Current methods used to compare project options in impact assessment studies raise the problem of evaluation aggregation. Moreover, stakeholders question the transparency of these methods and the limited number of true options analyzed. To avoid these deficiencies and related controversies, a multicriteria decision aid approach has been designed and applied to an actual project already submitted to the current environmental impact procedure (impact assessment study and environmental mediation). Emphasis has been put on the broadening of the options based on the analysis of issues raised by the stakeholders, and on an iterative aggregation procedure of evaluation criteria, using the multicriteria decision aid method ELECTRE III. Furthermore, this method considers the stakeholders value System (weighted criteria and preference thresholds). The case study is the prolongation of the highway 30, between highways 10 and 20 in the Montréal region (Québec, Canada).