A Tri-level Model with Environmental Considerations in Highway Alignment Optimization

Highway Alignment Optimization (HAO) process is a complex combinatorial optimization problem in which several conflicting factors, such as highway costs, user preferences, and environmentally sensitive factors will have to be simultaneously considered. In previous works, single level optimization, bi-level optimization, and multiobjective optimization approaches have been developed to optimize three-dimensional highway alignments. One drawback in previous approaches is that environmental factors, such as vehicular emissions were not adequately considered in conjunction with other factors (such as user preferences and highway costs) in the optimization process. This paper builds upon the authors' previous works and proposes two separate approaches for considering the environmental emissions in the highway alignment optimization process. The first approach involves a separate analysis of user and system optimum in which a conceptual formulation of various environmental factors are presented. In the second approach, a novel tri-level optimization framework is proposed for optimizing highway alignments. At the upper level, optimization is performed using the traditional criteria of cost minimization. At the intermediate level, total systems emission is minimized. Finally, at the lower level, the user equilibrium traffic flow is optimized. The developed approaches are illustrated through case study examples. The proposed approaches will be beneficial for designing highway alignments while considering environmental emissions. Additional refinement to the formulation and sensitivity analyses can be undertaken in future works.