Empirical model and side friction factor effect on desired operating speed at horizontal curved road

The road surface and its geometric condition affect the vehicle's desired operating speed. This paper is framed to examine an empirical model for the desired operating speed due to the variation of the side friction factor at horizontal road curvature across the design period. The minimum radius equation developed by the American Association of State Highway and Transportation Officials (AASHTO) was used to analyze the desired operating speed at the confessed minimum radius, maximum side friction factor, and superelevation. This empirical formula indicated that speed and side friction factors had a direct relationship. Even though this empirical equation cannot define the relationship between desired speed and side friction across the design period except at the opening phase of the road, based on the research gap indicated above, this study proposes an empirical model used to explore the desired operating speed that resulted from the variation of the side friction factor across the design period. To have a structured flow of ideas, a theoretical model was used to create the approach of the specific research inquiry. For exploration purposes, this study used a maximum superelevation of 8 %, the same minimum radius, and the AASTHO recommended side friction factor. Based on the proposed empirical model, this study confirmed that the desired speed increases and decreases as side friction decreases and increases, respectively. Except that vehicular movement is in a static state, at an unrealistic side friction factor (f = 0), and at a critical zone.