Analysis of Applying Nonlinear Superelevation Attaining in Spiral-Spiral Alignments in Suburban Highways Based on Quarter Car Suspension Model

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 superelevation, horizontal alignments are more prone to accidents than other parts of the highways. Superelevation attaining based on current methods has linear 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 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 (connection point of the two spirals) and then a sudden linear descent to the end of the second spiral can cause problems for 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 the 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 the two spirals, vertical acceleration above the ride comfort threshold (0.8 m/s2), based on vehicle speed, will be very probable (even three times the mentioned limit). On the other hand, by using non-linear methods as combined parabolic and cubic equations, the mentioned vertical acceleration will be reduced to less than 0.8 m/s2. As an example, for design speed of 100 km/hr, and using linear method, the vertical acceleration will be 2.5 m/s2, while with nonlinear method it reduces to 0.8 m/s2.