The 1985 European Union Directive on Environmental Impact Assessment was transported into Irish law in 1988. The resulting complex environmental impact statements often contain large volumes of information on relevant impacts, yet make relatively little effort to organize this information into a decision-assisting framework that enables the relative merits of the options to be comprehensively compared. This paper describes a practical application of the ELECTRE III decision-aid system to a preliminary environmental appraisal carried out recently on the Port Access and Eastern Relief Route (PAERR), a new motorway proposed for Dublin City. The case study illustrates how the ELECTRE III framework takes complex information from the environmental appraisal and uses it to rank the various project alternatives considered. The paper describes new techniques put forward by the writers for use within the ELECTRE III model. A weighting system previously developed by the writers is used for estimating the relative importance of criteria. Also, the writers' own methods are used for estimating indifference, preference, and veto thresholds. These estimates allow the uncertainties inherent in each set of criterion valuations to be incorporated directly into the decision model. Both these techniques make the model more readily applicable to the decision problem under examination within this paper.