Past work has indicated that on wet roads the rafted of decrease of skidding resistance with speed is largely dependent on the coarseness of the road surface texture. On smooth, fine-gained surfaces the decrease is much less rapid. Tests have now been carried out to examine more closely the way in which coefficients decrease with speed up to 80 mile/h and to attempt to find a quantitative measure of surface texture which will indicate the Drafted of fall in coefficient with speed. Two techniques of measuring texture have been examined: the sand patch' method from which a measure of 'texture depth' can be obtained, and a new stereophotogrametric technique which records the surface profile. A statistical correlation has been obtained between the percentage decrease in coefficient between 30 and 80 mile/h and texture depth, which measures the coarseness of texture. Recommendations for texture depth requirements for fast roads are given: to restrict the decrease to less than about 25 per cent, a texture depth of more than 0.025 in appears necessary A better correlation with percentage decrease in coefficient is obtained, by a measure of the surface of the surface profile (the "Profile ratio") which takes into account the shape of projections in the surface.
Abstract