This paper describes a method to evaluate the environmental impact due to highway improvement with a newly proposed inconsistency minimization model and development of a highway improvement programme for minimizing the vehicular emissions by explaining a case study. Scenario analysis is carried out using User Equilibrium Assignment method with modified BPR function based on Wardrop's equal travel time principle and simple vehicular exhaust emission formula. The new consideration in this study is that capacity of link and parameter of BPR function are changeable according to maintenance level. The amount of user cost and environmental emissions savings due to highway improvement or maintenance is estimated by using International Roughness Index (IRI) which is one of the indicators for highway service level. Finally the priority of highway links under budget constraint for maintenance will be evaluated according to the amount of savings. Optimum amount of maintenance budget is also estimated by trading off between total user cost and highway maintenance cost.