Road Surface Characteristics and Skidding Resistance

Abstract

Abstract When roads are wet their skidding resistance is reduced by the lubricating action of the film of water on the road. Under some circumstances this reduction may be substantial and the extent of it is largely dependent on the characteristics of the road surface. The first requirement for a good skidding resistance on wet roads is to facilitate break through of the water film in order to establish areas of dry contact between the road and the tire. Drainage channels, provided by the large scale texture of the road or by a pattern on the tire, assist in getting rid of the main bulk of water and are of increasing importance the higher the speed. The penetration of the remaining water film can be achieved only if there are sufficient fine scale sharp edges in the road on which high pressures (about 1000 lb/in2) are built up. The existence of such fine scale sharpness gives the surfaces a harsh feel. When vehicles are travelling at speeds of about 30 mph the fine scale texture of the road is the dominant factor determining skidding resistance. However, as they travel faster, it becomes increasingly difficult to penetrate the water film in the time available, however harsh the surface. At high speeds the requirements for a good skidding resistance are therefore different. The resistance to skidding arises to a larger extent from energy losses in the rubber of the tire as the surface of the tread is deformed by projections in the road surface and, although the physical properties of the tread rubber are important in this respect, it is essential to have sufficiently large and angular projections in the road surface to deform the tread, even though a water film may still be present on the surface. At higher speeds the coarseness of texture becomes as important as its harshness.