Free-flow Speed Changes in the Vicinity of Horizontal Curves

Context-sensitive design principles have been highly promoted in recent years. The application of these principles might lead to situations where design standards cannot be met because of restricting local conditions. In such cases, horizontal curves have reduced design speeds compared to the adjacent tangents, requiring drivers to reduce speeds to negotiate the curvature change. The design of transition sections takes into account safety, comfort, and road appearance. Spiral curves are often used to provide a gradual change in the alignment curvature and to preclude speed changes while negotiating the curvature change. Thus far, there is no empirical basis or research that supports the design of spirals or tangent-to-curve transition sections when the horizontal curve and the adjacent tangents have different design speeds. Driver behavior before and after horizontal curves was evaluated. The results establish that 65.5% of the deceleration transition and 71.6% of the acceleration transition occur on the preceding and the following tangent segments to a curve, respectively. The results also establish that the mean deceleration rate and the mean acceleration rate are 0.033 (ft/s)/ft and 0.022 (ft/s)/ft, respectively. These rates are approximately 2.4 ft/sq s and 1.6 ft/sq s, respectively, for a 10-mph reduction. Speed-predicting models for transition sections are also provided. These models can be used to design the length of spirals or the transition sections when there is a difference in the design speed between the curve and the adjacent tangents. The models can also be used to assess the design consistency of two-lane rural highways.