The effect of traffic and aggregate on the skid resistance of bituminious surfacings

The polishing characteristics of the aggregate used in a bituminous road surfacing are a major factor in determining its resistance to skidding. The amount and weight of traffic is also a determining factor on heavily trafficked roads. Analysis of measurements made on roads over the last 10 years has shown a direct correlation between traffic intensity and the skidding characteristics of the wet road surface, a correlation that varies consistently with the polishing characteristics of the aggregate. Thus it has been possible to derive an empirical formula which indicates the expected resistance to skidding of the surfacing from a knowledge of the polished-stone value of the aggregate and the expected traffic on the road. This formula will be of use to engineers in selecting aggregate to obtain surfacings of a required resistance to skidding under given circumstances. It applies only to surfacings which present a continuous mosaic of coarse aggregate to the motor tyre, i.e. to bituminous surface dressings and to bituminous premixes with a substantial proportion of exposed aggregate in the surface.