In 1967 a full-scale experiment was started on motorway M40, high Wycombe by-pass to compare a range of bituminous surfacings in their ability to maintain a good, high-speed, skid-resistance in wet conditions. One of the main criteria for this aspect of performance is surface texture. Deep textures of 1.5 mm or more were maintained throughout the 12 years of observation by the mastic asphalts, the rolled asphalts and the pervious macadam. Surface dressings started with texture depths of well over 3mm but these dropped rapidly during the first year of service. The dressings on a fine cold asphalt substrate levelled out at about 1.8mm, those on an asphaltic concrete substrate at 1mm and those on an unchipped rolled asphalt containing 40 per cent of coarse aggregate at about 0.8mm. For all the materials tested the percentage drop in braking force coefficient from 50 to 130 km/h was approximately equal to 40 - (20 x texture depth in mm). (Author/publisher)
Abstract