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 takes into consideration the influence of this character. In particular, the model includes the influence that the horizontal alignment has on driver speed behavior through the use of the curvature change rate index that is used to divide the road into sections with relatively uniform horizontal alignment. The model estimates the desired speed which represents the maximum operating speed that can be achieved and maintained along a 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 which 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 which was developed without considering the influence of the horizontal alignment, underline the importance of including this influence in the operating speed prediction models used for design consistency evaluations.
Abstract