This report describes the latest stage in a programme of research, undertaken by TRL for the Transport Research Foundation (TRF) to investigate the possibility of carrying out or aiding skid resistance measurement without contact with the road surface.
An aggregate specimen was polished in the laboratory to provide nine discrete levels of polish, with associated measurements of friction. At each level of polishing, high resolution images were taken and analysed. Although the correlation between parameters derived from the images and friction that has been demonstrated before was shown to hold, the analysis also showed that there are likely to be more factors involved in the changing appearance of the surface than just its texture.
At each polishing stage, replicas of the aggregate surface were taken. As well as providing a permanent record of the surface texture, the replicas allowed inspection by scanning electron microscopy (which is not possible on the aggregate specimen because it is too big for the SEM chamber). Clear changes in the surface texture, apparently due to the polishing process, were detectable at various scales.
Surface texture was measured using an optical microscope that combines the small depth of focus of an optical system with vertical scanning to provide topographical information derived from the variation of focus. Surface topography was analysed with three different software packages, using a number of standard roughness parameters. The effect of various wavelength filters was explored and this suggested that determination of the scale of texture most relevant to skid resistance is important, even within the relatively narrow range present on a single stone surface. It was shown that that the distribution of peaks, rather than their heights, is also likely to be a relevant factor.
Recommendations are made for further use of the specimens prepared in this work and for development of the methodology that will allow more conclusive texture measurement analysis in future.

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