In temperate climatic conditions the water depths on wet roads are generally low, typically less than 1 mm. In this paper we examine the various types of road surface and the manner in which they can be classified in terms of macro and micro-texture. We propose a simplified representation of the tyre road interface in which the tyre footprint is divided into two zones, a dry zone in which dry road friction levels are obtained and an initial wet zone in which there is a water layer between the tyre and road and which gives no retardation. A generalised relation for the variation in the size of the wet zone with speed is proposed. The model is applied to published data for road surfaces of differing characteristics with fully treaded and smooth tyres. The model is shown to give a good representation of the variation in locked wheel retardation with speed and highlights the sensitivity of stopping distance to variations in road surface and tyre tread depth. The data shows the key role of the micro-texture of the road surface in minimising stopping distance and also shows that when the road surface has a rough macro-texture the stopping distance is comparatively insensitive to tyre tread depth. Finally a simple physical model for tyre to wet road contact is introduced.
