Hamid behbahani - Department of civil engineering, Iran university of science and technology,Tehran,Iran
Sanaz kazemzadeh

Each year a large amount of traffic accidents occurs on two-lane rural highways specially on curved sections due to lack of geometric design consistency. Design consistency is one of the most significant methods in highway safety evaluation. An effective criteria to consider design consistency is through operating speed prediction and speed profile presentation. This paper develops operating speed models for combined horizontal and vertical curves on two-lane rural highways. Two types of combinations were considered: a horizontal curve combined with a sag vertical curve and a horizontal curve combined with a crest vertical curve. Regression analysis were used to develop operating speed models based on data collected on 10 combined curves in Karaj-Chalus rural highway. Validation results is presented and speed profiles were drawn for three one-kilometer road segments. In order to consider the relationship between safety and design consistency, collision crash rate models in both cases were presented using accident data in 14 combined curves. The results show that the most significant predictors for operating speed are radius of horizontal curve, horizontal distance between the point of horizontal intersection and the point of vertical intersection, length of vertical curve (rate of curvature), gradients and algebraic difference in grades and for collisions are operating speed differential in successive sections, degree of curve and roadway width

combined horizontal and vertical curves, design consistency, regression analysis, speed profile