Tyre grip on wet roads generally decreases with increasing speed. Its value at a given speed depends on many factors. The purpose of this report is to examine (1) how the variation of friction with speed affects stopping distances and (2) the problem of extracting information on the stopping distances experienced by vehicles from road friction measurements using standard equipment, such as scrim. A brief review is given of present understanding of wet road-tyre friction. Based on this, an examination of the relationship between scrim measurements and those of the average tyre reveals that, while no useful quantitative statements can be made, certain qualitative guidelines can be laid down. A convenient formula is derived for stopping distances which takes account of the decrease of tyre grip with speed. Tables of stopping distances based on this formula are presented. One conclusion is that stopping distances from speeds above about 80 km/h are greater, often considerably greater than would be expected from road-tyre grip measured at 50 km/h.
Abstract