach.