Many different devices have been developed to measure skid resistance or road surface friction. They all assess skid resistance by measuring friction between rubber and the wet road surface in some way. However, there is no absolute value of skid resistance against
which a measuring device can be compared. While it is possible to make test surfaces or identify in-service roads that have levels of skid resistance within a certain range, it is not possible to tell in advance what the actual skid resistance will be – especially as the
measured value varies from one device to another anyway. The development of true reference surfaces would be a significant step forward on the road towards a harmonised approach to skid resistance measurement and reporting.
The purpose of this report is to review the topic of test surfaces used for checking and calibrating skid resistance measuring equipment and the potential use of reference surfaces to contribute to the harmonisation purposes. The outcomes of this part of the study, in conjunction with deliverables D04 and D05 will then feed into the next stage of the TYROSAFE project to develop a road map for future harmonisation.
For a survey of current practice, a detailed questionnaire was sent to project partners and, through them, manufacturers, operators of test equipment and those organisations responsible for equipment accreditation were approached. The purpose of this questionnaire
was to see how different countries use test surfaces on roads or test tracks to calibrate their measurement devices, in the absence of “true” reference surfaces with known skidresistance characteristics.
The general conclusion drawn from this part of the work was that many different surfaces are used as test surfaces, most often (but not exclusively) made from conventional road-building materials. However, because they do not have access to test tracks (there are not many of these in Europe), most organisations use in-service roads for their calibration checks.
Consequently, the selection of test surfaces is not based just on a specific combination of friction and texture levels but on what is readily available on the road networks concerned.
Another consequence of the usage of in-service roads is that the range of friction levels that can be used is limited and low-friction surfaces are missing.
The second aspect covered by this report was the potential use of purpose-made reference surfaces that would have predictable, stable and reproducible skid resistance characteristics.
This topic was covered extensively in the FEHRL project HERMES and a literature review for TYROSAFE did not reveal any further published information on the topic. The HERMES report made suggestions as to what the general characteristics of such surfaces might be and how their construction might be approached. However, it was clear then, and remains so now, that research is still necessary to develop such surfaces from a practical point of view.
The choice of suitable materials to achieve predictable and stable performance will be difficult, so work to identify materials that could be reliably specified for use for the reference surfaces will need to be a fundamental aspect for research. The challenge is to find suitable
combinations of a regular and repeatable form and level of macrotexture with appropriate treatments or additives to provide predictable, controlled and durable microtexture. This, in turn, will need to be combined with consideration of the contribution of different test tyre compounds and their potential interaction with any proposed surfacing material.