Sight Distance , Signing , and Safety on Vertical Curves

Sight distance obviously affects safety. Although this statement is universally accepted, the relationship between sight distance and safety has not yet been successfully quantified. One reason for this is the obvious strong correlation between sight distance and alignment. Where roads have no vertical or horizontal alignment change, the available stopping sight distance (SSD) is not limited by road elements; such roads have been shown to be the safest. The greater accident experience on roads with horizontal and ventical curvature is not necessarily caused by limited SSD, but high-accident frequency locations quite often have SSD problems. This study began as part of a National Cooperative Highway Research Program project investigating the parameters affecting stopping sight distance.’ This analysis extends the data and scope of that study considerably.