Physics explained easily: What exactly does the new mobile road sensor MARWIS output “Friction” mean? How does this affect the driving behavior of a vehicle? Here, this will be explained by Karl Schedler.
Out of the measurement of weather-related parameters on the road or airfield (such as ice percentage, water film height and surface temperature), the MARWIS
calculates which average friction arises.
The term friction can be translated into traction or friction coefficient between tire and road surface. The friction coefficient is defined as the ratio between the brake force and the normal force affecting the tire surface area in physical terms (through the weight of the vehicle loading the tire). This definition is equivalent to another definition which may be better applicable in practice: The “friction” is the ratio of the average braking deceleration (at full braking!) to the acceleration due to gravity (gravitational acceleration measures very roughly 10 m/s2). The braking deceleration is a negative acceleration (therefore a change of velocity per time unit) and also expressed in “m/s2”. The relation is a dimensionless number.
If you know these connections, the change of the friction value can help to estimate the impact on traffic, because it allows specifying the extension of the stopping distance for a given initial speed. Friction is thus a measure of how slippery the road is.