This study forms part of an extended research programme of the Working Group on Tyres, Road-surfaces and Skidding accidents. According to the terms of reference a statistical relationship had to be established between the skidding resistance of a road-surface and the number of accidents per million vehicle kilometres, based on available data. The research procedure for studying skidding resistance had to be developed taking into consideration its possible use in future by the authorities in establishing a traffic safety programme. Wherever possible, the research results should be directly applicable for establishing the necessary counter measures. The research has been carried out on the basic data referring to the years 1965 and 1966. It covers nearly all important road sectors of the Dutch motorway system in which two road types have been distinguished. Road type I, comprises mainly roads with (separate) dual carriageways, while Road type 11 comprise all other types of roads, such as dual carriageways, and single lane roads. The data on road-surface skidding resistance, used in this study, refer to wet road-surfaces. The high degree of skidding resistance of dry road-surfaces cannot normally be matched by any kind of wet road-surface. Thus also the available volume data had to be related to wet road-surfaces, and this was only possible by analysing material collected during times of precipitation. The manner in which road-surfaces dry up is not yet sufficiently understood. The accident data contain a direct indication of the weather conditions. Thus, the terms of reference also included, and mainly concentrated on, the atmospheric conditions of rain. Accidents on road-surfaces made extremely slippery by snow, sleet, frost, were excluded from the investigation. The relative road risk referred to in the terms of reference, is expressed in the study in two ways. The term "accident quotient" indicates the relationship between the number of accidents, occurring on a given road-sector in a given period of time, and the total number of vehicle kilometres for the same road-sector and the same period of time. When the number of cars of a given category, involved in accidents, is compared with the total number of vehicle kilometres covered by the given category, the quotient of involvement is established. The quotient of involvement in the present study is not only established for the total of all cars per road type, but also separately for passenger cars and goods vehicles. The above-mentioned quotients have been calculated for all classes of road-surface skidding resistance from fairly well detailed basic material. In addition, further classifications have been made within each skidding resistance class, for all occurring classes of the total hourly volume of traffic.
Abstract