Consideration of Sight Distance in Placement of Concrete Barriers on Horizontal Curves of Roads

Abstract

Sight distance is an important criterion in highway geometric design. Many design elements, such as horizontal and vertical curvatures and offsets to lateral obstructions, must be checked so that drivers have sufficient sight distance to operate safely. One of the obstacles that can restrict sight distance is the roadside or median concrete barriers that are frequently placed along the road edges for safety. There are common types and dimensions for such barriers, such as New Jersey, F-shape, and constant-slope concrete barriers. Such barriers may cause a significant reduction in the available sight distance on horizontal curves, especially in the lane closest to the barriers. However, there is a clear lack of a reliable, accurate tool to determine the available sight distance when the sight distance is restricted by road barriers in three-dimensional combined alignments. This paper uses a software package previously developed by the authors to examine closely the trends of available stopping sight distance and the effects of the overlapping vertical alignment. The results confirm previous findings that the available sight distance depends on the type of the vertical alignment and the curvature of crest or sag vertical curves overlapping on the horizontal curve. In addition, it is shown that the recommendations in current design guides could lead to barrier placement that compromises the available sight distance. Charts were prepared as an easy-to-use tool to estimate the available stopping sight distance on horizontal curves when overlapping with a specific vertical alignment.