New Formulas of Side Friction Factor Based on Three-Dimensional Model in Horizontal Curves for Various Vehicles Ali Abdi Kordani 1 , Amirarsalan Mehrara Molan 2 , Saeed Monajjem 3 1 Assistant Professor, Department of Civil Engineering, Imam Khomeini International University, Qazvin, Iran. Tel: 0098-912-6085308; Email: aliabdi001@yahoo.com 2 Researcher, Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran, Tel: 0098-912-6358911; Fax: 0098-21-88578803; Email: a.mehrara@srbiau.ac.ir 3 Associate Professor, Department of Civil Engineering, K. N. Tossi University of Technology, Tehran, Iran, Tel: 0098-21-88779474 (EXT. 409); Email: monajjem@kntu.ac.ir ABSTRACT This research has been conducted to discover the correlation between side friction factor (demand) and longitudinal grade which is located in horizontal curves in three-dimensional simulation model. In this way, we present a series of models in order to assess these factors based on design speed, longitudinal grade, and vehicle type (Sedan, SUV, and Truck). The research involves (1) conducting a series of multi-body simulation tests using CarSim and TruckSim, (2) multiplex regression analysis to acquire relations between variables, and (3) recommending models for the side friction factor. Based on the results, side friction factors (demand) are bigger than downgrades for all vehicle types when cornering. Another important result is the significant differences of side friction factors for passenger cars in compared to heavy vehicles. Keywords: Side Friction Factor, Horizontal Curve, Longitudinal Grade, Three- Dimensional Simulation Modeling INTRODUCTION According to literature review and accident reports, horizontal curves are the most hazardous sections of highways. The collision rates on such curves are between 1.5 and 4 times greater than on similar straight sections (Aram 2010). The threat of accident posed by horizontal curves is even greater when they are located on steep longitudinal grades. The requirement for a combination of horizontal and vertical alignments is quite common in highway design, for example, in roads through mountainous terrain and on highway interchange ramps (Mannering et al. 2009).