The Evaluation of Friction Demand Factor in Loop Ramps of Interchange Facilities

Abstract

The aim of this study is to evaluate the side friction demand factor in the loop ramps of interchange facilities. The substantial exclusivity of these ramps is the existence of horizontal curves, combined with longitudinal grades. In this study, CarSim and TruckSim software packages, as simulation tools, are applied. Both passenger cars and heavy vehicles are used. The vehicles used in simulations were Hatchback and Sedan (as passenger cars) and Truck (as heavy vehicle). In addition, two various types of loop ramps, including Curve-Curve-Curve and Spiral-Curve-Spiral, in two different conditions (braking and no-braking) are examined. The results showed that the side friction demand factor values assumed by AASHTO Green book (as a main geometric design guideline) are uncertain. In the condition of no-braking, the differences between AASHTO values and the simulation results for uphill and downhill states are 24% and 18%, respectively. In braking condition, similar differences for uphill and downhill are 124% and 135%, respectively. Additionally, based on the regression analysis of the simulation results, the appropriate side friction demand factor models were achieved for different conditions. The findings of the study verify the necessity of revising the friction demand values, especially for the design of interchange loop ramps.