# Road Friction; What is it?

Written by Maritza Hernandez,

Friction and traction are key to avoid sliding vehicles and spinning wheels.

Friction is defined as the rubbing of one object or surface against another. The friction between your vehicles’ tires and the road will determine maximum acceleration and minimum stopping distance. The force of friction depends on the force pushing the objects or surfaces together and the coefficient of friction. The coefficient of friction is the relationship between the force required to move the surfaces against each other and the pressure to stay in contact while in motion.

The Coefficient of Friction (COF)

The coefficient of friction defines the slipperiness of a road surface. The lower the COF value, the more slippery the material. So, for example, two ice cubes pressed together will slide easily, two pieces of rubber will not, due to the gripping contact area increasing and consequently the friction.

However, COF is an empirical measure, i.e. it is a result of practical experiments and comparisons rather than theoretical calculations. Thus, the COF value is an approximation because of the many calculation variables. Yet, it is a practical measure since the value gives us an idea of slipperiness.

Is there a specific COF value for roads?

COF values for different specific road pavement would come in handy. Why aren’t there such? Although the materials used are similar, the dominant variable will be the texture or roughness of the pavement. So, how is this measurement accomplished when the surfaces are so different?

Using laser technology, it is possible to measure the macro-texture and get an idea of the ruggedness and frictional features of road pavement. Also, testing can be done with a surface friction tester that measures the resistance experienced by the wheel and stopping distance.

Is there a scale for road friction?

Currently, there is no standardized scale for road friction.

The measurement of road friction involves considering the cumulative effects of snow, ice, loose materials like sand and the texture of pavement on wheel traction. Many devices and methods have been developed around the world to measure the friction and texture of road surfaces. Adequate comparisons and harmonization of different research and test results will improve road safety development over time. Ideally, there should be an international common definition and scale for road friction.

However, different initiatives are driving development such as NASA, FAA, and Transport Canada's 5-year joint research effort to focus on proving technology concepts for a better understanding of runway friction, Virginia Tech’s research effort to compare and harmonize texture and skid resistance measurements plus there are numerous research efforts by different countries and agencies to better understand the behavior of different friction testing devices and the influence of texture, speed, and other external conditions on their measurements.

What do we do now?

Ultimately, safety is all about how we drive. Regardless of engineering methods, scales and values, slowing down and being attentive is the best way to keep traction and improve safety when roads are icy and slippery. However, losing traction may still occur so drivers should err on the side of caution and leave sufficient distance for reaction time and less than perfect road conditions. In addition to cautious driving, installing traction devices such as automatic snow chains can keep you on the road and to your destination safely.