Mechanistic Interpretation of Braking Distance Specifications and Pavement Friction Requirements

Providing a safe stopping distance is an important requirement in highway and street geometric design. A key element to this requirement is the availability of adequate tire-pavement skid resistance. This aspect has not been addressed explicitly in the existing practices of determining braking distance, a major component of stopping distance. In practice, the common methods of evaluating safe braking distances do not reflect the effects of factors related to tire, pavement surface, and the presence of water on the pavement surface. Today, equipped with a better knowledge on the mechanism of wet-weather skid resistance, a rational mechanistic interpretation of automobile braking distances can be made in relation to the characteristics of tire-pavement skid resistance behavior. This interpretation is achieved by using basic mechanics principles and finite-element skid resistance simulation modeling to study the implications of braking distance specifications and their relationships with pavement friction management. As an illustration, the relationship between the AASHTO stopping distance requirements and the skid resistance threshold level adopted by several state pavement management authorities was examined. In the analysis, the effects of wheel load, tire inflation pressure, and water-film thickness and the variation of skid resistance with vehicle speed were considered. The results of the analysis highlight the need to maintain consistency between geometric design stopping distance requirements and pavement friction management to achieve safe vehicular operations.