ACCIDENT RATES AND SPEED CONSISTENCY ON HORIZONTAL CURVES IN SINGLE CARRIAGEWAY RURAL ROADS

Horizontal road curves are major geometric discontinuities that increase the difficulty in the driving task and generate problems that may lead to a higher accident risk. In Portugal, roughly one third of the reported injury accidents outside intersections on rural roads of the main road network occurred on curves. A similar situation is reported in other countries, such as France, Finland and the Czech Republic. A study on the influence of some road characteristics on Portuguese driver behavior and accident occurrence in curves on single carriageway minor rural roads was recently completed at the Laboratorio Nacional de Engenharia Civil (LNEC). It was hypothesized that the drivers' expectancies can be represented by the traffic speed pattern at each road segment and that speed can be used as a link variable between the road characteristics and the accident risk. In situ measurements of road characteristics and traffic speeds were made, and their results related with the reported injury accident figures. In this paper, an overall descripion of the study is reported. The criteria used for selection of sites for speed and geometric road characteristics measurement campaigns are presented, together with the description of the conditions used for driver selection. A brief discussion of the main results of the study of the influence of some road characteristics in driver behavior is given. The log-linear accident frequency models fitted to estimated speed consistency and measured road characteristics are presented, together with the models for estimating the relation between accident rates on curves and the preceding tangent sections.