This study is a part of a pluridisciplinary project on the assessment of the environmental impacts of an urban mobility plan and their socio-economic consequences. One of the main objectives is to be able to build and to compare noise maps of large cities, related to various mobility plan parameters. Considering the computational time that is required for making classical noise maps a simplified approach of the French standard NMPB2008 method of sound propagation in urban areas has been proposed. A striking feature of the proposed approach is that the model is implemented within a two-dimensional geographic information system software (OrbisGIS). Incidentally a specific attention has been paid to the optimization of algorithms in order to reduce computational times and memory footprint. The method is then able to produce noise maps in few hours on a personal computer, for very large domains, around several millions of squared meters. A validation is presented by comparing noise maps produced by the proposed approach with regular noise maps of Nantes Metropole using the French standard method. Lastly, several mobility plans have been compared by considering their impacts on the spreading of noise in the urban environment.