Flattening of Horizontal Curves on Rural Two‐Lane Highways

Current practices in resurfacing, restoration, and rehabilitation of highways generally emphasize lane‐ and shoulder‐width improvements. These practices neglect the possibility that flattening curves by reducing curvature may achieve more cost‐effective designs. At the present time, guidelines for considering curve flattening on rural two‐lane highways are not well established. Because accident characteristics on such highways can vary substantially from one state to another, it is desirable for each state to develop its own guidelines. As a preliminary effort, this study examines the conditions under which curve flattening on rural two‐lane highways in the State of New York may become cost‐effective. The study is based primarily on data reported for 155 horizontal curves in the State of New York. These curves have gentle grades and uniform geometric design. Their accident rates are governed primarily by the curvatures of the curves, and to a lesser extent by the traffic exposures. Raising the superelevation rates of these curves is unlikely to bring about a significant reduction in accidents. Flattening curves with curvatures less than 11° to achieve cost‐effective improvements is difficult.