I Evaluation of Design Consistency of Twtine Rural

Despite the growth in traffic volumes, average operating speeds on rural highways have increased by as much as 8 kilometers per hour (kmh) in the past decade.1 But in most cases, horizontal and vertical alignments of these roads have remained unchanged. Together these two events have created geometric inconsistencies in certain locations because of the required curvature (demand) being greater than the available curvature (supply). There is still little agreement on the safety effects of horizontal geometric inconsistencies. Even the relationships between accidents and horizontal alignment established by different researchers4,s are considerably different from one another. However, several researchers2,3 have defined the inconsistency of a road section in terms of the difference between free-flow operating speed (VO)and design speed (V~) of vehicles in that section. The simplicity and premises underlying those definitions of consistency allow road agencies to use them to evaluate roadway safety or identify hazardous locations. The key to using this consistency approach is to be able to accurately estimate expected Vo. This article discusses the estimation of expected VO in horizontal curves. Based on a study of speeds on a twolane rural highway in Utah, it is shown