Influence of the General Character of Horizontal Alignment on Operating Speed of Two-Lane Rural Roads

Since the speed on curves and tangents depends not only on the characteristics of the individual alignment elements but also on the general character of the road alignment, this study developed a speed prediction model for two-lane rural roads that also considers the influence of this character. In particular, the model includes the influence that horizontal alignment has on driver speed behavior through the use of the curvature change rate index. This index is used to divide the road into sections with relatively uniform horizontal alignment and to estimate the desired speed that represents the maximum operating speed that can be achieved and maintained along each road section. Moreover, the model estimates the operating speed on curves using four equations. Each equation refers to a different curvature change rate range and is used for curves that belong to a road section that has the curvature change rate that falls within this range. The use of the curvature change rate in the speed prediction model makes it possible to effectively apply the model to roads with very different alignments, since the predicted speeds on tangents and on curves are, in any case, consistent with the general character of these alignments. The significant differences between the speeds estimated using this model, and the speeds estimated using the analogous model that was developed without considering the influence of the horizontal alignment, underscore the importance of including this influence in the operating speed prediction models used for design consistency evaluations.