It is known from previous research that skid resistance can vary significantly on a daily basis. The cause of this short· term variation may be attributed to external factors such as amount and intensity of rainfall and contamination effects of debris on the road surface. For this study, daily skid-resistance measurements were taken on the road surface according to ASTM E274-77 and a sample of dust and debris was collected from the road surface and analyzed for size distribution. It was found that the major factor that influenced the short-term skid resistance was dust particles in a certain range of sizes found on the road.

Road-surface friction characteristics vary throughout the year due to factors such as pavement polishing or wear, amount of precipitation, and road-surf ace contamination (1). Each of these factors separately plays an important part in determining the skid resistance of a road surface, but there is also a strong interaction among them that affects pavement friction characteristics. Pavement polishing increases with traffic density for a given pavement. Polishing alone will lower skid resistance of pavements due to the overall effect of wearing down of the pavement texture. The grinding action between the tire and the road surface creates a very fine abrasive compound made up of tire and road-surface compounds together with ground-up particles of wind-blown soil and debris. These particles may adhere to tires, become airborne as fugitive dust emissions, or remain on the road surface. The particles that remain on the road and in the tire path affect the skid resistance by filling in some of the small asperities. The small asperities that are smaller than 500 µm are defined as the pavement microtexture i those larger than 500 µm are defined as the macrotexture (~)• The greater the particle loading in the tire path, the lower the skid resistance at low tire slip speeds. The size of these particles is in the microtexture range of the pavement and they provide a crude form of lubrication on the road surface by preventing good contact between the tire and the pavement at the microtexture level. This is an important factor in skid resistance at tire slip speeds less than 16 km/h (10 miles/h). The effect of rain on pavement skid resistance has received more attention than the effect of contaminants. The amount of water between the tire and the pavement and how easily it is channeled away has a very significant effect on skid resistance. The action of channeling the water from the tire-pavement contact will produce a cleansing effect on the road surface. This cleansing effect, which washes away the fine aggregate created by the road polishing, will increase the skid resistance of the road surface. It also provides for maximum contact between the tire and actual pavement surface and results in a maximum skid resistance for the road surface.