عنوان مقاله [English]
Horizontal alignments are one of the most important parts of road infrastructure. Safety and driving comfort in these areas has always been the focus of researchers. Due to the geometric nature and especially the change in the curvature of the road and the existence of superelevations, horizontal alignments are more prone to accidents than other parts of highways. Superelevation attaining based on current methods has liner form. Spiral transition curves as one of the most practical options for connecting between a circular alignment and a straight path, in the absence of a circular part (due to geometric constraints or the designer's initial choice) can be considered as a spiral-spiral combination. In this case, linear increase in path edge elevation from start to top (the point of connection of the two spirals) and then a sudden linear descent to the end of the second spiral can cause problems for the safety and comfort of the vehicle passengers. In this study, in order to solve this problem, the use of nonlinear superelevation attaining at the junction of two spiral alignments has been investigated. For this purpose, the quarter car suspension model was used to study the vertical acceleration .According to the research results, by applying linear superelevation attaining in the spiral-spiral alignments, at the starting point of the spiral or at the junction of two spirals, vertical acceleration above the ride comfort range (0.8 m/s2), based on vehicle speed, will be very probable (even three times the value of 0.8 m/s2). In the other hand by using non-linear methods as combined parabolic and cubic equations, the mentioned accelerations will be reduced to values less than 0.8 m/s2.