“…The design variables depend on the specific problem and on the mathematical (geometric) model. In this sense, works published so far can be classified into three main groups: those focusing on the horizontal alignment (Casal et al., ; Easa and Mehmood, ; Lee et al., ; Mondal et al., ), those centering their attention into the vertical alignment (see Fwa et al., ; Hare et al., , ; Beiranvand et al., ), and those studying both alignments simultaneously (Bosurgi et al., ; Chew et al., ; Hirpa et al., ; Jong and Schonfeld, ; Li et al., ; Shafahi and Bagherian, ). For each of them, the chosen variables depend on the geometric model used to define the layout: within the horizontal alignment it is possible to consider, only main axes (Lee et al., ), axes and circular curves (Hirpa et al., ; Mondal et al., ; Jong and Schonfeld, ; Shafahi and Bagherian, ), or axes, circular curves, and transition curves (Casal et al., ; Kang et al., ); within the vertical alignment, vertical curves may be considered (Jong and Schonfeld, ; Shafahi and Bagherian, ), or may not (Hirpa et al., ).…”