Skid Terminology - Some Thoughts

Skid Resistance vs Friction

We often find ourselves talking about pavement friction in the pavement management and design arenas because, I believe, pavement skid resistance is so similar in concept to the frictional properties of two materials in contact. For example, the lateral force encountered is proportional to the perpendicular (normal) force (I am not convinced it is a linear proportionality in this case) and the sliding value is typically less than the static (rolling) one. What we typically measure, however, is something quite different from traditional friction. We attempt to measure the lateral force relationship between tire and pavement much as a typical user would encounter it during use. The force we measure relates to surface properties that are much larger than the molecular sized features associated with traditional friction. The lateral force we are measuring are considered to be primarily related to the micro and macro texture features of the two surfaces and the material properties of the test tire employed. To complicate matters further, we apply a surface lubricant (water) to the interface during measurement that significantly impacts the interaction between the two surfaces and the resultant measured force.

It is for these reasons that I suggest we, as a community, agree to refer to this pavement metric as Skid Resistance and the scale for the measured property , when measured by a locked wheel testing device, as Skid Number. I believe that other, non-locking type measurement equipment should also utilize the term Skid rather than Friction in the name of their output value since many of the same concerns apply to these alternate measurement devices.