The Friction Diagram Method for analyzing the skidding risk for road design

This paper shows a new method, the Friction Diagram Method, for analyzing the safety of a road segment related to the skidding risk. The Friction Diagram Method represents an update of the 3rd safety criterion of Lamm, that defines the road consistency of a road section on a curve, by comparison between the side friction factor assumed “fRA” and the side friction factor demanded “fRD”. The first represents the force of friction that the road can provide the vehicle, while the second represents the force of friction that the vehicle requires from the road. The Friction Diagram Method improves the concepts proposed by Lamm and introduces the new concept of the “Friction Capital” that is the real maximum skid resistance that the road can provide to a vehicle taking into account all the factors that influence it: vehicle characteristics, vehicle dynamics, vertical curves, inertial actions, environmental conditions and road geometry. The “Friction Capital” determined in this way varies greatly along the road and neither the driver, nor the designer were able to detect its value or its variations. In fact, while a user drives along a road, he uses a portion of the capital available, but he does not know how much of this capital he is using. The output of the Friction Diagram Method is the Friction Diagram that represents, along the road, the percentage of “Friction Capital” that a vehicle is using traveling on a given road. Therefore, the Friction Diagram is a tool that allows the designer to analyze the skidding risk during the design steps, and to inform the driver about the really safety condition of a road. In particular the proposed method allows to define the Friction Diagram, and then the skidding risk, according to all types of vehicle by using the “Design Critical Vehicle”, which represents the vehicle that maximize the consumption of Friction Capital and then give to the designer the opportunity to test the level of skidding risk for each type of vehicle with a single diagram.